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CoVO 3 High-Pressure Polymorphs: To Order or Not to Order? ADVANCED SCIENCE (WEINHEIM, BADEN-WURTTEMBERG, GERMANY) 2024; 11:e2307766. [PMID: 38103011 PMCID: PMC10916632 DOI: 10.1002/advs.202307766] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/17/2023] [Revised: 11/21/2023] [Indexed: 12/17/2023]
Abstract
Materials properties are determined by their compositions and structures. In ABO3 oxides different cation orderings lead to mainly perovskite- or corundum like derivatives with exciting physical properties. Sometimes, a material can be stabilized in more than one structural modification, providing a unique opportunity to explore structure-properties relationship. Here, CoVO3 obtained in both ilmenite-(CoVO3 -I) and LiNbO3 -type (CoVO3 -II) polymorphs at moderate (8-12 GPa) and high pressures (22 GPa), respectively are presented. Their distinctive cation distributions affect drastically the magnetic properties as CoVO3 -II shows a cluster-glass behavior while CoVO3 -I hosts a honeycomb zigzag magnetic structure in the cobalt network. First principles calculations show that the influence of vanadium is crucial for CoVO3 -I, although it is previously considered as non-magnetic in a dimerized spin-singlet state. Contrarily, CoVO3 -II shows two independent interpenetrating antiferromagnetic Co- and ferromagnetic V-hcp sublattices, which intrinsically frustrate any possible magnetic order. CoVO3 -II is also remarkable as the first oxide crystallizing with the LiNbO3 -type structure where both metals contain free d electrons. CoVO3 polymorphs pinpoint therefore as well to a much broader phase field of high-pressure A-site Cobaltites.
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(CN 3H 6)[Fe IIFe III(SO 4) 3(H 2O) 3]: A Framework Iron Sulfate with a Mixed S = 2 and S = 5/2 Honeycomb Lattice. Inorg Chem 2023; 62:17625-17633. [PMID: 37844565 DOI: 10.1021/acs.inorgchem.3c02109] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2023]
Abstract
A new guanidinium-templated hydrated iron sulfate, [CN3H6][FeIIFeIII(SO4)3(H2O)3] (1), was prepared from strongly acidic aqueous solutions. Its crystal structure is comprised from FeIIIO6 and FeIIO3(H2O)3 octahedra linked by sulfate bridges forming a [FeIIFeII(SO4)3(H2O)3]- 3D framework with a layer-by-layer ordering of ferric and ferrous cations. The structural topology of the framework is related to the anhydrous rhombohedral mikasaite Fe2(SO4)3. The removal of part of the sulfate tetrahedra and the partial replacement of the Fe3+ cations in the [Fe3+2(SO4)3]0 framework by Fe2+ provide a negative charge and allow the incorporation of the protonated organic species in the voids. The compound 1 has been characterized by single-crystal X-ray diffraction, TG and DSC analyses, UV-vis-NIR spectroscopy, magnetic susceptibility, Mössbauer spectroscopy, IR and Raman spectroscopy, and density functional band-structure calculations. The magnetic behavior of 1 shows an interplay of FeII (S = 2) and FeIII (S = 5/2) sublattices that exhibit different types of antiferromagnetic couplings, one FeIII-FeIII (J1 ∼ 6.1 K) and two FeII-FeIII couplings (J2 ∼ 1 K, J3 ∼ 5.9 K) within corrugated honeycomb layers. These ferrimagnetic layers are coupled antiparallel to each other, resulting in an overall antiferromagnetic order below TN = 31 K.
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Effect of antifluorite layer on the magnetic order in Eu-based 1111 compounds, EuTAsF (T = Zn, Mn, and Fe). Phys Chem Chem Phys 2023; 25:4862-4871. [PMID: 36692371 DOI: 10.1039/d2cp04863a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
The 1111 compounds with an alternating sequence of fluorite and antifluorite layers serve as structural hosts for the vast family of Fe-based superconductors. Here, we use neutron powder diffraction and density-functional-theory (DFT) band-structure calculations to study magnetic order of Eu2+ in the [EuF]+ fluorite layers depending on the nature of the [TAs]- antifluorite layer that can be non-magnetic semiconducting (T = Zn), magnetic semiconducting (T = Mn), or magnetic metallic (T = Fe). Antiferromagnetic transitions at TN ∼ 2.4-3 K due to an ordering of the Eu2+ magnetic moments were confirmed in all three EuTAsF compounds. Whereas in EuTAsF (T = Zn and Mn), the commensurate k1 = (½ ½ 0) stripe order pattern with magnetic moments within the a-b plane is observed, the order in EuFeAsF is incommensurate with k = (0 0.961(1) ½) and represents a cycloid of Eu2+ magnetic moments confined within the bc-plane. Additionally, the Mn2+ sublattice in EuMnAsF features a robust G-type antiferromagnetic order that persists at least up to room temperature, with magnetic moments along the c-direction. Although DFT calculations suggest stripe antiferromagnetic order in the Fe-sublattice of EuFeAsF as the ground state, neutron diffraction reveals no evidence of long-range magnetic order associated with Fe. We show that the frustrating interplane interaction J3 between the adjacent [EuF]+ layers is comparable with in-plane J1-J2 interactions already in the case of semiconducting fluorite layers [TAs]- (T = Zn and Mn) and becomes dominant in the case of the metallic [FeAs]- ones. The latter, along with a slight orthorhombic distortion, is proposed to be the origin of the incommensurate magnetic structure observed in EuFeAsF.
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Chemical Vapor Transport Synthesis of Cu(VO) 2(AsO 4) 2 With Two Distinct Spin-1/2 Magnetic Ions. Inorg Chem 2022; 61:16539-16548. [PMID: 36226856 DOI: 10.1021/acs.inorgchem.2c00293] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
A new copper vanadyl arsenate, Cu(VO)2(AsO4)2, was synthesized via the chemical vapor transport method. Cu(VO)2(AsO4)2 adopts an original structure type. It is characterized by layers formed by edge-sharing and corner-sharing V-centered octahedra resulting in a unique topology that was hitherto not reported for vanadates. Single CuO6 octahedra connect vanadate layers into a rigid framework. The thermal expansion of the framework studied by the single-crystal HT X-ray diffraction is reported. The magnetic behavior of Cu(VO)2(AsO4)2 shows an interplay of ferromagnetic V4+-V4+ and antiferromagnetic Cu2+-V4+ interactions that result in a ferrimagnetic long-range order below TC = 66 K.
