half-Heusler Compounds Research Articles

Analysing the structural, mechanical and thermal stability in lithium-based half-Heusler compounds LiAlSi and LiAlGe: a first-principles study

Analysing the structural, mechanical and thermal stability in lithium-based half-Heusler compounds LiAlSi and LiAlGe: a first-principles study

Half-Heusler alloys as emerging high power density thermoelectric cooling materials

To achieve optimal thermoelectric performance, it is crucial to manipulate the scattering processes within materials to decouple the transport of phonons and electrons. In half-Heusler (hH) compounds, selective defect reduction can significantly improve performance due to the weak electron-acoustic phonon interaction. This study utilized Sb-pressure controlled annealing process to modulate the microstructure and point defects of Nb0.55Ta0.40Ti0.05FeSb compound, resulting in a 100% increase in carrier mobility and a maximum power factor of 78 µW cm−1 K−2, approaching the theoretical prediction for NbFeSb single crystal. This approach yielded the highest average zT of ~0.86 among hH in the temperature range of 300-873 K. The use of this material led to a 210% enhancement in cooling power density compared to Bi2Te3-based devices and a conversion efficiency of 12%. These results demonstrate a promising strategy for optimizing hH materials for near-room-temperature thermoelectric applications.

Machine learning prediction of thermal and elastic properties of double half-Heusler alloys

Double half-Heusler alloys are promising materials for applications as magnetocaloric materials, topological insulators, but especially thermoelectric materials. Four different elements in their composition provide a wide range of possible compositions, which, on the other hand, is difficult to study directly by applying traditional first-principles approaches to large number of compositions. In this work, based on the gradient boosting method, regression models are constructed that allow rapid prediction of the lattice thermal conductivity, as well as a number of other thermal and elastic properties, based on the composition and crystal structure of a compound. This made it possible for the first time to calculate the lattice thermal conductivity, as well as Grüneisen parameter, Debye temperature, and elastic moduli for a number of double half-Heusler compounds. We observe that the predicted thermal conductivity is in better agreement with the experimental data than the results of density functional theory calculations available in the literature. Half-Heusler compounds with thermal conductivity values lower than those previously known have been found. In addition, we have analyzed the importance of various features for predicting each of the studied properties, and the effect of the crystallographic symmetry of the compound on the prediction accuracy.

Opening the Bandgap of Metallic Half-Heuslers via the Introduction of d-d Orbital Interactions.

Half-Heusler compounds with semiconducting behavior have been developed as high-performance thermoelectric materials for power generation. Many half-Heusler compounds also exhibit metallic behavior without a bandgap and thus inferior thermoelectric performance. Here, taking metallic half-Heusler MgNiSb as an example, a bandgap opening strategy is proposed by introducing the d-d orbital interactions, which enables the opening of the bandgap and the improvement of the thermoelectric performance. The width of the bandgap can be engineered by tuning the strength of the d-d orbital interactions. The conduction type and the carrier density can also be modulated in the Mg1- x Tix NiSb system. Both improved n-type and p-type thermoelectric properties are realized, which are much higher than that of the metallic MgNiSb. The proposed bandgap opening strategy can be employed to design and develop new half-Heusler semiconductors for functional and energy applications.

Thermoelectric Properties Investigation of Ni/Co Doped ZrCoBi Half-Heusler Alloy

Half-Heusler (HH) thermoelectric (TE) composites have been extensively inspected due to their excellent TE properties in the medium- to high-temperature range. First-principle calculations make it easier to discover or improve more HH compounds. This article presents an ab initio theoretical evaluation of TE properties of Half-Heusler alloy, when doped with Nickel (Ni), using FP-LAPW and the semi classic Boltzmann theory. Thermoelectric parameters were calculated using BoltzTraP code, like Seebeck coefficient ( ), electrical conductivity to relaxation time ratio ( ), electronic thermal conductivity to relaxation time ratio ( ), thermoelectric power factor to relaxation time ratio ( ), and the dimensionless figure-of-merit ( ) in a temperature range of . Calculated Seebeck coefficient reveals that the studied alloys show a tendency to conduct as p-type with balanced TE performance between both charge carriers (holes and electrons). A high electronic thermal conductivity value is found, which predicts a potential use in heat sink applications for the investigated alloys. Obtained results, such as a high thermoelectric power factor and , postulate that alloys could have potential thermoelectric applications.

