Na2Ga7: A Zintl-Wade Phase Related to "α-Tetragonal Boron"

Na₂Ga₇ crystallizes with the orthorhombic space group Pnma (no. 62; a = 14.8580(6) Å, b = 8.6766(6) Å, and c = 11.6105(5) Å; Z = 8) and constitutes a filled variant of the Li₂B₁₂Si₂ structure type. The crystal structure consists of a network of icosahedral Ga₁₂ units with 12 exohedral bonds and four-bonded Ga atoms in which the Na atoms occupy the channels and cavities. The atomic arrangement is consistent with the Zintl [(4b)Ga]^- and Wade [(12b)Ga₁₂]^2- electron counting approach. The compound forms peritectically from Na₇Ga₁₃ and the melt at 501 °C and does not show a homogeneity range. The band structure calculations predict semiconducting behavior consistent with the electron balance [Na⁺]₄[(Ga₁₂)^2-][Ga^-]₂. Magnetic susceptibility measurements show that Na₂Ga₇ is diamagnetic.

Flexomagnetism and vertically graded Néel temperature of antiferromagnetic Cr2O3 thin films

Antiferromagnetic insulators are a prospective materials platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A magnetomechanical coupling in antiferromagnets offers vast advantages in the control and manipulation of the primary order parameter yet remains largely unexplored. Here, we discover a new member in the family of flexoeffects in thin films of Cr2O3. We demonstrate that a gradient of mechanical strain can impact the magnetic phase transition resulting in the distribution of the Néel temperature along the thickness of a 50-nm-thick film. The inhomogeneous reduction of the antiferromagnetic order parameter induces a flexomagnetic coefficient of about 15 μB nm−2. The antiferromagnetic ordering in the inhomogeneously strained films can persist up to 100 °C, rendering Cr2O3 relevant for industrial electronics applications. Strain gradient in Cr2O3 thin films enables fundamental research on magnetomechanics and thermodynamics of antiferromagnetic solitons, spin waves and artificial spin ice systems in magnetic materials with continuously graded parameters.

Defect Nanostructure and its Impact on Magnetism of α-Cr2 O3 Thin Films

Thin films of the magnetoelectric insulator α-Cr₂ O₃ are technologically relevant for energy-efficient magnetic memory devices controlled by electric fields. In contrast to single crystals, the quality of thin Cr₂ O₃ films is usually compromised by the presence of point defects and their agglomerations at grain boundaries, putting into question their application potential. Here, the impact of the defect nanostructure, including sparse small-volume defects and their complexes is studied on the magnetic properties of Cr₂ O₃ thin films. By tuning the deposition temperature, the type, size, and relative concentration of defects is tailored, which is analyzed using the positron annihilation spectroscopy complemented with electron microscopy studies. The structural characterization is correlated with magnetotransport measurements and nitrogen-vacancy microscopy of antiferromagnetic domain patterns. Defects pin antiferromagnetic domain walls and stabilize complex multidomain states with a domain size in the sub-micrometer range. Despite their influence on the domain configuration, neither small open-volume defects nor grain boundaries in Cr₂ O₃ thin films affect the Néel temperature in a broad range of deposition parameters. The results pave the way toward the realization of spin-orbitronic devices where magnetic domain patterns can be tailored based on defect nanostructures without affecting their operation temperature.

The Intermetallic Semiconductor ht-IrGa3: a Material in the in-Transformation State

The compound IrGa₃ was synthesized by direct reaction of the elements. It is formed as a high-temperature phase in the Ir-Ga system. Single-crystal X-ray diffraction analysis confirms the tetragonal symmetry (space group P4₂ /mnm, No. 136) with a = 6.4623(1) Å and c = 6.5688(2) Å and reveals strong disorder in the crystal structure, reflected in the huge values and anisotropy of the atomic displacement parameters. A model for the real crystal structure of ht-IrGa₃ is derived by the split-position approach from the single-crystal X-ray diffraction data and confirmed by an atomic-resolution transmission electron microscopy study. Temperature-dependent electrical resistivity measurements evidence semiconductor behavior with a band gap of 30 meV. A thermoelectric characterization was performed for ht-IrGa₃ and for the solid solution IrGa_3-x Zn _x .

