half-Heusler Alloys Research Articles

Large magnetoresistance and temperature-driven spin filter effect in spin valve based on half Heusler alloy.

High spin-injection-efficiency (SIE) and thermal spin-filter-effect (SFE) from a magnetic material to a barrier material are crucial to the high performance of a spintronic device and a spin caloritronic device, respectively. By performing a nonequilibrium Green's function combined with first-principles calculations, we study the voltage-driven and temperature-driven spin transport properties of a half Heusler alloy RuCrAs based spin valve with different atom-terminated interfaces. The spin valve with a CrAs-top (or Ru-top) interface structure has an ultrahigh equilibrium magnetoresistance (MR) ratio of ∼1.56 × 109% (or ∼5.14 × 108%), ∼100% SIE, a large MR ratio, and high spin current intensity under bias voltage, suggesting that it has a great potential application in spintronic devices. The spin valve with the CrAs-top (or CrAs-bri) interface structure has a perfect SFE due to its very high spin polarization of temperature-driven currents, and it is useful in spin caloritronic devices.

Search for semiconducting materials among 18-electron half-Heusler alloys

A high throughput search for semiconductors among 153 half-Heusler phases with 18 valence electrons was performed. The alloys considered were as follows: III–X–XV, IV–X–XIV, IV–IX–XV, V–IX–XIV, and V–VIII–XV, wherein the particular groups of the elements were confined as shown: III (Sc, Y, La, Ga, In), IV (Ti, Zr, Hf), V (V, Nb, Ta), VIII (Fe, Ru, Os), X (Ni, Pd, Pt), XIV (Ge, Sn, Pb), and XV (As, Sb, Bi). The generalized gradient approximation (GGA) and modified Becke–Johnson GGA (MBJGGA) approaches were employed in order to calculate the band structures as well as determine the types and widths of band gaps in the materials. Furthermore, the strength of spin–orbit coupling in some systems was discussed based on the heavy- and light-hole energy split-offs. Some compounds are expected to be topological insulators, e.g., the novel Pd-based arsenide (LuPdAs). The set of 24 novel semiconductors following the ‘10 kBT rule’, which suggests their potential for application in thermoelectrics, was revealed. The results of this work should encourage further experimental efforts in synthesis of novel half-Heusler phases.

Prediction of electronic structural properties of semi-Heusler CoHfZ (Z: Sb, Sn)

Co-based half-Heusler alloys CoHfZ (Z: Sb, Sn) are investigated under density functional formalism by using the Quantum Espresso and BoltzTrap packages. Unit cell relaxation and ground state energy optimization are carried out for different possible phases considered for MgAgAs-type crystal structure for these alloys. The equilibrium lattice parameter is obtained and then the density of states and band structural calculations are performed. An indirect semiconducting band gap is found for both the spin-up and spin-down charge carriers with a gap of 1.12 eV and 0.66 eV for CoHfSb and CoHfSn, respectively. For CoHfSn alloy, a few bands are found to just cross the Fermi level suggesting the p-type semiconducting behaviour for this alloy. A maximum Seebeck coefficient of 110 μV/K is estimated for CoHfSn half-Heusler alloy.

Enhanced Thermoelectric Properties of Zr0.85–xHfxNb0.15–yTayCoSb Medium-Entropy Alloys: Tradeoff between “What to Alloy” and “How Much to Alloy”

Although configurational entropy is regarded to be a gene-like performance indicator for thermoelectric (TE) materials, increasing the configurational entropy alone does not guarantee a high TE figure of merit (ZT). Therefore, determining protocols for designing medium- and high-entropy TE materials with high ZT values is imperative. Herein, we provide a strategy for designing high ZT n-type ZrCoSb-based medium-entropy (ME) half-Heusler (HH) alloys and highlight the tradeoff between “what to alloy” and “how much to alloy”. As expected, an intrinsically low lattice thermal conductivity of ∼2.24 W m–1 K–1 was obtained at 923 K for the Zr0.45Hf0.4Nb0.06Ta0.09CoSb ME HH alloy owing to the synergy between Zr-site disorder, anharmonicity, refined grain size, and entropy-driven multiscale defects. Entropy-driven quantum traps lead to energy-filter effects that concurrently provide optimized Seebeck coefficients and power factors. These favorable modifications enable the Zr0.45Hf0.4Nb0.06Ta0.09CoSb ME HH alloy to exhibit a high peak ZT of ∼0.57. The developed design concept provides an effective strategy for enhancing the ZT values of medium- and high-entropy TE materials.