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Composition dependent polymorphism and superconductivity in Y 3+x{Rh,Ir} 4Ge 13-x. Dalton Trans 2022; 51:4734-4748. [PMID: 35244111 DOI: 10.1039/d2dt00167e] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Polymorphism is observed in the Y3+xRh4Ge13-x series. The decrease of Y-content leads to the transformation of the primitive cubic Y3.6Rh4Ge12.4 [x = 0.6, space group Pm3̄n, a = 8.96095(9) Å], revealing a strongly disordered structure of the Yb3Rh4Sn13 Remeika prototype, into a body-centred cubic structure [La3Rh4Sn13 structure type, space group I4132, a = 17.90876(6) Å] for x = 0.4 and further into a tetragonal arrangement (Lu3Ir4Ge13 structure type, space group I41/amd, a = 17.86453(4) Å, a = 17.91076(6) Å) for the stoichiometric (i.e. x = 0) Y3Rh4Ge13. Analogous symmetry lowering is found within the Y3+xIr4Ge13-x series, where the compound with Y-content x = 0.6 is crystallizing with La3Rh4Sn13 structure type [a = 17.90833(8) Å] and the stoichiometric Y3Ir4Ge13 is isostructural with the Rh-analogue [a = 17.89411(9) Å, a = 17.9353(1) Å]. The structural relationships of these derivatives of the Remeika prototype are discussed. Compounds from the Y3+xRh4Ge13-x series are found to be weakly-coupled BCS-like superconductors with Tc = 1.25, 0.43 and 0.6, for x = 0.6, 0.4 and 0, respectively. They also reveal low thermal conductivity (<1.5 W K-1 m-1 in the temperature range 1.8-350 K) and small Seebeck coefficients. The latter are common for metallic systems. Y3Rh4Ge13 undergoes a first-order phase transition at Tf = 177 K, with signatures compatible to a charge density wave scenario. The electronic structure calculations confirm the instability of the idealized Yb3Rh4Sn13-like structural arrangements for Y3Rh4Ge13 and Y3Ir4Ge13.
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Semiconducting and Metallic Compounds within the IrIn 3 Structure Type: Stability and Chemical Bonding. Inorg Chem 2022; 61:3274-3280. [PMID: 35138836 DOI: 10.1021/acs.inorgchem.1c03843] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Narrow-gap semiconductors are very rare among intermetallic compounds. They appear only when two factors come together: strong hybridization of valence orbitals in the vicinity of the Fermi level and an appropriate number of valence electrons. Surprisingly, the IrIn3 family of intermetallics contains a number of semiconductors, including 17 e- FeGa3, RuGa3, OsGa3, and RuIn3, for which the d-p hybridization gap opens at the Fermi energy. We present comprehensive total energy electronic-structure calculations and crystal orbital Hamilton population analysis of the stable IrIn3-type compounds with semiconducting and metallic properties. The calculated electronic structures possess two pseudogaps and one real gap at the magic valence electron count of 15, 17, and 18 e- per formula unit. When the Fermi level is located in these gaps, the antibonding states are minimized. Total energies calculated for the isomorphous compounds suggest that the metallic state with 18 e- leads to a comparable or even higher thermodynamic stability than the semiconducting state with 17 e-.
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From ( S = 1) Spin Hexamer to Spin Tetradecamer by CuO Interstitials in A 2Cu 3O(CuO) x(SO 4) 3 (A = alkali). Inorg Chem 2021; 60:18185-18191. [PMID: 34812626 DOI: 10.1021/acs.inorgchem.1c02808] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
(Na,K)2Cu3O(SO4)3 compounds form structural chains of Cu6 hexameric units with nominal S = 1 spins due to the interplay between inner strong antiferromagnetic and ferromagnetic exchanges. We show here that the lattice relaxation after the replacement of alkali by larger Rb and Cs ones is accompanied by the insertion of neutral CuO species into (Rb,Cs)2Cu3O(CuO)x(SO4)3 phases. Structurally, interstitial CuO links the next two Cu6 units in longer Cu14 tetradecameric ones. For A = Cs (x = 0.5), the cationic ordering is perfect inside a double-cell superstructure. Magnetically, the original Cu14 units consist of frustrated fragments of an S = 1/2 spin ladder, with ferromagnetic rung-like but antiferromagnetic leg-like and next-nearest neighbor couplings. It returns S = 1 Cu14 spin clusters, effective around 100 K. Our density functional theory calculations and susceptibility fits also show that at low temperatures they interact in two-dimensional lattices, despite the existence of short inter-Cu-Cu distances between the next two clusters along pseudo-one-dimensional chains.
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Abstract
In contrast to the corundum-type A2X3 structure, which has only one crystallographic site available for trivalent cations (e.g., in hematite), the closely related ABX3 ilmenite-type structure comprises two different octahedrally coordinated positions that are usually filled with differently charged ions (e.g., in Fe2+Ti4+O3 ilmenite). Here, we report a synthesis of the first binary ilmenite-type compound fabricated from a simple transition-metal oxide (Mn2O3) at high-pressure high-temperature (HP-HT) conditions. We experimentally established that, at normal conditions, the ilmenite-type Mn2+Mn4+O3 (ε-Mn2O3) is an n-type semiconductor with an indirect narrow band gap of Eg = 0.55 eV. Comparative investigations of the electronic properties of ε-Mn2O3 and previously discovered quadruple perovskite ζ-Mn2O3 phase were performed using X-ray absorption near edge spectroscopy. Magnetic susceptibility measurements reveal an antiferromagnetic ordering in ε-Mn2O3 below 210 K. The synthesis of ε-Mn2O3 indicates that HP-HT conditions can induce a charge disproportionation in simple transition-metal oxides A2O3, and potentially various mixed-valence polymorphs of these oxides, for example, with ilmenite-type, LiNbO3-type, perovskite-type, and other structures, could be stabilized at HP-HT conditions.
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Abstract
We synthesized single crystals of marokite (CaMn2O4)-type orthorhombic manganese (II,III) oxide, γ-Mn3O4, in a multianvil apparatus at pressures of 10-24 GPa. The magnetic, electronic, and optical properties of the crystals were investigated at ambient pressure. It was found that γ-Mn3O4 is a semiconductor with an indirect band gap Eg of 0.96 eV and two antiferromagnetic transitions (TN) at ∼200 and ∼55 K. The phase stability of the γ-Mn3O4 crystals was examined in the pressure range of 0-60 GPa using single-crystal X-ray diffraction and Raman spectroscopy. A bulk modulus of γ-Mn3O4 was determined to be B0 = 235.3(2) GPa with B' = 2.6(6). The γ-Mn3O4 phase persisted over the whole pressure range studied and did not transform or decompose upon laser heating of the sample to ∼3500 K at 60 GPa. This result seems surprising, given the high-pressure structural diversity of iron oxides with similar stoichiometries. With an increase in pressure, the degree of distortion of MnO6 polyhedra decreased. Furthermore, there are signs indicating a limited charge transfer between the Mn3+ ions in the octahedra and the Mn2+ ions in the trigonal prisms. Our results demonstrate that the high-pressure behavior of the structural, electronic, and chemical properties of manganese oxides strongly differs from that of iron oxides with similar stoichiometries.
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Abstract
Mixed-valent transition-metal compounds display complex structural, electronic and magnetic properties, which often intricately coexist. Here, we report the new ternary oxide GaV4O8, a structural sibling of skyrmion-hosting lacunar spinels. GaV4O8 contains a vanadium trimer and an original spin-orbital-charge texture that forms upon the structural phase transition at TS = 68 K followed by the magnetic transition at TN = 35 K. The texture arises from the coexistence of orbital molecules on the vanadium trimers and localized electrons on the remaining vanadium atoms. Such hybrid electrons create opportunities for novel types of spin, charge, and orbital order in mixed-valent transition-metal compounds.
