Looking for new thermoelectric materials among the TMX intermetallics using high-throughput calculations

Within 4 different crystal structures, 2280 ternary intermetallic configurations have been investigated via high-throughput density functional theory calculations in order to discover new semiconducting materials. The screening is restricted to intermetallics with the equimolar composition TMX, where T is a transition metal from the Ti, V, Cr columns, Sr, Ba, Y and La, M an element from the first line of transition metals and X a sp elements (Al, P, Si, Sn and Sb), i.e. to a list of 24 possible elements. Since the calculations are done combinatorically, every possible ternary composition is considered, even those not reported in the literature. All these TMX configurations are investigated in the 4 most reported structure-types: TiNiSi, MgAgAs, BeZrSi and ZrNiAl. With an excellent agreement between calculations and literature for the reported stable phases, we identify 472 possible stable compounds among which 21 are predicted as non-metallic. Among these 21 compositions, 4 could be considered as new semiconductors

Ferromagnetically correlated clusters in semi-metallic Ru2NbAl Heusler alloy

In this work, we report the structural, magnetic and electrical and thermal transport properties of the Heusler-type alloy Ru2NbAl. From the detailed analysis of magnetization data, we infer the presence of superparamagnetically interacting clusters with a Pauli paramagnetic background, while short-range ferromagnetic interaction is developed among the clusters below 5 K. The presence of this ferromagnetic interaction is confirmed through heat capacity measurements. The relatively small value of electronic contribution to specific heat, gamma (~2.7 mJ/mol-K2), as well as the linear nature of temperature dependence of Seebeck coefficient indicate a semi-metallic ground state with a pseudo-gap that is also supported by our electronic structure calculations. The activated nature of resistivity is reflected in the observed negative temperature coefficient and has its origin in the charge carrier localization due to antisite defects, inferred from magnetic measurements as well as structural analysis. Although the absolute value of thermoelectric figure of merit is rather low (ZT = 5.2*10-3) in Ru2NbAl, it is the largest among all the reported non-doped full Heusler alloys.Comment: 25 pages, 14 figure

Superparamagnetic and metal-like Ru2TiGe: a propitious thermoelectric material

We report a study of structural, magnetic, heat capacity and thermoelectric properties of a Rubased Heusler alloy, Ru2TiGe. The magnetic measurements reveal that at higher temperatures, diamagnetic and Pauli paramagnetic contributions dominate the magnetic behaviour whereas, at lower temperatures (T<= 20 K), superparamagnetic interaction among clusters is observed. Effect of such magnetic defects is also evident in the electrical resistivity behaviour at lower temperatures. Though the temperature dependence of resistivity exhibits a metal-like nature, the large value of Seebeck coefficient leads to an appreciable power factor of the order of 1 mW/mK2 at 300 K. Large power factor as well as low thermal conductivity results in a value of ZT = 0.025 at 390 K for Ru2TiGe that is orders of magnitude higher than that of the other pure Heusler alloys and point towards its high potential for practical thermoelectric applications

Coexisting structural disorder and robust spin-polarization in half-metallic FeMnVAl

Half-metallic ferromagnets (HMF) are on one of the most promising materials in the field of spintronics due to their unique band structure consisting of one spin sub-band having metallic characteristics along with another sub-band with semiconductor-like behavior. In this work, we report the synthesis of a novel quaternary Heusler alloy FeMnVAl and have studied the structural, magnetic, transport, and electronic properties complemented with first-principles calculations. Among different possible structurally ordered arrangements, the optimal structure is identified by theoretical energy minimization. The corresponding spin-polarized band structure calculations indicates the presence of a half-metallic ferromagnetic ground state. A detailed and careful investigation of the x-ray diffraction data, M\"{o}ssbauer and nuclear magnetic resonance spectra suggest the presence of site-disorder between the Fe and Mn atoms in the stable ordered structure of the system. The magnetic susceptibility measurement clearly establishes a ferromagnetic-like transition below $\sim$213 K. The ${^{57}}$Fe M\"{o}ssbauer spectrometry measurements suggest only the Mn-spins could be responsible for the magnetic order, which is consistent with our theoretical calculation. Surprisingly, the density-functional-theory calculations reveal that the spin-polarization value is almost immunized (92.4\% ${\rightarrow}$ 90.4\%) from the Mn-Fe structural disorder, even when nonmagnetic Fe and moment carrying Mn sites are entangled inseparably. Robustness of spin polarization and half metallicity in the studied FeMnVAl compound comprising structural disorder is thus quite interesting and could provide a new direction to investigate and understand the exact role of disorders on spin polarization in these class of materials, over the available knowledge.Comment: 12 page