First-principles theoretical analysis of magnetically tunable topological semimetallic states in antiferromagnetic DyPdBi

A number of compounds in the family of rare-earth half-Heusler alloys have been predicted to be topologically nontrivial semimetals, classified as Weyl, triple-point, and Dirac semimetals based on the multiplicity of degeneracy of the nodal points or crossing of linearly dispersed electronic bands. Here, we present a first-principles theoretical characterization of the electronic topology of the antiferromagnetic half-Heusler alloy DyPdBi. In the antiferromagnetic state preserving ${C}_{3v}$ symmetry of the crystal, DyPdBi is a triple-point semimetal hosting four triply degenerate nodes along the threefold symmetry axis of the Brillouin zone. In contrast, the antiferromagnetic state of DyPdBi with local magnetic moments on Dy rotated to a direction perpendicular to the ${C}_{3}$ axis breaks the threefold rotational symmetry, and hosts four Weyl nodes in its Brillouin zone. Our calculations of the Berry curvature of their electronic states clearly show that the triple-point fermions of DyPdBi exhibit a signature peak in the anomalous Hall conductivity, while the Weyl fermions do not contribute to anomalous Hall conductivity. As these two topologically distinct magnetic states are separated by a small energy difference of $\ensuremath{\approx}$15 meV, we expect them to be switchable with a magnetic field or spin torque and distinguishable experimentally from anomalous Hall conductance.

Half metallic Heusler alloys XMnGe (X = Ti, Zr, Hf) for spin flip and thermoelectric device application – Material computations

The ground state properties of XMnGe (X = Ti, Zr, Hf) magnetic half Heusler alloys have been investigated through density functional theory. Two different exchange correlation functionals (GGA and mBJ) are used to study the reported materials for comparison. By using these approximation methods, we have calculated corresponding lattice constants and ground state energies. The band structure and density of state calculations have been done by computing dispersion curves. From mBJ potential, the reported alloys are predicted to be half metals with semiconducting nature in spin up state and metallic nature in spin down state. The respective band gap for XMnGe (X = Ti, Zr, Hf) is 1.06 e V, 1.13 eV and 1.03 eV. The studied cubic alloys show ductile character and also exhibit directional property. The calculated Seebeck coefficient of XMnGe (X = Ti, Zr, Hf) is −109.4 μV/K, 101.9 μV/K and 128.5 μV/K at 1100 K, 1300 K and 1100 K respectively and the corresponding power factor is 2 × 1012 W/m.K [2].s, 1.4 × 1012 W/m.K [2].s and 1.3 × 1012 W/m.K2.s at respective same temperature. Since, these alloys exhibit half metallic behaviour and also have good thermoelectric nature, they can be used for spin-based electronics and thermoelectric energy applications.

Properties of the double half-heusler alloy ScNbNi2Sn2 with respect to structural, electronic, optical, and thermoelectric aspects

In the current work, the structural, electronic, thermoelectrics, and optical characteristics of the double half Heusler (DHH) ScNbNi2Sn2 compound are reported for the first time using density functional theory (DFT). The computed band structures show typical semiconductor behavior with an indirect bandgap (0.47 eV) using EV-GGA approximation. We also investigated the optical properties such as the dielectric function, optical conductivity, refractive index. Boltzmann's semiclassical theory attempts to explain a simulation concept in the BoltzTrap software, and the findings were presented and analyzed in terms of electrical conductivity, electronic and lattice thermal conductivities, the Seebeck coefficient, and the Figure of merit over a 50 K–1000 K temperature range. At room temperature, with a low magnitude of lattice thermal conductivity (κL) (5.30776 W/m. K) and a maximum value of the merit factor (ZT) is 0.64 at 900 K for ScNbNi2Sn2 compound is observed. These findings suggest that our material may be a viable option for use in high-temperature thermoelectric devices. We calculated (S, (σ/τ), (ke/τ)) along the x, y, and z axes utilizing the EV-GGA method. We found out that our compound is thermoelectrically anisotropic. We have also studied in EV-GGA the effect of the carrier concentration on the Seebeck coefficnet at T = 600 K. The maximum value of S is 356.9905 μV/K with n=3.04×1019Cm−3 within GGA and EV-GGA respectively.