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Cooperative Cluster Jahn-Teller Effect as a Possible Route to Antiferroelectricity. PHYSICAL REVIEW LETTERS 2021; 126:187601. [PMID: 34018769 DOI: 10.1103/physrevlett.126.187601] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/16/2020] [Revised: 03/02/2021] [Accepted: 04/08/2021] [Indexed: 06/12/2023]
Abstract
We report the observation of an antipolar phase in cubic GaNb_{4}S_{8} driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb_{4}S_{4} molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at T_{JT}=31 K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above T_{JT}. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb_{4}S_{8} may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferroelectric.
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Magnetic structures of Fe 32+δGe 33As 2 and Fe 32+δ'Ge 35-xP x intermetallic compounds: a neutron diffraction and 57Fe Mössbauer spectroscopy study. Dalton Trans 2021; 50:2210-2220. [PMID: 33502418 DOI: 10.1039/d0dt03923c] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Fe32+δGe33As2 and Fe32+δ'Ge35-xPx are quasi-binary intermetallic compounds that possess a rare variant of intergrowth-type crystal structure, which is a combination of the column shaped Co2Al5 and MgFe6Ge6 structure type blocks. The compounds are antiferromagnets with the Néel temperatures around 125 K. Neutron powder diffraction experiments on the samples with δ≈ 0.1, δ'≈ 0.5 and x≈ 3 reveal commensurate magnetic ordering of low symmetry in both compounds and a non-monotonic change in the intensities of magnetic reflections. On the other hand, temperature dependence of the hyperfine fields obtained from 57Fe Mössbauer spectroscopy indicates a gradual, monotonic increase in local magnetic fields upon cooling. We interpret these results as a spin reorientation within the Co2Al5-type block of the crystal structure, with the possible formation of a non-collinear magnetic order at low temperatures. Between the compounds, the reorientation occurs at significantly different temperatures, however the resulting magnetic structures themselves are similar as well as the average values of the magnetic moments and the hyperfine fields.
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Semiconducting and superconducting Mo–Ga frameworks: total energy and chemical bonding. Inorg Chem Front 2021. [DOI: 10.1039/d0qi01515f] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
Abstract
The superconducting Mo4Ga21 structure type is derived from the electron-precise MoGa4 cubic framework.
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Abstract
Poly- and single-crystalline samples of In0.67□0.33In2S4 thiospinel were obtained by various powder metallurgical and chemical vapor transport methods, respectively. All synthesized samples contained β-In0.67□0.33In2S4 modification only, independent of the synthesis procedure. High-resolution powder X-ray diffraction (PXRD) experiments at 80 K enabled the observation of split tetragonal reflections (completely overlapped at room temperature), which prove the correctness of the crystal structure model accepted for the β-polymorph. Combining single-crystal XRD, transmission electron microscopy and selected-area electron diffraction studies, the presence of three twin domains in the as-grown crystals was confirmed. A high temperature PXRD study revealed both abrupt (in full widths at half maxima of main reflections and in unit-cell volume) and gradual (in intensity of satellites and c/a ratio) changes in the vicinity of the α-β phase transition. These observations, together with a clear endothermic peak in the heat capacity, the magnitude of enthalpy/entropy change and the temperature dependence of electrical resistivity (associated with hysteresis), hinted towards the 1st order type of transition. Three scenarios, based on Rietveld refinement analysis, were considered for the description of the crystal structure evolution from β- to α-modification, including the (3+3)D-modulated cubic structure at 693 K as an intermediate state during the β-α transformation. The Seebeck coefficient, electrical resistivity and thermal conductivity were not only influenced by phase transition, but also by annealing conditions (S-poor or S-rich atmosphere). Density functional theory calculations predicted semiconducting behavior of In0.67□0.33In2S4, as well as instability of the fictitious InIn2S4 thiospinel.
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Thermodynamic Perspective on Field-Induced Behavior of α-RuCl_{3}. PHYSICAL REVIEW LETTERS 2020; 125:097203. [PMID: 32915615 DOI: 10.1103/physrevlett.125.097203] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/06/2020] [Accepted: 07/28/2020] [Indexed: 06/11/2023]
Abstract
Measurements of the magnetic Grüneisen parameter (Γ_{B}) and specific heat on the Kitaev material candidate α-RuCl_{3} are used to access in-plane field and temperature dependence of the entropy up to 12 T and down to 1 K. No signatures corresponding to phase transitions are detected beyond the boundary of the magnetically ordered region, but only a shoulderlike anomaly in Γ_{B}, involving an entropy increment as small as 10^{-5}Rlog2. These observations put into question the presence of a phase transition between the purported quantum spin liquid and the field-polarized state of α-RuCl_{3}. We show theoretically that at low temperatures Γ_{B} is sensitive to crossings in the lowest excitations within gapped phases, and identify the measured shoulderlike anomaly as being of such origin. Exact diagonalization calculations demonstrate that the shoulderlike anomaly can be reproduced in extended Kitaev models that gain proximity to an additional phase at finite field without entering it. We discuss manifestations of this proximity in other measurements.
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Synthesis, electronic structure and physical properties of two new layered compounds, EuFAgSe and EuFAg 1-δTe, featuring the active redox pair Eu 2+/Ag . Dalton Trans 2020; 49:7426-7435. [PMID: 32432284 DOI: 10.1039/d0dt01504k] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Systematic studies of the ZrCuSiAs (also LaOAgS or 1111) structure type resulted in the synthesis of two new fluoride chalcogenides, EuFAgSe and EuFAg1-δTe, whereas their sulfide analog, EuFAgS, could not be obtained. Both new compounds are tetragonal, P4/nmm, with cell parameters a = 4.1542(1) Å, c = 9.2182(1) Å for the selenide and a = 4.3255(1) Å, c = 9.5486(1) Å for the telluride. Rietveld refinement reveals a significant silver deficiency in the telluride (δ = 0.05), while the selenide is nearly stoichiometric. Both compounds are semiconductors as shown by diffuse reflectance spectroscopy and confirmed by density-functional calculations of the band structure. Magnetism of both compounds is predominantly driven by Eu2+, as indicated by magnetic susceptibility measurements and corroborated by 151Eu Mössbauer spectroscopy. EuFAg1-δTe and EuFAgSe are paramagnetic down to 1.8 K.
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Spin liquids in geometrically perfect triangular antiferromagnets. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2020; 32:224004. [PMID: 32015221 DOI: 10.1088/1361-648x/ab724e] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
The cradle of quantum spin liquids, triangular antiferromagnets show strong proclivity to magnetic order and require deliberate tuning to stabilize a spin-liquid state. In this brief review, we juxtapose recent theoretical developments that trace the parameter regime of the spin-liquid phase, with experimental results for Co-based and Yb-based triangular antiferromagnets. Unconventional spin dynamics arising from both ordered and disordered ground states are discussed, and the notion of a geometrically perfect triangular system is scrutinized to demonstrate non-trivial imperfections that may assist magnetic frustration in stabilizing dynamic spin states with peculiar excitations.
