Bulk ferromagnetism in cleavable van der Waals telluride NbFeTe2

A van der Waals telluride, NbFeTe2, has been synthesized using chemical vapor transport reactions. The optimized synthetic conditions yield high-quality single crystals with a novel monoclinic crystal structure. Monoclinic NbFeTe2 demonstrates a (100) cleavage plane, bulk ferromagnetism below 87 K, and a metallic ground state-the necessary prerequisites for needed spintronics technologies.

Mn2Ga2S5 and Mn2Al2Se5 van der Waals Chalcogenides: A Source of Atomically Thin Nanomaterials

Layered chalcogenides containing 3d transition metals are promising for the development of two-dimensional nanomaterials with interesting magnetic properties. Both mechanical and solution-based exfoliation of atomically thin layers is possible due to the low-energy van der Waals bonds. In this paper, we present the synthesis and crystal structures of the Mn2Ga2S5 and Mn2Al2Se5 layered chalcogenides. For Mn2Ga2S5, we report magnetic properties, as well as the exfoliation of nanofilms and nanoscrolls. The synthesis of both polycrystalline phases and single crystals is described, and their chemical stability in air is studied. Crystal structures are probed via powder X-ray diffraction and high-resolution transmission electron microscopy. The new compound Mn2Al2Se5 is isomorphous with Mn2Ga2S5 crystallizing in the Mg2Al2Se5 structure type. The crystal structure is built by the ABCBCA sequence of hexagonal close-packing layers of chalcogen atoms, where Mn2+ and Al3+/Ga3+ species preferentially occupy octahedral and tetrahedral voids, respectively. Mn2Ga2S5 exhibits an antiferromagnetic-like transition at 13 K accompanied by the ferromagnetic hysteresis of magnetization. Significant frustration of the magnetic system may yield spin-glass behavior at low temperatures. The exfoliation of Mn2Ga2S5 layers was performed in a non-polar solvent. Nanolayers and nanoscrolls were observed using high-resolution transmission electron microscopy. Fragments of micron-sized crystallites with a thickness of 70-100 nanometers were deposited on a glass surface, as evidenced by atomic force microscopy.

Impact of Ge Doping on Structural and Magnetic Ordering in RMnxGa3 and R4Mn1-xGa12-yGey (R = Tb, Dy; x ≤ 0.25, y ≈ 1.0-3.3)

Single crystals of RMnxGa3 and their new quaternary derivatives R4Mn1-xGa12-yGey (R = Tb, Dy, x ≤ 0.25, y ≈ 1.0-3.3) were grown from a Ga flux. The compounds are derivatives of cubic RGa3 phases, with Mn atoms filling the Ga6 voids. RMnxGa3 formally adopts a cubic ABO3 perovskite structure, in which the presence of Mn atoms results in a shift of the neighboring Ga atoms from their ideal position. A partial substitution of Ga by Ge leads to a higher Mn content, resulting in structural ordering of the latter and the formation of the superstructure phases R4Mn1-xGa12-yGey, which can be formally described in the Y4PdGa12 structure type. The presence of Mn vacancies, which was observed for R = Tb, and Ga/Ge mixing lead to a noticeable deviation from the idealized structure. The compounds contain two magnetic sublattices: the R sublattice, which orders antiferromagnetically near 20 K, and the Mn sublattice, which orders ferromagnetically at TC = 125-225 K with the Ge doping resulting in higher TC. The two sublattices are not independent, as the Mn sublattice induces partial ferromagnetic ordering of the rare earth atoms below TC, at least for the Ge-doped phases. Near TN, both magnetic susceptibility and heat capacity reveal complex behavior, indicating changes in magnetic structures below TN.

Itinerant ferromagnet Re4-xMnxGe7-δ (x = 0.9-1.5, δ = 0.42-0.44) with incommensurate chimney-ladder structure stabilised at ambient pressure

Re_4-xMn_xGe_7-δ (x = 0.9-1.5, δ = 0.42-0.44) is a new member of the Nowotny chimney-ladder family of compounds and features an incommensurate composite structure of transition element T (Re and Mn) and Ge substructures. Our theoretical calculations indicate metallic conductivity and ferromagnetic ordering, the latter being experimentally observed below 157 K.