Challenging the Durability of Intermetallic Mo-Ni Compounds in the Hydrogen Evolution Reaction

Molybdenum-nickel materials are catalysts of industrial interest for the hydrogen evolution reaction (HER). Well-characterized surfaces of the single-phase intermetallic compounds Ni₇Mo₇, Ni₃Mo, and Ni₄Mo were subjected to accelerated durability tests (ADTs) and thorough characterization to unravel whether crystallographic ordering affects the activity. Their intrinsic instability leads to molybdenum leaching, resulting in higher specific surface areas and nickel-enriched surfaces. These are more prone to form Ni(OH)₂ layers, which leads to deactivation of the Mo-Ni materials. The crystal structure of the intermetallic compounds has, due to the intrinsic instability of the materials in alkaline media, no effect on the activity. Ni₇Mo₇, identified earlier as durable, proves to be highly unstable in the applied ADTs. The results show that the enhanced activity of unsupported bulk Mo-Ni electrodes can solely be ascribed to increased specific surface areas.

Crystal structure, phase transition and properties of indium(III) sulfide

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.

Al2 Pt for Oxygen Evolution in Water Splitting: A Strategy for Creating Multifunctionality in Electrocatalysis

The production of hydrogen via water electrolysis is feasible only if effective and stable catalysts for the oxygen evolution reaction (OER) are available. Intermetallic compounds with well-defined crystal and electronic structures as well as particular chemical bonding features are suggested here to act as precursors for new composite materials with attractive catalytic properties. Al2 Pt combines a characteristic inorganic crystal structure (anti-fluorite type) and a strongly polar chemical bonding with the advantage of elemental platinum in terms of stability against dissolution under OER conditions. We describe here the unforeseen performance of a surface nanocomposite architecture resulting from the self-organized transformation of the bulk intermetallic precursor Al2 Pt in OER.

Structural Peculiarities and Thermoelectric Study of Iron Indium Thiospinel

The homogeneity range of ternary iron indium thiospinel at 873 K was investigated. A detailed study was focused on two distinct series (y=z): 1) a previously reported charge-balanced (In0.67+0.33y □0.33-0.33y )tetr [In2-z Fez ]oct S4 (A1-series; □ stands for vacancy; the abbreviations "tetr" and "oct" indicate atoms occupying tetrahedral 8a and octahedral 16d sites, respectively) and 2) a new charge-unbalanced (In0.67+y □0.33-y )tetr [In2-z Fez ]oct S4 (A2-series). Fe atoms were confirmed to exclusively occupy an octahedral position in both series. An unusual reduction of the unit cell parameter with increasing Fe content is explained by differences in the ionic radii between Fe and In, as well as by an additional electrostatic attraction originating from charge imbalance (latter only in A2-series). The studied compound is an n-type semiconductor, and its charge carrier concentration increases or decreases for larger Fe content within the A1- and A2-series, respectively. The thermal conductivity κtot is significantly reduced upon increasing vacancy concentration, whereas the change of power factor is insufficient to drastically improve the thermoelectric figure of merit.

Indium thiospinel In1-x□xIn2S4- structural characterization and thermoelectric properties

A detailed study of polycrystalline indium-based In_1-x□_xIn₂S₄ (x = 0.16, 0.22, 0.28, and 0.33) thiospinel is presented (□- vacancy). Comprehensive investigation of synthesis conditions, phase composition and thermoelectric properties was performed by means of various diffraction, microscopic and spectroscopic methods. Single-phase α- and β-In_1-x□_xIn₂S₄ were found in samples with 0.16 ≤x≤ 0.22 and x = 0.33 (In₂S₃), respectively. In contrast, it is shown that In_0.72□_0.28In₂S₄ contains both α- and β-polymorphic modifications. Consequently, the thermoelectric characterization of well-defined α- and β-In_1-x□_xIn₂S₄ is conducted for the first time. α-In_1-x□_xIn₂S₄ (x = 0.16 and 0.22) revealed n-type semiconducting behavior, a large Seebeck coefficient (>|200|μV K^-1) and moderate charge carrier mobility on the level of ∼20 cm² V^-1 s^-1 at room temperature (RT). Decreases in charge carrier concentration (increase of electrical resistivity) and thermal conductivity (even below 0.6 W m^-1 K^-1 at 760 K) for larger In-content are observed. Although β-In_0.67□_0.33In₂S₄ (β-In₂S₃) is a distinct polymorphic modification, it followed the abovementioned trend in thermal conductivity and displayed significantly higher charge carrier mobility (∼10⁴ cm² V^-1 s^-1 at RT). These findings indicate that structural disorder in the α-modification affects both electronic and thermal properties in this thiospinel. The reduction of thermal conductivity counterbalances a lowered power factor and, thus, the thermoelectric figure of merit ZT_max = 0.2 at 760 K is nearly the same for both α- and β-In_1-x□_xIn₂S₄.