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Two Linear Regimes in Optical Conductivity of a Type-I Weyl Semimetal: The Case of Elemental Tellurium. PHYSICAL REVIEW LETTERS 2020; 124:136402. [PMID: 32302162 DOI: 10.1103/physrevlett.124.136402] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/21/2019] [Revised: 02/07/2020] [Accepted: 03/03/2020] [Indexed: 06/11/2023]
Abstract
Employing high-pressure infrared spectroscopy we unveil the Weyl semimetal phase of elemental Te and its topological properties. The linear frequency dependence of the optical conductivity provides clear evidence for metallization of trigonal tellurium (Te-I) and the linear band dispersion above 3.0 GPa. This semimetallic Weyl phase can be tuned by increasing pressure further: a kink separates two linear regimes in the optical conductivity (at 3.7 GPa), a signature proposed for Type-II Weyl semimetals with tilted cones; this however reveals a different origin in trigonal tellurium. Our density-functional calculations do not reveal any significant tilting and suggest that Te-I remains in the Type-I Weyl phase, but with two valence bands in the vicinity of the Fermi level. Their interplay gives rise to the peculiar optical conductivity behavior with more than one linear regime. Pressure above 4.3 GPa stabilizes the more complex Te-II and Te-III polymorphs, which are robust metals.
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Innentitelbild: A Room‐Temperature Verwey‐type Transition in Iron Oxide, Fe
5
O
6
(Angew. Chem. 14/2020). Angew Chem Int Ed Engl 2020. [DOI: 10.1002/ange.202001375] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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Inside Cover: A Room‐Temperature Verwey‐type Transition in Iron Oxide, Fe
5
O
6
(Angew. Chem. Int. Ed. 14/2020). Angew Chem Int Ed Engl 2020. [DOI: 10.1002/anie.202001375] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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Cu 9O 2(VO 4) 4Cl 2, the First Copper Oxychloride Vanadate: Mineralogically Inspired Synthesis and Magnetic Behavior. Inorg Chem 2020; 59:2136-2143. [PMID: 31999107 DOI: 10.1021/acs.inorgchem.9b02565] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
A new spin-1/2 frustrated antiferromagnet, Cu9O2(VO4)4Cl2, was synthesized via chemical vapor transport method that emulates mineral formation in volcanic fumaroles. Cu9O2(VO4)4Cl2 is the first copper oxychloride vanadate obtained in the ternary CuO-V2O5-CuCl2 anhydrous system. Copper ions constitute a three-dimensional complex framework with a topological structure novel for synthetic compounds but similar to that in the fumarolic mineral yaroshevskite. All of the oxygen atoms except for the O7 site are strongly bonded in the VO4 tetrahedra. The O7 site belongs to an additional oxygen atom (Oa) being tetrahedrally coordinated by four Cu atoms, thus forming the OCu4 tetrahedra. The structural formula can be represented as Cu3[Cu6O2](VO4)4Cl2 highlighting oxocentered units in the structure. IR spectra reveal several absorption bands at 526, 578, and 601 cm-1 interpreted as a characteristic feature of the OCu4 tetrahedra. Cu9O2(VO4)4Cl2 reveals ferrimagnetic behavior with the Curie temperature TC = 24 K and the uncompensated moment of Mr ∼ 1.9 μB/f.u.
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A Room-Temperature Verwey-type Transition in Iron Oxide, Fe 5 O 6. Angew Chem Int Ed Engl 2020; 59:5632-5636. [PMID: 31899577 PMCID: PMC7154779 DOI: 10.1002/anie.201914988] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/24/2019] [Revised: 12/30/2019] [Indexed: 12/02/2022]
Abstract
Functional oxides whose physicochemical properties may be reversibly changed at standard conditions are potential candidates for the use in next‐generation nanoelectronic devices. To date, vanadium dioxide (VO2) is the only known simple transition‐metal oxide that demonstrates a near‐room‐temperature metal–insulator transition that may be used in such appliances. In this work, we synthesized and investigated the crystals of a novel mixed‐valent iron oxide with an unconventional Fe5O6 stoichiometry. Near 275 K, Fe5O6 undergoes a Verwey‐type charge‐ordering transition that is concurrent with a dimerization in the iron chains and a following formation of new Fe−Fe chemical bonds. This unique feature highlights Fe5O6 as a promising candidate for the use in innovative applications. We established that the minimal Fe−Fe distance in the octahedral chains is a key parameter that determines the type and temperature of charge ordering. This model provides new insights into charge‐ordering phenomena in transition‐metal oxides in general.
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EuNi2P4, the first magnetic unconventional clathrate prepared via a mechanochemically assisted route. Inorg Chem Front 2020. [DOI: 10.1039/c9qi01566c] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The first magnetic unconventional clathrate EuNi2P4 has been prepared and its thermodynamic properties have been investigated.
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Li 2(Se 2O 5)(H 2O) 1.5·CuCl 2, a salt-inclusion diselenite structurally based on tetranuclear Li 4 complexes. Dalton Trans 2020; 49:7790-7795. [DOI: 10.1039/d0dt01260b] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
A new lithium copper diselenite chloride hydrate, Li2Se2O5(H2O)1.5·CuCl2, was prepared from aqueous solution.
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Endohedral Cluster Superconductors in the Mo-Ga-Sn System Explored by the Joint Flux Technique. Inorg Chem 2019; 58:15552-15561. [PMID: 31657910 DOI: 10.1021/acs.inorgchem.9b02598] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Endohedral Ga cluster compounds feature nontrivial superconducting states including the two-gap superconductivity similar in nature to MgB2. We use the joint flux synthetic technique to introduce Sn into the Ga matrix and tune the valence electron count in the two new endohedral cluster superconductors Mo8Ga41-xSnx and Mo4Ga21-x-δSnx with critical temperatures of Tc = 8.7 and 5.85 K, respectively. While the former compound is a derivative of the previously known Mo8Ga41 superconductor, where Sn atoms are enclosed inside the Sn@Ga6 octahedral clusters, the latter is a new architecture built upon Mo@Ga9Sn clusters, Ga@Ga12 cuboctahedra, and Sn4 squares. We show that this novel Mo4Ga21-x-δSnx superconductor features strong electron-phonon coupling with the large ratio of 2Δ(0)/(kBTc) = 4.1 similar to that of the Mo8Ga41 superconductor with the closely related crystal structure.