High spin-polarization in a disordered novel quaternary Heusler alloy FeMnVGa

In this work, we report the successful synthesis of a Fe-based novel half-metallic quaternary Heusler alloy FeMnVGa and its structural, magnetic and transport properties probed through different experimental methods and theoretical technique. Density functional theory (DFT) calculations performed on different types of structure reveal that Type-2 ordered structure (space group: F-43m, Ga at 4a, V at 4b, Mn at 4c and Fe at 4d) possess minimum energy among all the ordered variants. Ab-initio simulations in Type 2 ordered structure further reveal that the compound is half-metallic ferromagnet (HMF) having a large spin-polarization (89.9 %). Neutron diffraction reveal that the compound crystalizes in disordered Type-2 structure (space group: Fm-3m) in which Ga occupy at 4a, V at 4b and Fe/Mn occupy 4c/4d sites with 50:50 proportions. The structural disorder is further confirmed by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS),57Fe Mossbauer spectrometry results and DFT calculations. Magnetisation studies suggest that the compound orders ferromagnetically below TC ~ 293 K and the saturation magnetization follows Slater-Pauling rule. Mossbauer spectrometry, along with neutron diffraction suggest that Mn is the major contributor to the total magnetism in the compound consistent with the theoretical calculations. First principle calculations indicate that spin-polarization remain high (81.3 %) even in the presence of such large atomic disorder. The robustness of the HMF property in presence of disorder is a quite unique characteristic over other reported HMF in literature and make this compound quiet promising for spintronics applications

Review of the Thermoelectric Properties in Nanostructured Fe2VAl

Besides alloying, nanostructuring was implemented to improve the thermoelectric properties in Fe2VAl. This Heusler alloy indeed displays a thermoelectric figure of merit too small for applications (ZT ~ 0.1 at 300 K) which is caused by a large lattice thermal conductivity (&lambda;L = 27 W&middot;m&minus;1&middot;K&minus;1 at 300 K). The effect of nanostructuring on the microstructure and on the thermoelectric properties of alloyed Fe2VAl are therefore reviewed. By mechanical alloying followed by spark plasma sintering, the average grain size (D) was decreased to D ~ 300&ndash;400 nm in Fe2VAl0.9Si0.1, Fe2VAl0.9Si0.07Sb0.03, Fe2V1.05Al0.95, and Fe2V0.9W0.1Al. As expected, phonon scattering at the numerous grain boundaries lead to a strong decrease in the lattice thermal conductivity, which reached values as small as &lambda;L = 3.3 W&middot;m&minus;1&middot;K&minus;1. However, in all the reviewed examples, the thermoelectric figure of merit (ZT) is only marginally or not even improved when comparing to non-nanostructured samples because the electrical resistivity also increases upon nanostructuring. A significantly improved ZT = 0.3 at 500 K was only recently observed in severely deformed Fe2VAl0.95Ta0.05 by high pressure torsion because the very fine microstructure (D ~ 100 nm) strongly enhanced the thermal conductivity reduction

Optimization of Criteria for an Efficient Screening of New Thermoelectric Compounds: The TiNiSi Structure-Type as a Case-Study

International audience7 High-throughput calculations can be applied to a large number of compounds, in order to 8 discover new useful materials. In the present work, ternary intermetallic compounds are 9 investigated, to find new potentially interesting materials for thermoelectric applications. The 10 screening of stable non-metallic compounds required for such applications is performed by 11 calculating their electronic structure, using DFT methods. In the first section, the study of the 12 density of states at the Fermi level, of pure elements, binary and ternary compounds, leads to 13 empirically chose the selection criterion to distinguish metals from non-metals. In the second 14 section, the TiNiSi structure-type is used as a case-study application, through the investigation 15 of 570 possible compositions. The screening leads to the selection of 12 possible 16 semiconductors. The Seebeck coefficient and the lattice thermal conductivity of the selected 17 1 Corresponding author: [email protected] 2 compounds are calculated in order to identify the most promising ones. Among them, TiNiSi, 18 TaNiP or HfCoP are shown to be worth a detailed experimental investigation. 1

Numerical and experimental determination of the thermal conductivity of pristine and substituted Fe2VAl

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Phase diagram and order-disorder transitions in Y0.9Gd0.1Fe2Hx hydrides (x ≥ 2.9)

International audienceY0.9Gd0.1Fe2, which crystallize in a C15 cubic structure, can absorb up to 5 H/f.u. and its pressure-composition isotherm displays a multiplateau behavior related to the existence of several hydrides with different crystal structures. At room temperature Y0.9Gd0.1Fe2Hx hydrides (2.9 ≤ x ≤ 5) crystallize in three phases with cubic structure (C1, C2 and C3), two phases with monoclinic structures (M1 and M2), and one phase with orthorhombic structure (O), with the following sequence for increasing H concentration: C1, M1, C2, M2, C3, O. Each phase exists as single phase within a H homogeneity range, and they are separated from each other by two-phase domains. The reductions of crystal symmetry are related to various hydrogen orders into interstitial sites. Weak superstructure peaks were indexed by doubling the cubic cell parameter of the cubic C2 phase. Upon heating, the monoclinic M1 and M2 and the cubic C2 phases undergo order-disorder (O-D) transitions toward a disordered cubic structure CDis. These O-D transitions are reversible with thermal hysteresis effects. The cubic C3 and orthorhombic O phases transform into a disordered cubic phasecompanied by H desorption

Nanomatériaux thermoélectriques

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