Thermal and Electronic Transport Properties of the Half-Heusler Phase ScNiSb

Thermoelectric properties of the half-Heusler phase ScNiSb (space group F 4 ¯ 3m) were studied on a polycrystalline single-phase sample obtained by arc-melting and spark-plasma-sintering techniques. Measurements of the thermopower, electrical resistivity, and thermal conductivity were performed in the wide temperature range 2-950 K. The material appeared as a p-type conductor, with a fairly large, positive Seebeck coefficient of about 240 μV K^-1 near 450 K. Nevertheless, the measured electrical resistivity values were relatively high (83 μΩm at 350 K), resulting in a rather small magnitude of the power factor (less than 1 × 10^-3 W m^-1 K^-2) in the temperature range examined. Furthermore, the thermal conductivity was high, with a local minimum of about 6 W m^-1 K^-1 occurring near 600 K. As a result, the dimensionless thermoelectric figure of merit showed a maximum of 0.1 at 810 K. This work suggests that ScNiSb could be a promising base compound for obtaining thermoelectric materials for energy conversion at high temperatures.

Structural stability and thermoelectric performance of high quality synthetic and natural pyrites (FeS2)

Single crystalline pyrite of high quality reveals good thermal- and bad electrical conductivities resulting in poor thermoelectric performance.

Spark Plasma Sintering (SPS)-Assisted Synthesis and Thermoelectric Characterization of Magnéli Phase V6O11

The Magnéli phase V₆O₁₁ was synthesized in gram amounts from a powder mixture of V₆O₁₁/V₇O₁₃ and vanadium metal, using the spark plasma sintering (SPS) technique. Its structure was determined with synchrotron X-ray powder diffraction data from a phase-pure sample synthesized by conventional solid-state synthesis. A special feature of Magnéli-type oxides is a combination of crystallographic shear and intrinsic disorder that leads to relatively low lattice thermal conductivities. SPS prepared V₆O₁₁ has a relatively low thermal conductivity of κ = 2.72 ± 0.06 W (m K)^-1 while being a n-type conductor with an electrical conductivity of σ = 0.039 ± 0.005 (μΩ m)^-1, a Seebeck coefficient of α = -(35 ± 2) μV K^-1, which leads to a power factor of PF = 4.9 ± 0.8 × 10^-5W (m K²)^-1 at ∼600 K. Advances in the application of Magnéli phases are mostly hindered by synthetic and processing challenges, especially when metastable and nanostructured materials such as V₆O₁₁ are involved. This study gives insight into the complications of SPS-assisted synthesis of complex oxide materials, provides new information about the thermal and electrical properties of vanadium oxides at high temperatures, and supports the concept of reducing the thermal conductivity of materials with structural building blocks such as crystallographic shear (CS) planes.

Two-gap superconductivity in Ag1-x Mo6S8 Chevrel phase

The superconducting properties of [Formula: see text]Mo₆S₈ [[Formula: see text]] Chevrel phase [[Formula: see text] K] are studied on a sample compacted by spark plasma sintering. Both lower ([Formula: see text] mT) and the upper [[Formula: see text] T] critical magnetic fields are obtained from magnetization and electrical resistivity measurements for the first time. The analysis of the low-temperature electronic specific heat indicates [Formula: see text]Mo₆S₈ to be a two band superconductor with the energy gaps [Formula: see text] meV (95%) and [Formula: see text] meV (5%). Theoretical DFT calculations reveal a much stronger electron-phonon coupling in the studied Chevrel phase compared to earlier reports. Similar to MgB₂, the Fermi surface of studied Chevrel phase is formed by two hole-like and one electron-like bands.