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Synthesis, crystal and electronic structures of Pt-rich phosphides EuPt 3P and EuPt 6P 2. Dalton Trans 2019; 48:15272-15282. [PMID: 31580356 DOI: 10.1039/c9dt02845e] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Two new ternary Pt-rich phosphides, EuPt6P2 and EuPt3P, have been prepared via a two-step solid state reaction. Their crystal structures have been determined from powder XRD data. EuPt6P2 is isostructural to SrPt6P2 (cubic, Pa3[combining macron], a = 8.4603(1) Å); its crystal structure comprises corner-sharing Pt6P trigonal prisms hosting Eu2+ cations in the cuboctahedral voids of the framework. EuPt3P is isostructural to the SrPt3P anti-perovskite (P4/nmm, a = 5.7452(1) Å and c = 5.4212(1) Å). Magnetization measurements reveal the magnetic response caused by the Eu2+(4f7) cations. EuPt6P2 is paramagnetic exhibiting no phase transitions down to 1.8 K, whereas EuPt3P orders ferromagnetically below 19 K. Similar to SrPt6P2 and SrPt3P, the new compounds are metallic with states near the Fermi level predominantly formed by the 5d orbitals of Pt.
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Structure–magnetic property correlations in metal–formate frameworks at high pressure. Acta Crystallogr A Found Adv 2019. [DOI: 10.1107/s2053273319092556] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
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From endohedral cluster superconductors to approximant phases: synthesis, crystal and electronic structure, and physical properties of Mo 8Ga 41-xZn x and Mo 7Ga 52-xZn x. Dalton Trans 2019; 48:7853-7861. [PMID: 31073584 DOI: 10.1039/c8dt04982c] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Using the crystal-growth joint flux technique based on the combination of two aliovalent low-melt metals, gallium and zinc, we adjust the gross valence electron count in the Mo-Ga-Zn system and produce the Mo8Ga41-xZnx and Mo7Ga52-xZnx intermetallic compounds. Gradual reduction in the valence electron count first leads to the Zn for Ga substitution in the Mo8Ga41 endohedral cluster superconductor, accompanied by the formation of Zn-containing clusters in the crystal structure and by the gradual suppression of superconductivity. Mo8Ga41-xZnx with x = 7.2(2) exhibits superconducting properties below TC = 4 K, whereas there is no superconducting transition at temperatures above 2 K for the limiting composition of x = 11.3(2). Further, the Mo7Ga52-xZnx phase is formed from the flux with a higher content of Zn. Mo7Ga52-xZnx crystallizes in the Mo7Sn12Zn40 structure type with a narrow homogeneity range and exhibits metallic behavior with no sign of superconductivity down to at least 1.8 K. Its experimental valence electron count of 2.9 e per atom is below that of endohedral gallium cluster superconductors. Electronic structures of Mo8Ga41-xZnx and Mo7Ga52-xZnx feature the opening of a pseudogap slightly below the Fermi level indicating the specific stability of these structure types at the valence electron count of 3.2 e per atom and 2.7 e per atom, respectively.
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Rearrangement of Uncorrelated Valence Bonds Evidenced by Low-Energy Spin Excitations in YbMgGaO_{4}. PHYSICAL REVIEW LETTERS 2019; 122:137201. [PMID: 31012603 DOI: 10.1103/physrevlett.122.137201] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/23/2018] [Indexed: 05/02/2023]
Abstract
dc-magnetization data measured down to 40 mK speak against conventional freezing and reinstate YbMgGaO_{4} as a triangular spin-liquid candidate. Magnetic susceptibility measured parallel and perpendicular to the c axis reaches constant values below 0.1 and 0.2 K, respectively, thus indicating the presence of gapless low-energy spin excitations. We elucidate their nature in the triple-axis inelastic neutron scattering experiment that pinpoints the low-energy (E≤J_{0}∼0.2 meV) part of the excitation continuum present at low temperatures (T<J_{0}/k_{B}), but completely disappearing upon warming the system above T≫J_{0}/k_{B}. In contrast to the high-energy part at E>J_{0} that is rooted in the breaking of nearest-neighbor valence bonds and persists to temperatures well above J_{0}/k_{B}, the low-energy one originates from the rearrangement of the valence bonds and thus from the propagation of unpaired spins. We further extend this picture to herbertsmithite, the spin-liquid candidate on the kagome lattice, and argue that such a hierarchy of magnetic excitations may be a universal feature of quantum spin liquids.
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Crystal Growth of Intermetallics from the Joint Flux: Exploratory Synthesis through the Control of Valence Electron Count. Inorg Chem 2019; 58:1561-1570. [PMID: 30615431 DOI: 10.1021/acs.inorgchem.8b03083] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
In this study, we modify the flux-growth method for the purpose of exploratory synthesis of ternary intermetallic compounds. Our concept is based on the assumption that valence electron count plays a crucial role in the stability of polar intermetallic compounds of different structure types. Control of the valence electron count parameter is made possible through the use of an excess of two metals having a different number of valence electrons. By gradually changing the ratio between these metals in the joint flux, we scan the gross number of valence electrons and explore the crystallization of new compounds. In the ternary system Re-Ga-Zn, we detect compounds belonging to three structure types, ReGa5, PtHg4, and V8Ga41, while gradually increasing the content of Zn metal in the flux. Two new compounds, ReGa3Zn and Re8Ga41- xZn x with x = 21.2(5), are obtained in the form of high-quality single crystals, and the former compound shows the narrow-gap semiconducting behavior favorable for high thermoelectric performance.
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Crystal Structures and Low-Dimensional Ferromagnetism of Sodium Nickel Phosphates Na 5Ni 2(PO 4) 3·H 2O and Na 6Ni 2(PO 4) 3OH. Inorg Chem 2019; 58:610-621. [PMID: 30565920 DOI: 10.1021/acs.inorgchem.8b02791] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Two new sodium nickel phosphates, Na5Ni2(PO4)3·H2O (I) and Na6Ni2(PO4)3OH (II), have been synthesized hydrothermally and characterized by synchrotron X-ray diffraction, electron diffraction, low-temperature thermodynamic and magnetic measurements, and ab initio calculations. Unlike the majority of Ni2+ compounds, I and II show predominant ferromagnetic exchange couplings. I crystallizes in the monoclinic space group P21/ n ( a = 14.0395(4) Å, b = 5.1847(14) Å, c = 16.4739(4) Å, β = 110.4186(14)°) and features chains of ferromagnetically coupled Ni2+ ions. In II with the orthorhombic space group Pcmb ( a = 7.5007(15) Å, b = 21.4661(4) Å, c = 7.1732(15) Å), the ferromagnetically coupled Ni2+ ions form dimers arranged on a spin ladder. Both compounds represent rare examples of quasi-one-dimensional ferromagnets. Structural features behind this unusual magnetic behavior are discussed.