The thermal and mechanical properties of Li-based Half Heusler alloys LiAlZ(Z = Si, Ge) using Quasi-Harmonic Approach

Using the Density Functional Density (DFT) method coupled with a Quasi-Harmonic Approximation (QHA), we studied the structural stability and thermal properties of Li-based Half Heusler alloys LiAlZ (Z= Si and Ge). The obtained results show that type I for both compounds is the most stable of all the examined structural phase types i (i = I, II, III) and have semiconductor behavior with narrow indirect band gaps. The study of the elastic constants and phonon curves confirmed that these compounds are mechanically and dynamically stable. The obtained results of total free energy are considered while estimating the thermal properties as the lattice parameters, volume thermal expansion, heat capacity, electronic and entropy in the range of temperature from 0 K to 1000 K. The thermal properties show that half Heuslers LiAlZ are thermodynamically stable, and the vibrational contribution is dominant compared to configurational and electronic contributions in the heat capacity and entropy. In contrast, the vibrational contribution is very small to the configurational and electronic free energy contributions.

Ink casting and 3D-extrusion printing of the thermoelectric half-Heusler alloy Nb1-xCoSb

Additive manufacturing of lattices with ∼600 μm diameter struts is achieved via extrusion printing of an ink containing prealloyed powder of the half-Heusler alloy Nb1-xCoSb, followed by debinding and vacuum sintering to a relative density of ∼70-80%. The ink can also be poured and cast into square blocks, which, after debinding and sintering, achieve a similar relative density. Sintering in a Sb-rich atmosphere reduces Sb sublimation and decomposition of the half-Heusler phase. This is reflected in electrical conductivity values, for ink-cast specimens, similar to those of a sample created by vacuum hot-pressing of dry powders (1080 S/cm 298 K). However, a lower Seebeck coefficient for the ink-cast specimens (92 vs. 150 µV/K for a hot-pressed sample) leads to a reduced figure of merit (zTmax= 0.10±0.015 vs. 0.26±0.04 at 873 K for a hot-pressed sample), where secondary phases, NbSb2 and CoSb3 were observed; these might be minimized, or even eliminated, via further optimization of the sintering conditions.

First principles study on structural, mechanical, and thermoelectric properties of half-Heusler alloy (KLiTe)

In this study, the structural, optoelectronics, elastic, and transport properties of KLiTe were estimated from first principles computations using the FP-LAPW technique and semi-classical Boltzmann transport theory The exchange–correlation is addressed employing the generalized gradient approximation with Perdew-Burke-Ernzerhof scheme (GGA-PBE). In order to distinguish between the brittle and ductile properties, the Poisson and Pugh's ratio was calculated. The semiconductor KLiTe is demonstrated to have a direct band gap. The compound under investigation exhibits significant Seebeck coefficient and power factor values. The high-power factor of 8.3 × 1011 W/K2ms. that KLiTe exhibits at 1200 K makes it a potentially viable material for thermoelectric applications. Due to their high figure of merit values, the results obtained indicate that KLiTe could be a strong contender for thermoelectric generator applications at low and moderate temperature.

Enhancement of thermoelectric performances in n-type RbCrZ (Z = S, Se, Te) half-metallic ferromagnetic alloys via charge carrier concentration or chemical potential

We used first-principles simulations to examine the magnetic, elastic, and thermoelectric properties of RbCrZ (Z = S, Se, and Te) half-Heusler alloys. Our results indicate that RbCrZ (Z = S, Se, and Te) alloys are completely spin-polarized half-metallic ferromagnets in the ground state. Elastic constants provide mechanical stability. Unlike RbCrS and RbCrSe, RbCrTe is brittle and unable to withstand thermal shocks. RbCrS is the most anisotropic material, while RbCrSe is the most isotropic. The greatest ZT values in the spin-down channel for RbCrS, RbCrSe, and RbCrTe are 0.81355, 0.62249, and 1.02846, respectively. To attain this value, the charge carrier concentration must be lowered to n = - 3.18119 × 1022 cm−3 for RbCrS, n = - 5.32817 × 1022 cm−3 for RbCrSe, and for RbCrTe, the charge carrier concentration must be enhanced to n = - 4.99321 × 1022 cm−3. The decrease of the chemical potential of RbCrS, RbCrSe, and RbCrTe by 0.02, 0.084, and 0.0329 Ryd, respectively, resulted in identical values.

First Principles calculation of Half metallic proprieties of QCrAs (Q=Hf, Ti and Zr)

The structural, electrical, magnetic, mechanical, and thermodynamic properties of some novel half-Heusler alloys QCrAs(Q=Hf, Ti and Zr) are investigated using first principles calculations. The results show that the three half Heusler alloys are half metals and they can find application in spintronics industries. They possess magnetic moment of 3\mu_B. The mechanical properties shows that they are mechanically stable. The B/G ratio of the three half-Heusler alloys show that they are ductile in nature and the Poisson’s ratio reveal that the plasticity of TiCrAs and ZrCrAs are higher than that of HfCrAs. The Debye temperature and average sound velocity of ZrCrAs is observed to be higher than the other two alloys. This implies that the thermal conductivity of ZrCrAs is the highest.