Downscaling Effect on the Superconductivity of Pd3Bi2X2 (X = S or Se) Nanoparticles Prepared by Microwave-Assisted Polyol Synthesis

Pd3Bi2S2 and Pd3Bi2Se2 have been successfully prepared in the form of nanoparticles with diameters of ∼50 nm by microwave-assisted modified polyol synthesis at low temperatures. The composition and morphology of the samples have been studied by means of powder X-ray diffraction as well as electron microscopy methods, including X-ray intensity mapping on the nanoscale. Superconducting properties of the as-prepared samples have been characterized by electrical resistivity measurements down to low temperatures (∼0.2 K). Deviations from the bulk metallic behavior originating from the submicrometer nature of the samples were registered for both phases. A significant critical-field enhancement up to 1.4 T, i.e., 4 times higher than the value of the bulk material, has been revealed for Pd3Bi2Se2. At the same time, the critical temperature is suppressed to 0.7 K from the bulk value of ∼1 K. A superconducting transition at 0.4 K has been observed in nanocrystalline Pd3Bi2S2. Here, a zero-temperature upper critical field of ∼0.5 T has been estimated. Further, spark plasma-sintered Pd3Bi2S2 and Pd3Bi2Se2 samples have been investigated. Their superconducting properties are found to lie between those of the bulk and nanosized samples.

Solid solution Pb1−xEuxTe: constitution and thermoelectric behavior

The thermoelectric properties of polycrystalline materials on the basis of the solid solution Pb_1−xEu_xTe prepared by spark-plasma-sintering are characterized. The solid solution undergoes a metal–semiconductor transition in parallel to the p–n transition around 500 K.

Structural evolvement and thermoelectric properties of Cu(3-x)Sn(x)Se₃ compounds with diamond-like crystal structures

Polycrystalline samples of Cu(3-x)Sn(x)Se3 were synthesized in the composition range x = 0.87-1.05. A compositionally induced evolvement from tetragonal via cubic to monoclinic crystal structures is observed, when the composition changes from a Cu-rich to a Sn-rich one. The Cu(3-x)Sn(x)Se3 materials show a metal-to-semiconductor transition with increasing x. Electronic transport properties are governed by the charge-carrier concentration which is well described by a linear dispersion-band model. The lattice component of the thermal conductivity is practically independent of x which is attributed to the opposite influence of the atomic ordering and the inhomogeneous distribution of the Cu-Se or Sn-Se bonds with different polarities in the crystal structure. The highest thermoelectric figure of merit ZT of 0.34 is achieved for x = 1.025 at 700 K.

Synthesis, structure, and properties of two Zintl phases around the composition SrLiAs

Two atomic arrangements were found near the equiatomic composition in the strontium-lithium-arsenic system. Orthorhombic o-SrLiAs was synthesized by reaction of elemental components at 950 °C, followed by annealing at 800 °C and subsequent quenching in water. The hexagonal modification h-SrLi(1-x)As was obtained from annealing of o-SrLiAs at 550 °C in dynamic vacuum. The structures of both phases were determined by single-crystal X-ray diffraction: o-SrLiAs, structure type TiNiSi, space group Pnma, Pearson symbol oP12, a = 7.6458(2) Å, b = 4.5158(1) Å, c = 8.0403(3) Å, V = 277.61(2) Å(3), R(F) = 0.028 for 558 reflections; h-SrLi(1-x)As, structure type ZrBeSi, space group P6(3)/mmc, Pearson symbol hP6, a = 4.49277(9) Å, c = 8.0970(3) Å, V = 141.54(1) Å(3), RF = 0.026 for 113 reflections. The analysis of the electron density within the framework of the quantum theory of atoms in molecules revealed a charge transfer according to the Sr(1.3+)Li(0.8+)As(2.1-), in agreement with the electronegativities of the individual elements. The electron localizability indicator distribution indicated the formation of a 3D anionic framework [LiAs] in o-SrLiAs and a rather 2D anionic framework [LiAs] in h-SrLi(1-x)As. Magnetic susceptibility measurements point to a diamagnetic character of both phases, which verifies the calculated electronic density of states.