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Irreversible Made Reversible: Increasing the Electrochemical Capacity by Understanding the Structural Transformations of Na xCo 0.5Ti 0.5O 2. ACS APPLIED MATERIALS & INTERFACES 2018; 10:36108-36119. [PMID: 30251827 DOI: 10.1021/acsami.8b11609] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Two new structural forms of Na xCo0.5Ti0.5O2, the layered O3- and P3-forms, were synthesized and comprehensively characterized. Both materials show electrochemical activity as electrodes in Na-ion batteries. During cell charging (desodiation of the Na xCo0.5Ti0.5O2 cathode), we observed a structural phase transformation of O3-Na0.95Co0.5Ti0.5O2 into P3-Na xCo0.5Ti0.5O2, whereas no changes other than conventional unit cell volume shrinkage were detected for P3-Na0.65Co0.5Ti0.5O2. During Na insertion (cell discharging), the reconversion of the P3-form into O3-Na xCo0.5Ti0.5O2 was impeded for both materials and occurs well below 1 V versus Na+/Na only. The reconversion is hindered by the charge and spin transfers of Co (LS-Co3+ → HS-Co2+) and by a significant unit cell volume expansion at the P3 → O3 transformation, as revealed from the magnetization, crystallographic, and spectroscopic studies. As the kinetics of such transformations depend on numerous parameters such as time, temperature, and particle size, a large cell overpotential ensues. An extended cutoff voltage at 0.2 V versus Na+/Na during discharging allows to complete the P3 → O3 transformation and increases the specific discharging capacity to 200 mA h g-1. Moreover, a quasi-symmetrical full cell, based on the O3- and P3-forms, was designed, eliminating safety concerns associated with sodium anodes and delivering a discharge capacity of 130 mA h g-1.
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Pressure dependence of spin canting in ammonium metal formate antiferromagnets. Phys Chem Chem Phys 2018; 20:24465-24476. [PMID: 30221645 DOI: 10.1039/c8cp03761b] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
High-pressure single-crystal X-ray diffraction at ambient temperature and high-pressure SQUID measurements down to 2 K were performed up to ∼2.5 GPa on ammonium metal formates, [NH4][M(HCOO)3] where M = Mn2+, Fe2+, and Ni2+, in order to correlate structural variations to magnetic behaviour. Similar structural distortions and phase transitions were observed for all compounds, although the transition pressures varied with the size of the metal cation. The antiferromagnetic ordering in [NH4][M(HCOO)3] compounds was maintained as a function of pressure, and the magnetic ordering transition temperature changed within a few kelvins depending on the structural distortion and the metal cation involved. These compounds, in particular [NH4][Fe(HCOO)3], showed greatest sensitivity to the degree of spin canting upon compression, clearly visible from the twenty-fold increase in the low-temperature magnetisation for [NH4][Fe(HCOO)3] at 1.4 GPa, and the change from purely antiferromagnetic to weakly ferromagnetic ordering in [NH4][Mn(HCOO)3] at 1 GPa. The variation in the exchange couplings and spin canting was checked with density-functional calculations that reproduce well the increase in canted moment within [NH4][Fe(HCOO)3] upon compression, and suggest that the Dzyaloshinskii-Moriya (DM) interaction is evolving as a function of pressure. The pressure dependence of spin canting is found to be highly dependent on the metal cation, as magnetisation magnitudes did not change significantly for when M = Ni2+ or Mn2+. These results demonstrate that the overall magnetic behaviour of each phase upon compression was not only dependent on the structural distortions but also on the electronic configuration of the metal cation.
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Spin-induced multiferroicity in the binary perovskite manganite Mn 2O 3. Nat Commun 2018; 9:2996. [PMID: 30065294 PMCID: PMC6068161 DOI: 10.1038/s41467-018-05296-0] [Citation(s) in RCA: 31] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2017] [Accepted: 05/01/2018] [Indexed: 11/10/2022] Open
Abstract
The ABO3 perovskite oxides exhibit a wide range of interesting physical phenomena remaining in the focus of extensive scientific investigations and various industrial applications. In order to form a perovskite structure, the cations occupying the A and B positions in the lattice, as a rule, should be different. Nevertheless, the unique binary perovskite manganite Mn2O3 containing the same element in both A and B positions can be synthesized under high-pressure high-temperature conditions. Here, we show that this material exhibits magnetically driven ferroelectricity and a pronounced magnetoelectric effect at low temperatures. Neutron powder diffraction revealed two intricate antiferromagnetic structures below 100 K, driven by a strong interplay between spin, charge, and orbital degrees of freedom. The peculiar multiferroicity in the Mn2O3 perovskite is ascribed to a combined effect involving several mechanisms. Our work demonstrates the potential of binary perovskite oxides for creating materials with highly promising electric and magnetic properties. Multiferroic binary oxides with the perovskite structure have been very rare. Here, Cong et al. report magnetically-driven ferroelectricity and a large magnetoelectric effect in a binary perovskite compound Mn2O3 at low temperatures.
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Magnetic resonance as a local probe for kagomé magnetism in Barlowite Cu 4(OH) 6FBr. Sci Rep 2018; 8:10851. [PMID: 30022078 PMCID: PMC6052016 DOI: 10.1038/s41598-018-29080-8] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/28/2018] [Accepted: 07/04/2018] [Indexed: 11/24/2022] Open
Abstract
Temperature- and field-dependent 1H-, 19F-, and 79,81Br-NMR measurements together with zero - field 79,81Br-NQR measurements on polycrystalline samples of barlowite, Cu4(OH)6FBr are conducted to study the magnetism and possible structural distortions on a microscopic level. The temperature dependence of the 79,81Br-NMR spin-lattice relaxation rates 1/T1 indicate a phase transition at TN \documentclass[12pt]{minimal}
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\begin{document}$$\simeq $$\end{document}≃ 15 K which is of magnetic origin, but with an unusually weak slowing down of fluctuations below TN. Moreover, 1/T1T scales linear with the bulk susceptibility which indicates persisting spin fluctuations down to 2 K. Quadupolare resonance (NQR) studies reveal a pair of zero-field NQR- lines associated with the two isotopes of Br with the nuclear spins of I = 3/2. Quadrupole coupling constants of vQ ≃ 28.5 MHz and 24.7 MHz for 79Br- and 81Br-nuclei are determined from Br-NMR and the asymmetry parameter of the electric field gradient was estimated to η ≃ 0.2. The Br-NQR lines are consistent with our findings from Br-NMR and they are relatively broad, even above TN. This broadening and the relative large η value suggests a symmetry reduction at the Br- site reflecting the presence of a local distortion in the lattice. Our density-functional calculations show that the displacements of Cu2 atoms located between the kagome planes do not account for this relatively large η. On the other hand, full structural relaxation, including the deformation of kagome planes, leads to a better agreement with the experiment.
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Breakdown of Magnetic Order in the Pressurized Kitaev Iridate β-Li_{2}IrO_{3}. PHYSICAL REVIEW LETTERS 2018; 120:237202. [PMID: 29932706 DOI: 10.1103/physrevlett.120.237202] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/19/2018] [Indexed: 06/08/2023]
Abstract
Temperature-pressure phase diagram of the Kitaev hyperhoneycomb iridate β-Li_{2}IrO_{3} is explored using magnetization, thermal expansion, magnetostriction, and muon spin rotation measurements, as well as single-crystal x-ray diffraction under pressure and ab initio calculations. The Néel temperature of β-Li_{2}IrO_{3} increases with the slope of 0.9 K/GPa upon initial compression, but the reduction in the polarization field H_{c} reflects a growing instability of the incommensurate order. At 1.4 GPa, the ordered state breaks down upon a first-order transition, giving way to a new ground state marked by the coexistence of dynamically correlated and frozen spins. This partial freezing in the absence of any conspicuous structural defects may indicate the classical nature of the resulting pressure-induced spin liquid, an observation paralleled to the increase in the nearest-neighbor off-diagonal exchange Γ under pressure.