Layered van der Waals Chalcogenides FeAl2Se4, MnAl2S4, and MnAl2Se4: Atomically Thin Triangular Arrangement of Transition-Metal Atoms

Layered van der Waals (vdW) chalcogenides of 3d transition metals are a rich source of two-dimensional (2D) nanomaterials, in which atomically thin layers with the terminating chalcogen atoms exhibit promising functionality for novel spintronic devices. Here, we report on the synthesis, crystal growth, and magnetic properties of FeAl₂Se₄, MnAl₂S₄, and MnAl₂Se₄ ternary chalcogenides. Crystal structures are probed by powder X-ray diffraction, Mössbauer spectroscopy, and high-resolution transmission electron microscopy. We improve the structural models of FeAl₂Se₄ and MnAl₂S₄ and show that isostructural MnAl₂S₄ and MnAl₂Se₄ crystallize in the centrosymmetric R3̅̅m space group. In the crystal structure, transition metal and Al atoms mutually occupy the octahedral and tetrahedral voids of four close-packing chalcogen layers terminated by vdW gaps. The transition-metal atoms form a triangular arrangement inside the close-packing layers. As a result, FeAl₂Se₄ and MnAl₂S₄ show no long-range magnetic order in the studied temperature range. In the paramagnetic state, Fe and Mn possess effective magnetic moments of 4.99(2) and 5.405(6) μ_B, respectively. Furthermore, FeAl₂Se₄ enters a frozen spin-disordered state below 12 K.

Cleavable crystals, crystal structure, and magnetic properties of the NbFe1+xTe3 layered van der Waals telluride

Transition metal-based two-dimensional nanomaterials with competing magnetic states are at the cutting edge of spintronic and low-power memory devices. In this paper, we present a Fe-rich NbFe_1+xTe₃ layered telluride (x ≈ 0.5), which shows an interplay of spin-glass and antiferromagnetic states below the Néel temperature of 179 K. The compound has a layered crystal structure, where the NbFeTe₃ layers are terminated by the Te atoms and van der Waals gaps. Bulk single crystals grown by chemical vapor transport reactions possess the (1̄01) cleavage plane suitable for the exfoliation of two-dimensional nanomaterials. Combination of high-resolution transmission electron microscopy and powder X-ray diffraction reveals the zigzag ladders of Fe atoms inside the structural layers, as well as complementary zigzag chains of the partially occupied Fe positions in the interstitial region. Fe atoms carry large effective magnetic moment of 4.85(3)μ_B per atom in the paramagnetic state yielding intriguing magnetic properties of NbFe_1+xTe₃. They include frozen spin-glass state at low temperatures and spin-flop transition in high magnetic fields indicating promising flexibility of the magnetic system and its potential control by magnetic field or gate tuning in the spintronic devices and heterostructures.

Fe-Rich Ferromagnetic Cleavable Van der Waals Telluride Fe5AsTe2

Transition metal-based layered compounds with van der Waals gaps between the structural layers are a rich source of magnetic materials for spintronic applications. Bulk crystals can be cleaved, providing high-quality two-dimensional nanomaterials, which are promising for the manipulation of spins in spintronic devices and low power quantum logic interfaces. The layered van der Waals telluride Fe₅AsTe₂ can be synthesized by the high-temperature reaction of elements. In the crystal structure, Fe-rich structural layers with the composition of Fe_4.58(4)AsTe₂ are separated by the van der Waals gaps with no atoms in the interstitial region. Crystal growth employing chemical vapor transport reactions yields bulk cleavable crystals, which exhibit weak inherent ferromagnetism below the Curie temperature of T_C = 48 K. In the ordered state, the magnetization shows a dual-slope behavior in low magnetic fields, indicating the compensated or canted nature of magnetism. Magnetic susceptibility and magnetization measurements reveal perpendicular magnetic anisotropy. The large Rhodes-Wohlfarth ratio of 4.6 indicates the itinerant nature of ferromagnetism in Fe₅AsTe₂.

Ferromagnetic correlations in the layered van der Waals sulfide FeAl2S4

Transition metal-based layered compounds with van der Waals gaps between the adjacent layers are a source of two-dimensional (2D) nanomaterials with nontrivial transport and magnetic properties. 2D ferromagnets, both metals and semiconductors, can be leveraged to produce spin-polarized current in spintronic devices with tailored functionalities. Here, we report on the synthesis, crystal growth, crystal and electronic structure, and magnetic properties of the Fe-based FeAl₂S₄ layered sulfide. In the crystal structure, Fe and Al atoms mix on octahedral and tetrahedral sites between hexagonal layers of S atoms, which are terminated by the van der Waals gaps. Band structure calculations reveal strong electronic correlations within the semiconducting ground state, which induce ferromagnetism with the magnetic moment of 0.12μ_B per formula unit for a Hubbard interaction U = 5 eV and Hund's rule coupling J = 0.8 eV. Crystal growth employing chemical vapor transport reactions results in bulk cleavable crystals, which show paramagnetic Curie-Weiss behavior at high temperatures with the Fe²⁺ magnetic centers. At low temperatures, an anomaly is observed on the magnetic susceptibility curve, below which the magnetization shows ferromagnetic hysteresis, indicating the presence of ferromagnetic correlations in FeAl₂S₄.