First principle study of strain tunable electronic and optical properties of half-Heusler alloys XCoGe (X=V, Nb, Ta)

Half-Heusler alloys are gaining considerable attention for designing novel optoelectronic and renewable energy device applications. In the present work, we performed the first principle study of structural, electronic, and optical properties of XCoGe(X=V,Nb,andTa) half-Heusler alloys under external strain (0–8%). All compounds exhibit cubic crystal structures with an F-43m space group. The lattice constant and bond length increase (decrease) with increasing tensile (compressive) strain. The calculated formation energy of XCoGe systems is negative and increases (decreases) with varying the tensile (compressive) strain from 0 to 8%, indicating the stability of these systems. All XCoGe compounds possess an indirect band gap nature and are retained under strain. The band gap reduces under tensile strain and considerably widens by external compressive strain. The X-s state and Co-s state mainly contribute to the valence band maximum and conduction band minimum in XCoGe systems. Interestingly, the optical absorption spectra reveal maximum absorption in the visible energy range for both unstrained and strained systems. Interestingly, the red- and blue-shift under tensile and compressive strain in XCoGe make them good candidates for optoelectronic applications. The results pave the path for the practical utilization of XCoGe compounds for optoelectronic and light detection applications.

Combined Experimental and First Principles Study on Nanostructured NbFeSb Half-Heusler Alloy Synthesized by Mechanical Alloying

The Half-Heusler semiconductor alloys can be used efficiently as thermoelectric materials to transform the waste heat into useful electrical energy. The low-cost and large-scale production of suitable half-Heusler alloys are important in the present context. In this work, a nanostructured half-Heusler NbFeSb alloy is obtained by mechanical alloying with 15h of milling. The structural parameters of the sample are investigated by powder X-ray diffraction followed by Rietveld refinement. Differential scanning calorimetry indicates that the NbFeSb phase is stable up to about 420 K. The electrical resistivity is obtained as a function of temperature. A band gap of 0.37(3) eV is obtained from UV-Vis measurements. Density functional theory calculation shows an indirect band gap of 0.52 eV. Analyses of the obtained data indicate that structural defects and nanometric crystallites sizes present in the nanostructured NbFeSb produced by mechanical alloying do not degrade the electrical and optical properties of the compound.

Alloying effect on the lattice thermal conductivity of MNiSn half-Heusler alloys.

The lattice thermal conductivity of MNiSn (M = Ti, Zr, Hf) half-Heusler (HH) alloys was studied. Ab initio DFT calculations were used for the calculation of the material physical properties. A combination of the Slack model and Klemens analytical alloying model was used to simulate the lattice thermal conductivity as a function of composition and temperature. Our results emphasize the major role of point defect scattering in a single-phase state of HH alloys because of the mixing of elements in the M-sub-lattice, especially at the high working temperature of the thermoelectric material. We performed a series of calculations from pure unalloyed compounds to multicomponent compositions with five elements in the M sub-lattice of (Ti, Zr, Hf, Al, Sc)NiSn.

Approaching the minimum lattice thermal conductivity in TiCoSb half-Heusler alloys by intensified point-defect phonon scattering

Enhanced point defect phonons scattering through huge isovalent substitution substantially reduces the lattice thermal conductivity of half-Heusler alloys.

Preparation and thermoelectric properties of Sc-doped Ti<sub>1–<i>x</i></sub>NiSb half-Heusler alloys