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Magnetism of coupled spin tetrahedra in ilinskite-type KCu 5O 2(SeO 3) 2Cl 3. Sci Rep 2018; 8:2379. [PMID: 29402893 PMCID: PMC5799366 DOI: 10.1038/s41598-018-20350-z] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2017] [Accepted: 01/15/2018] [Indexed: 11/08/2022] Open
Abstract
Synthesis, thermodynamic properties, and microscopic magnetic model of ilinskite-type KCu5O2(SeO3)2Cl3 built by corner-sharing Cu4 tetrahedra are reported, and relevant magnetostructural correlations are discussed. Quasi-one-dimensional magnetic behavior with the short-range order around 50 K is rationalized in terms of weakly coupled spin ladders (tubes) having a complex topology formed upon fragmentation of the tetrahedral network. This fragmentation is rooted in the non-trivial effect of the SeO3 groups that render the Cu-O-Cu superexchange strongly ferromagnetic even at bridging angles exceeding 110°.
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Models and materials for generalized Kitaev magnetism. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2017; 29:493002. [PMID: 28914608 DOI: 10.1088/1361-648x/aa8cf5] [Citation(s) in RCA: 69] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The exactly solvable Kitaev model on the honeycomb lattice has recently received enormous attention linked to the hope of achieving novel spin-liquid states with fractionalized Majorana-like excitations. In this review, we analyze the mechanism proposed by Jackeli and Khaliullin to identify Kitaev materials based on spin-orbital dependent bond interactions and provide a comprehensive overview of its implications in real materials. We set the focus on experimental results and current theoretical understanding of planar honeycomb systems (Na2IrO3, α-Li2IrO3, and α-RuCl3), three-dimensional Kitaev materials (β- and γ-Li2IrO3), and other potential candidates, completing the review with the list of open questions awaiting new insights.
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Pressure-Induced Ferromagnetism due to an Anisotropic Electronic Topological Transition in Fe_{1.08}Te. PHYSICAL REVIEW LETTERS 2017; 119:227003. [PMID: 29286759 DOI: 10.1103/physrevlett.119.227003] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/09/2017] [Indexed: 06/07/2023]
Abstract
A rapid and anisotropic modification of the Fermi-surface shape can be associated with abrupt changes in crystalline lattice geometry or in the magnetic state of a material. We show that such an electronic topological transition is at the basis of the formation of an unusual pressure-induced tetragonal ferromagnetic phase in Fe_{1.08}Te. Around 2 GPa, the orthorhombic and incommensurate antiferromagnetic ground state of Fe_{1.08}Te is transformed upon increasing pressure into a tetragonal ferromagnetic state via a conventional first-order transition. On the other hand, an isostructural transition takes place from the paramagnetic high-temperature state into the ferromagnetic phase as a rare case of a "type-0" transformation with anisotropic properties. Electronic-structure calculations in combination with electrical resistivity, magnetization, and x-ray diffraction experiments show that the electronic system of Fe_{1.08}Te is instable with respect to profound topological transitions that can drive fundamental changes of the lattice anisotropy and the associated magnetic order.
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Abstract
Since its proposal by Anderson, resonating valence bonds (RVB) formed by a superposition of fluctuating singlet pairs have been a paradigmatic concept in understanding quantum spin liquids. Here, we show that excitations related to singlet breaking on nearest-neighbour bonds describe the high-energy part of the excitation spectrum in YbMgGaO4, the effective spin-1/2 frustrated antiferromagnet on the triangular lattice, as originally considered by Anderson. By a thorough single-crystal inelastic neutron scattering study, we demonstrate that nearest-neighbour RVB excitations account for the bulk of the spectral weight above 0.5 meV. This renders YbMgGaO4 the first experimental system where putative RVB correlations restricted to nearest neighbours are observed, and poses a fundamental question of how complex interactions on the triangular lattice conspire to form this unique many-body state. The signature of short range resonating valence bonds (RVB) to understand quantum spin liquids is yet to be explored. Here, Li et al. observe the putative RVB correlations restricted to nearest neighbours in YbMgGaO4, responsible for the high-energy spin excitations.
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Abstract
We investigated the structural, vibrational, magnetic, and electronic properties of the recently synthesized CaCo3V4O12 double perovskite with the high-spin (HS) Co2+ ions in a square-planar oxygen coordination at extreme conditions of high pressures and low temperatures. The single-crystal X-ray diffraction and Raman spectroscopy studies up to 60 GPa showed a conservation of its cubic crystal structure but indicated a crossover near 30 GPa. Above 30 GPa, we observed both an abnormally high "compressibility" of the Co-O bonds in the square-planar oxygen coordination and a huge anisotropic displacement of HS-Co2+ ions in the direction perpendicular to the oxygen planes. Although this effect is reminiscent of a continuous HS → LS transformation of the Co2+ ions, it did not result in the anticipated shrinkage of the cell volume because of a certain "stiffing" of the bonds of the Ca and V cations. We verified that the oxidation states of all the cations did not change across this crossover, and hence, no charge-transfer effects were involved. Consequently, we proposed that CaCo3V4O12 could undergo a phase transition at which the large HS-Co2+ ions were pushed out of the oxygen planes because of lattice compression. The antiferromagnetic transition in CaCo3V4O12 at 100 K was investigated by neutron powder diffraction at ambient pressure. We established that the magnetic moments of the Co2+ ions were aligned along one of the cubic axes, and the magnetic structure had a 2-fold periodicity along this axis, compared to the crystallographic one.
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Crystalline Electric-Field Randomness in the Triangular Lattice Spin-Liquid YbMgGaO_{4}. PHYSICAL REVIEW LETTERS 2017; 118:107202. [PMID: 28339219 DOI: 10.1103/physrevlett.118.107202] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/04/2016] [Indexed: 06/06/2023]
Abstract
We apply moderate-high-energy inelastic neutron scattering (INS) measurements to investigate Yb^{3+} crystalline electric field (CEF) levels in the triangular spin-liquid candidate YbMgGaO_{4}. Three CEF excitations from the ground-state Kramers doublet are centered at the energies ℏω=39, 61, and 97 meV in agreement with the effective spin-1/2 g factors and experimental heat capacity, but reveal sizable broadening. We argue that this broadening originates from the site mixing between Mg^{2+} and Ga^{3+} giving rise to a distribution of Yb-O distances and orientations and, thus, of CEF parameters that account for the peculiar energy profile of the CEF excitations. The CEF randomness gives rise to a distribution of the effective spin-1/2 g factors and explains the unprecedented broadening of low-energy magnetic excitations in the fully polarized ferromagnetic phase of YbMgGaO_{4}, although a distribution of magnetic couplings due to the Mg/Ga disorder may be important as well.