Semiconducting and Metallic Compounds within the IrIn3 Structure Type: Stability and Chemical Bonding

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^-.

Intermetallic Compound Re2Ga9Ge with Re- and Ge-Embedded Gallium Clusters: Synthesis, Crystal Structure, Chemical Bonding, and Physical Properties

Transition metal-based endohedral cluster intermetallic compounds are interesting electron phases, which frequently exhibit superconductivity with a peculiar interplay between the critical temperature and valence electron count. We present a new Re-based endohedral gallium cluster compound, Re₂Ga₉Ge. Its unique crystal structure (P4₂/mmc space group, a = 8.0452(3) Å, c = 6.7132(2) Å) is built by two types of gallium polyhedra: monocapped Archimedean antiprisms centered by rhenium atoms and tetrahedra containing a main-group element inside. The analysis of chemical bonding shows the presence of localized pairwise interactions between the p-block elements and the formation of multicenter bonds with the participation of d-orbitals of rhenium. In the electronic band structure, the Fermi level is located in a narrow pseudogap indicating the optimum band filling and thus explaining the virtual absence of a homogeneity range. The compound exhibits Pauli paramagnetism and metallic properties with unexpectedly low thermal conductivity. A sharp anomaly observed on the magnetic susceptibility and resistivity curves presumably indicates the electronic phase transition accompanied by charge ordering at the characteristic temperature of T * = 271 K in zero magnetic field.

Endohedral cluster intermetallic superconductors: at the frontier between chemistry and physics

When a transition metal combines with an excess of a p-metal, the latter forms endohedral clusters with the number of vertices up to 14. These clusters are the building units of endohedral cluster intermetallic compounds. Although discovered a few decades ago, they have gained renewed interest due to their peculiar crystal and electronic structures and frequently observed superconducting properties. Advances over recent years reveal that endohedral cluster architectures are flexible enough, enabling chemical substitutions and the formation of a series of structurally related phases, where the same clusters can be arranged in different ways. Within the structural series, the superconducting-state parameters, including critical temperature and magnetic field, can be controlled and finely tuned. Herein, we present the most recent results in the chemical properties and superconductivity of endohedral cluster intermetallics and provide an outlook for the field.

Semiconducting and superconducting Mo–Ga frameworks: total energy and chemical bonding

The superconducting Mo₄Ga₂₁ structure type is derived from the electron-precise MoGa₄ cubic framework.

Electron-Precise Semiconducting ReGa2Ge: Extending the IrIn3 Structure Type to Group 7 of the Periodic Table

Intermetallic compounds with semiconducting properties are rare, but they give rise to advanced materials for energy conversion and saving applications. Here, we present ReGa₂Ge, a new electron-precise narrow-gap intermetallic semiconductor. The compound crystallizes in the IrIn₃ structure type (space group P4₂/mnm, a = 6.5734(3) Å, c = 6.7450(8) Å, and Z = 4), where Re atoms occupy the Ir site, while Ga and Ge jointly populate the In sites. ^69,71Ga nuclear quadrupole resonance spectroscopy indicates nonstatistical partially ordered distribution of Ga and Ge over two available crystallographic sites; however, the Ga:Ge ratio is exactly 2:1 without noticeable homogeneity range. The stoichiometry of ReGa₂Ge ensures its precise valence electron count, which is 17 e^- per formula unit. Accordingly, a narrow energy gap opens up at the Fermi energy in the electronic structure. Electrical resistivity, Seebeck coefficient, and thermal conductivity are in agreement with the semiconducting behavior deduced from the electronic structure calculations and point to prospective thermoelectric properties at high temperatures. Bonding analysis reveals dominant covalency in Re-E (E = Ga, Ge) and Re-Re interactions.

Endohedral Cluster Superconductors in the Mo-Ga-Sn System Explored by the Joint Flux Technique

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 Mo₈Ga_41-xSn_x and Mo₄Ga_21-x-δSn_x with critical temperatures of T_c = 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 Mo₄Ga_21-x-δSn_x superconductor features strong electron-phonon coupling with the large ratio of 2Δ(0)/(k_BT_c) = 4.1 similar to that of the Mo₈Ga₄₁ superconductor with the closely related crystal structure.