The nominal composition TiNiSb with 19 valence electrons is demonstrated to be composed of off-stoichiometric half-Heusler phase and impurities. In this work, the Ti<sub>1–<i>x</i></sub>NiSb (<i>x</i> = 0, 0.10, 0.15, 0.20, 0.25) samples are prepared by ball milling and spark plasma sintering. The single-phase Ti<sub>0.9</sub>NiSb sample, deviating from the theoretical composition Ti<sub>0.75</sub>NiSb base on 18-electron rule, is obtained, which might be ascribed to the small defect formation energy of Ti filling the vacancy as well as our ball-milling preparation method. With the single-phase Ti<sub>0.9</sub>NiSb sample used as the base material, a small amount of Sc is used to partially replace Ti in order to further reduce the carrier concentration. Thus, the Ti<sub>1–<i>x</i>–<i>y</i></sub>Sc<sub><i>y</i></sub>NiSb (<i>x</i> = 0.10, 0.15; <i>y</i> = 0.03, 0.05) samples are designed to investigate the effect of Sc doping on the thermoelectric properties. The X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) analysis confirm the single-phase nature of the Ti<sub>1–<i>x</i>–<i>y</i></sub>Sc<sub><i>y</i></sub>NiSb samples. Energy-dispersive X-ray spectroscopy (EDS) results indicate that the actual compositions of the Ti<sub>1–<i>x</i>–<i>y</i></sub>Sc<sub><i>y</i></sub>NiSb samples are consistent well with their nominal compositions, and all elements are distributed uniformly in the sample. Moreover, the doping of Sc can increase the content of Ti vacancy while maintaining the single-phase structure, which could be attributed to the higher binding energy between Sc and Sb because the electronegativity of Sc is less than that of Ti. Both the substitution of Sc for Ti and the increase of the Ti vacancies significantly reduce the carrier concentration, which decreases from ~13.6 × 10<sup>21</sup> cm<sup>–3</sup> for Ti<sub>0.9</sub>NiSb to ~3.4 × 10<sup>21</sup> cm<sup>–3</sup> for Ti<sub>0.8</sub>Sc<sub>0.05</sub>NiSb. The reduced carrier concentration results in greatly increased Seebeck coefficient, therefore the Ti<sub>0.8</sub>Sc<sub>0.05</sub>NiSb sample achieves a power factor as high as 17.7 μW·cm<sup>-1</sup>·K<sup>-2</sup> at 973 K. Although the lattice thermal conductivity of Sc-doped sample increases slightly due to the reduction of electron–phonon scattering and the enhancement of chemical bonds, the total thermal conductivity decreases dramatically due to the electronic thermal conductivity decreasing greatly. Finally, the Ti<sub>0.8</sub>Sc<sub>0.05</sub>NiSb sample reaches a <i>ZT</i> value of ~0.42 at 973 K, which is 180% higher than that of Ti<sub>0.9</sub>NiSb sample. Despite the fact that the thermoelectric performance of our sample is still inferior to those of the state-of-the-art off-stoichiometric 19-electron half-Heusler alloys, this work demonstrates that the thermoelectric performance of Ti<sub>1–<i>x</i></sub>NiSb can be further improved by non-isoelectronic doping.

Tailoring the intrinsic magneto-electronic, mechanical, thermo-physical and thermoelectric response of cobalt-based Heusler alloys: an ab initio insight.

We conducted a comprehensive analysis of the fundamental properties of CoHfSi and CoHfGe half-Heusler alloys using density functional theory simulations implemented in Wien2k. To begin, structural optimization revealed that both alloys effectively adopt a cubic C1b structure, with Y1 as the dominant ferromagnetic phase. Electronic properties were computed using various approximation schemes, including the Generalized Gradient Approximation and the modified Becke-Johnson potential. The examination of electronic band structures and their accompanying density of states using the modified Becke-Johnson functional approach unveiled their half-metallic nature. In this context, the spin-up channel exhibited semiconductor behaviour, while the spin-down channel displayed metallic characteristics. Additionally, the spin-splitting observed in their resulting band structures contributed to a net magnetism within their lattice structure, making them promising candidates for spintronic applications. We also scrutinized Seebeck coefficients, electrical conductivity, thermal conductivity, and power factor to gain a better understanding of their thermoelectric properties.

Enhancing the thermoelectric performance of a Ti2FeNiSb2 double half-Heusler alloy through excess Ni-induced full-Heusler nanoprecipitates

The excessed Ni will embed full-hesuler TiNi2Sb nanoprecipitates into the matrix, which can improve the thermoelectric performance by scattering low-energy carriers and phonons.

Tantalum half-Heusler alloys RbTaSi and RbTaGe: potential candidates for desirable thermoelectric and spintronic applications.

Heusler alloys have drawn the interest of researchers due to their possible technical significances and multifunctional use. Herein, a thorough theoretical analysis using "density functional theory (DFT)" is performed to investigate the general physical features of RbTaSi and RbTaGe alloys. The "generalised gradient approximation (GGA)" and "Tran-Blaha modified Becke-Johnson (TB-mBJ) potential" has been incorporated to model the electronic structures of RbTaSi and RbTaGe. The structural optimization results signify that these materials are stable in the ferromagnetic phase with a cubic F4̄3m structure, which is supported by the computed elastic parameters. In addition, cohesive energy and microhardness signify strong bonding. The spin-polarisation bands and density of states indicate the half-metallic nature of these materials. These materials have spin magnetic moment 2μB, thereby emphasizing the use of these alloys for spintronic applications. Transport and thermodynamic properties have been calculated, and their temperature dependence is illustrated. The behavior of transport coefficients with temperature futher implies the presence of half-metallic nature.