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Crystal structure and spin-trimer magnetism of Rb 2.3(H 2O) 0.8Mn 3[B 4P 6O 24(O,OH) 2]. Dalton Trans 2017; 46:2957-2965. [PMID: 28197612 DOI: 10.1039/c6dt04241d] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The novel borophosphate Rb2.3(H2O)0.8Mn3[B4P6O24(O,OH)2] was prepared under hydrothermal conditions at 553 K. Its crystal structure was determined using single-crystal X-ray diffraction data obtained from a non-merohedral twin and refined against F2 to R = 0.057. The compound crystallizes in the orthorhombic space group Pbcn, with unit-cell parameters a = 20.076(2) Å, b = 9.151(1) Å, c = 12.257(1) Å, V = 2251.8(2) Å3, and Z = 4. The title compound is the first example of a borophosphate with manganese ions adopting both octahedral and tetrahedral coordinations. Its unique crystal structure is formed by borophosphate slabs and chains of Mn2+-centered polyhedra sharing edges and vertices. These 2D and 1D fragments interconnect into a framework with open channels that accommodate Rb+ cations and water molecules. Topological relationships between borophosphates built from three-membered rings of two borate and one phosphate tetrahedra sharing oxygen vertices, amended by additional PO4 and HPO4 tetrahedra, are discussed. The temperature dependence of the magnetic susceptibility of Rb2.3(H2O)0.8Mn3[B4P6O24(O,OH)2] reveals predominant antiferromagnetic exchange interactions and the high-temperature effective magnetic moment corresponding to the high-spin S = 5/2 state of Mn2+ ions. At 12.5 K, a magnetic transition is evidenced by ac-susceptibility and specific heat measurements. A spin-trimer model with the leading exchange interaction J ∼ 3.2 K is derived from density-functional band-structure calculations and accounts for all experimental observations.
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Composition-dependent charge transfer and phase separation in the V 1-xRe xO 2 solid solution. Dalton Trans 2017; 46:1606-1617. [PMID: 28091651 DOI: 10.1039/c6dt04389e] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The substitution of vanadium in vanadium dioxide VO2 influences the critical temperatures of structural and metal-to-insulator transitions in different ways depending on the valence of the dopant. Rhenium adopts valence states between +4 and +7 in an octahedral oxygen surrounding and is particularly interesting in this context. Structural investigation of V1-xRexO2 solid solutions (0.01 ≤ x ≤ 0.30) between 80 and 1200 K using synchrotron X-ray powder diffraction revealed only two polymorphs that resemble VO2: the low-temperature monoclinic MoO2-type form (space group P21/c), and the tetragonal rutile-like form (space group P42/mnm). However, for compositions with 0.03 < x ≤ 0.15 a phase separation in the solid solution was observed below 1000 K upon cooling down from 1200 K, giving rise to two isostructural phases with slightly different lattice parameters. This is reflected in the appearance of two metal-to-insulator transition temperatures detected by magnetization and specific heat measurements. Comprehensive X-ray photoelectron spectroscopy studies showed that an increased amount of Re leads to a change in the Re valence state from solely Re6+ at a low doping level (≤3 at% Re) via mixed-valence states Re4+/Re6+ for at least 0.03 < x ≤ 0.10, up to nearly pure Re4+ in V0.70Re0.30O2. Thus, compositions V1-xRexO2 with only one valence state of Re in the material (Re6+ or Re4+) can be obtained as a single phase, while intermediate compositions are subjected to a phase separation, presumably due to different valence states of Re.
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Nontrivial Recurrent Intergrowth Structure and Unusual Magnetic Behavior of Intermetallic Compound Fe 32+δGe 33As 2. Inorg Chem 2016; 55:12953-12961. [PMID: 27989149 DOI: 10.1021/acs.inorgchem.6b02412] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
A new phase Fe32+δGe33As2 (δ ≤ 0.136) was obtained by two-step synthesis from the elements. Fe32+δGe33As2 crystallizes in its own structure type (space group P6/mmm, Z = 1, a = 11.919(3) Å, c = 7.558(4) Å) that can be described as a recurrent two-dimensional intergrowth of two intermetallic structure types, MgFe6Ge6 and Co2Al5. Their blocks are represented by infinite columns in the structure. No visible structural changes were observed in the temperature range from 10 to 300 K. At 125 K, Fe32+δGe33As2 undergoes an antiferromagnetic-like transition, while above 150 K it shows a typical Curie-Weiss paramagnetic behavior. Below the transition temperature, a peculiar field-dependent magnetic susceptibility, that shows a significant increase of the susceptibility upon increasing the magnetic field, and a change in transport properties have been observed. Above 140 K, Fe32+δGe33As2 reveals a metallic behavior, in agreement with electronic structure calculation, while below this point the resistivity nonmonotonically increases upon cooling. The Seebeck coefficient is positive, indicating that holes are the major charge carriers, and shows a broad maximum around 57 K.
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Structural and Thermodynamic Stability of the "1111" Structure Type: A Case Study of the EuFZnPn Series. Inorg Chem 2016; 55:12409-12418. [PMID: 27934435 DOI: 10.1021/acs.inorgchem.6b02297] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Two new compounds with the LaOAgS structure, EuFZnAs (1) and EuFZnSb (2), were obtained via solid state reaction. Both compounds are tetragonal (P4/nmm) with the cell parameters a = 4.1000(1) Å and c = 9.0811(1) Å for 1 and a = 4.2852(1)Å and c = 9.4238(1)Å for 2. The absence of their phosphide analog can be explained based on crystal chemical considerations as well as on quantum-chemical estimates of their thermodynamic stability with respect to EuF2 and EuZn2Pn2. The magnetic response of 1 and 2 is ascribed to the presence of Eu2+ ions. Both compounds are paramagnetic down to low temperatures, where they order antiferromagnetically at ∼5 K and ∼3 K, respectively. They are narrow-gap semiconductors, and EuFZnSb demonstrates a relatively high value of the Seebeck coefficient.
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New Fe-based layered telluride Fe 3-δAs 1-yTe 2: synthesis, crystal structure and physical properties. Dalton Trans 2016; 45:16938-16947. [PMID: 27722484 DOI: 10.1039/c6dt02721k] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
A new ternary telluride, Fe3-δAs1-yTe2, was synthesized from elements at 600 °C. It crystallizes in the hexagonal P63/mmc space group with the unit cell parameters a = 3.85091(9) Å and c = 17.1367(4) Å for δ = 0.3 and y = 0.04. Its layered crystal structure contains partially occupied intralayer and interlayer Fe positions, which give rise to significant nonstoichiometry: Fe3-δAs1-yTe2 was found to possess the homogeneity range of 0.25 < δ < 0.45 and y = 0.04. Regions of local vacancy ordering alternate with regions of randomly distributed vacancies, so that the ordering of Fe atoms and vacancies is not complete in the average structure. Clear evidence of the magnetic phase transition is obtained by thermodynamic measurements, Mössbauer spectroscopy, and neutron powder diffraction. Magnetic susceptibility measurements reveal weak ferromagnetism below TC = 123 K with a net moment of MS∼ 0.1μB/Fe at T = 2 K. This transition is confirmed by differential scanning calorimetry. Additionally, neutron powder diffraction indicates the onset of a complex AFM-like magnetic ordering below 100 K.
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