Elastic, anisotropic, lattice dynamics and electronic properties of XNiM and XNi2M (X = Ti, Zr, Hf; M = Sn, Ge, Si): DFT comparison study

The ab-initio calculations were performed using density functional theory (DFT) as implemented in the Quantum ESPRESSO (QE) code. Lattice parameter mismatch was minimal (2.5 % - 3 %) in TiNiSn/TiNi2Sn, ZrNiSn/ZrNi2Sn, and HfNiSn/HfNi2Sn compounds. Half-Heusler compounds had shorter bond lengths and smaller bulk moduli than their full-Heusler counterparts. Directional dependencies of the Young’s and shear moduli were investigated using elastic constants. TiNiGe, HfNiSn, and ZrNiSn had a greater universal anisotropy index among the half-Heusler compounds. Half-Heusler, TiNi2Sn, TiNi2Si, ZrNi2Sn, and HfNi2Sn compounds are reported to be dynamically stable. The electronic structure computations show that TiNiSn, ZrNiSn, and HfNiSn compounds have the least indirect energy gaps of 0.4508 eV, 0.5019 eV, and 0.3847 eV respectively. TiNiSn, ZrNiSn, and HfNiSn compounds are good candidates for minimizing the thermal conductivity of thermoelectric devices. TiNi2Sn, ZrNi2Sn, and HfNi2Sn are good candidates as possible contact electrodes for TiNiSn, ZrNiSn, and HfNiSn.

Recent trends of ternary, quaternary half-Heusler thermoelectric materials and contact resistance effect on their power output efficiency

Climate change isone of the most pressing global issues, owing to the high carbon footprint generated during conventional power generation via coal consumption and gasoline-fuelled exhaust. Furthermore, nearly half of thetotal energy is lost as heat. Suitable thermoelectric (TE) devices can convert waste heat into green/clean energy. Moreover, these devices have no moving parts, require no maintenance, and are environmentally friendly. A better TE device's thermoelectric performance (zT) should be highly cost-effective for process scaling. Because of their low cost, narrow band gap, better contact engineering, decent electrical properties, and high mechanical performance, Half-Heusler (HH) compounds (space group Fm-3 m) have been considered the best candidate materials worldwide. However, these materials have a high thermal conductivity. The high thermoelectric performance of HH compounds was achieved through iso-electronic doping and nanostructuring, which reduced thermal conductivity significantly. The current report briefly overviews17, 18, 19, and 21 valence electron count (VEC) HH materials and their effects on thermal conductivity and zT. Finally, using cumulative temperature dependence (CTD) and constant property models (CPM), the theoretical estimation of power density and efficiency was calculated by considering the materials' thermoelectric performanceand contact resistance.

Selective Scatterings of Phonons and Electrons in Defective Half-Heusler Nb1- δ CoSb for the Figure of Merit zT > 1.

The recently developed defective 19-electron half-Heusler (HH) compounds, represented by Nb1- δ CoSb, possess massive intrinsic vacancies at the cation site and thus intrinsically low lattice thermal conductivity that is desirable for thermoelectric (TE) applications. Yet the TE performance of defective HHs with a maximum figure of merit (zT) <1.0 is still inferior to that of the conventional 18-electron ones. Here, a peak zT exceeding unity is obtained at 1123 K for both Nb0.7 Ta0.13 CoSb and Nb0.6 Ta0.23 CoSb, a benchmark value for defective 19-electron HHs. The improved zT results from the achievement of selective scatterings of phonons and electrons in defective Nb0.83 CoSb, using lanthanide contraction as a design factor to select alloying elements that can strongly impede the phonon propagation but weakly disturb the periodic potential. Despite the massive vacancies induced strong point defect scattering of phonons in Nb0.83 CoSb, Ta alloying is still found effective in suppressing lattice thermal conductivity while maintaining the carrier mobility almost unchanged. In comparison, V alloying significantly deteriorates the carrier transport and thus the TE performance. These results enlarge the category of high-performance HH TE materials beyond the conventional 18-electron ones and highlight the effectiveness of selective scatterings of phonons and electrons in developing TE materials even with massive vacancies.