Homo- and heterovalent substitutions in the new clathrates I Si30P16Te(8-x)Se(x) and Si(30+x)P(16-x)Te(8-x)Br(x): synthesis, crystal structure, and thermoelectric properties

The new cationic clathrates I Si(30)P(16)Te(8-x)Se(x) and Si(30+x)P(16-x)Te(8-x)Br(x) were synthesized by the standard ampule technique. The Si(30)P(16)Te(8-x)Se(x) (x = 0-2.3) clathrates crystallize in the cubic space group Pm3̅n with the unit cell parameter a ranging from 9.9382(2) to 9.9696(1) Å. In the case of the Si(30+x)P(16-x)Te(8-x)Br(x) (x = 1-6.4) clathrates, the lattice parameter varies from 9.9720(8) to 10.0405(1) Å; at lower Si/P ratios (x = 1-3) the ordering of bromine atoms induces the splitting of the guest positions and causes the transformation from the space group Pm3n to Pm3. Irrespective of the structure peculiarities, the normal temperature motion of the guest atoms inside the oversized cages of the framework is observed. The title clathrates possess very low thermal expansion coefficients ranging from 6.6 × 10(-6) to 1.0 × 10(-5) K(-1) in the temperature range of 298-1100 K. The characteristic Debye temperature is about 490 K. Measurements of the electrical resistivity and thermopower showed typical behavior of p-type thermally activated semiconductors, whereas the temperature behavior of the thermal conductivity is glasslike and in general consistent with the PGEC concept. The highest value of the thermoelectric figure of merit (ZT = 0.1) was achieved for the Br-bearing clathrate Si(32.1(2))P(13.9(2))Te(6.6(2))Br(1.0(1)) at 750 K.

Facile general route toward tunable Magnéli nanostructures and their use as thermoelectric metal oxide/carbon nanocomposites

Engineering nanoscale interfaces is a requisite for harnessing electrical and thermal transports within nanostructured materials, especially those destined for thermoelectric applications requiring an unusual combination of low thermal conductivity and electrical resistivity. Nanocomposites open up possibilities in this area, but are still bound to a very narrow range of materials. Here, we report a new approach combining the sol-gel process toward hybrid materials with spark plasma sintering (SPS) to yield functional nanocomposites based on substoichiometric titanium oxides Ti(n)O(2n-1), so-called Magnéli phases. The potential of this new approach is demonstrated by three results. First, multiple Ti(n)O(2n-1) compounds (n = 3, 4, 5, 6, 8) are obtained for the first time as sole nano-Magnéli crystalline phases with controlled specific surface areas from 55 to 300 m(2)·g(-1), classified as potential thermoelectric n-type metal oxides and paving the way toward advanced systems for energy-harvesting devices and optoelectronics. Second, this work combines the use of sol-gel and SPS processes to yield percolated nanocomposites based on metal oxide nanoparticles embedded in a carbon matrix with low electrical resistivity (2 × 10(-4) Ω·m for a Ti(4)O(7) compound) and reduced thermal conductivity (1 W·m(-1)·K(-1)) with respect to bulk phases. Finally, the discovered materials are reliable with thermoelectric figures of merit (ZT = 0.08) relatively high for n-type Ti-O-based systems and metal oxides. Thereby this study represents a proof of concept for the development of promising, cheaper, and more efficient thermoelectric conversion devices.

Thermoelectric properties of lead chalcogenide core-shell nanostructures

We present the full thermoelectric characterization of nanostructured bulk PbTe and PbTe-PbSe samples fabricated from colloidal core-shell nanoparticles followed by spark plasma sintering. An unusually large thermopower is found in both materials, and the possibility of energy filtering as opposed to grain boundary scattering as an explanation is discussed. A decreased Debye temperature and an increased molar specific heat are in accordance with recent predictions for nanostructured materials. On the basis of these results we propose suitable core-shell material combinations for future thermoelectric materials of large electric conductivities in combination with an increased thermopower by energy filtering.

ZT enhancement in solution-grown Sb(2-x)BixTe3 nanoplatelets

We report a solution-processed, ligand-supported synthesis of 15-20 nm thick Sb(2-x)BixTe3 nanoplatelets. After complete ligand removal by a facile NH3-based etching procedure, the platelets are spark plasma sintered to a p-type nanostructured bulk material with preserved crystal grain sizes. Due to this nanostructure, the total thermal conductivity is reduced by 60% in combination with a reduction in electric conductivity of as low as 20% as compared to the bulk material demonstrating the feasibility of the phonon-glass electron-crystal concept. An enhancement in the dimensionless thermoelectric figure of merit of up to 15% over state-of-the-art bulk materials is achieved, meanwhile, shifting the maximum to significantly higher temperatures.