Boosting Water Oxidation through In Situ Electroconversion of Manganese Gallide: An Intermetallic Precursor Approach

For the first time, the manganese gallide (MnGa₄) served as an intermetallic precursor, which upon in situ electroconversion in alkaline media produced high‐performance and long‐term‐stable MnO_x‐based electrocatalysts for water oxidation. Unexpectedly, its electrocorrosion (with the concomitant loss of Ga) leads simultaneously to three crystalline types of MnO_x minerals with distinct structures and induced defects: birnessite δ‐MnO₂, feitknechtite β‐MnOOH, and hausmannite α‐Mn₃O₄. The abundance and intrinsic stabilization of Mn^III/Mn^IV active sites in the three MnO_x phases explains the superior efficiency and durability of the system for electrocatalytic water oxidation. After electrophoretic deposition of the MnGa₄ precursor on conductive nickel foam (NF), a low overpotential of 291 mV, comparable to that of precious‐metal‐based catalysts, could be achieved at a current density of 10 mA cm⁻² with a durability of more than five days.

Single-gap superconductivity in Mo8Ga41

In this paper, the potential existence of two-gap superconductivity in Mo₈Ga₄₁ is addressed in detail by means of thermodynamic and spectroscopic measurements. A combination of highly sensitive bulk and surface probes, specifically ac-calorimetry and scanning tunneling spectroscopy (STS), are utilized on the same piece of crystal and reveal the presence of only one intrinsic gap in the system featuring strong electron-phonon coupling. Minute traces of additional superconducting phases detected by STS and also in the heat capacity measured in high magnetic fields on a high-quality and seemingly single-phase crystal might mimic the multigap superconductivity of Mo₈Ga₄₁ suggested recently in several studies.

From endohedral cluster superconductors to approximant phases: synthesis, crystal and electronic structure, and physical properties of Mo8Ga41-xZnx and Mo7Ga52-xZnx

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 Mo₈Ga_41-xZn_x and Mo₇Ga_52-xZn_x 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. Mo₈Ga_41-xZn_x with x = 7.2(2) exhibits superconducting properties below T_C = 4 K, whereas there is no superconducting transition at temperatures above 2 K for the limiting composition of x = 11.3(2). Further, the Mo₇Ga_52-xZn_x phase is formed from the flux with a higher content of Zn. Mo₇Ga_52-xZn_x 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 Mo₈Ga_41-xZn_x and Mo₇Ga_52-xZn_x 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.

ReGaGe2: an intermetallic compound with semiconducting properties and localized bonding

ReGaGe2 is a new member of the family of intermetallic compounds with non-metallic properties. It displays highly localized covalent bonding patterns. Its electronic structure is governed by mixing of Re d orbitals with the s and p orbitals of Ga and Ge and features the Fermi level falling into the opened band gap, ensuring experimentally confirmed semiconducting properties.

ReGa0.4Ge0.6: Intermetallic Compound with Pronounced Covalency in the Bonding Pattern

We report synthesis, crystal and electronic structure, and transport properties of new intermetallic compound ReGa_0.4Ge_0.6, which was obtained by two-step ampule method from the elements. ReGa_0.4Ge_0.6 crystallizes in its own structure type (space group I4/ mmm, a = 2.89222(3) Å, c = 15.1663(3) Å, and Z = 4) which can be described as a sequential alternation of blocks of rhenium atoms and blocks of gallium and germanium atoms. Chemical bonding analysis reveals pronounced covalency of Re-Re, Re-E, and E-E (E = Ga and Ge) interactions and an interesting bonding pattern that includes many variations of localized bonding within a single compound, including pairwise homo- and heterometallic bonding, three-centered homometallic and four-centered bonding, and possibly even more delocalized bonding, which is not often encountered in such a simple intermetallic compound. Metallic behavior is confirmed by electronic structure calculations and by measurements of electrical resistivity.

Crystal Growth of Intermetallics from the Joint Flux: Exploratory Synthesis through the Control of Valence Electron Count

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 Re₈Ga_{41- x}Zn _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.

Magnetism of coupled spin tetrahedra in ilinskite-type KCu5O2(SeO3)2Cl3

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°.

Nontrivial Recurrent Intergrowth Structure and Unusual Magnetic Behavior of Intermetallic Compound Fe32+δGe33As2

A new phase Fe_32+δGe₃₃As₂ (δ ≤ 0.136) was obtained by two-step synthesis from the elements. Fe_32+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, Fe_32+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, Fe_32+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.

Structural and Thermodynamic Stability of the "1111" Structure Type: A Case Study of the EuFZnPn Series

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.

New Fe-based layered telluride Fe3-δAs1-yTe2: synthesis, crystal structure and physical properties

A new ternary telluride, Fe_3-δAs_1-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: Fe_3-δAs_1-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 T_C = 123 K with a net moment of M_S∼ 0.1μ_B/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.