Thermoelectric performance of PdTiSn half Heusler compound facilitated by structural and dynamical stability: A first principle study

Structural optimization, band structure and stability (dynamical, chemical, and mechanical) of PdTiSn half-Heusler compound are examined by implementing Density Functional Theory. The electronic band structure of PdTiSn reveals that it has indirect semiconducting nature with an bandgap of 0.79 eV. The analysis of phonon properties, and formation energy confirms its dynamical, and chemical stability. The calculated thermal conductivity of 9.53 W/mK at 300 K and the figure of merit of 0.44 both demonstrate that this material has good thermoelectric properties.

Spin-polarized two-dimensional electron/hole gas at the interface of nonmagnetic semiconducting half-Heusler compounds: Modified Slater-Pauling rule for half-metallicity at the interface

Half-Heusler compounds with 18 valence electrons per unit cell are well-known nonmagnetic semiconductors. Employing first-principles electronic band-structure calculations, we study the interface properties of the half-Heusler heterojunctions based on FeVSb, CoTiSb, CoVSn, and NiTiSn compounds, which belong to this category of materials. Our results show that several of these heterojunction interfaces become not only metallic but also magnetic. The emergence of spin-polarization is accompanied by the formation of two-dimensional electron gas or hole gas at the interface. A qualitative analysis of the origin of the spin polarization at the interfaces suggests that strong correlations are responsible. For the cases of magnetic interfaces where half-metallicity is also present, we propose a modified Slater-Pauling rule similar to the one for bulk half-metallic half-Heusler compounds. Additionally, we calculate exchange parameters, Curie temperatures, and magnetic anisotropy energies for magnetic interfaces. Our study, along with recent experimental evidence confirming the presence of two-dimensional electron gas at CoTiSb/NiTiSn heterojunctions, may motivate future efforts to explore and realize device applications using these heterojunctions.

Half-metallicity from CrCoSi and MnCoSi half-Heusler alloys to their derivative double half-Heusler CrMnCo2Si2

The structural, elastic, electronic, and magnetic properties of the CrCoSi, MnCoSi parent half-Heusler (HH) alloys, and their CrMnCo2Si2 derivative double half-Heusler (DHH) compound are studied, utilizing the augmented plane wave method, which is based on density functional theory and implemented in the WIEN2k code. The stability of HH structure of the CrCoSi and MnCoSi alloys has been checked for their non-magnetic and ferromagnetic phases, leading to that the latter phase of the type I arrangement is the most stable. The CrMnCo2Si2 DHH alloy, derivative from the found structural and magnetic ground states of CrCoSi and MnCoSi HH alloys, is constructed and investigated. This DDH as well as its CrCoSi parent HH are found to be resistant to deformation and can be classified as ductile materials, whereas the MnCoSi compound is brittle. By the gradient generalized approximation (GGA), the electronic structures of CrCoSi, MnCoSi, and CrMnCo2Si2compounds exhibit a metallic behavior in the spin-up channel and a semiconducting behavior in the spin-dn channel, with band gaps (half-metallic gaps) of 0.851(0.020), 0.852(0.021), and 0.531(0.002) eV, respectively. The half-metallicity of CrMnCo2Si2 DHH is retained with smaller (larger) band gap (half-metallic gap) of 0.38(0.106) eV than that of GGA, using GGA + U approximation. In addition, the total magnetic moments are found to be 1, 2, and 3 µB for CrCoSi, MnCoSi, and CrMnCo2Si2, respectively. Therefore, these alloys can be good candidates for spinitronic applications due to their half-metallicity.

Anharmonicity effects and thermal expansion of thermoelectric (M,M’,M”)NiSn (M,M’,M”=Ti, Zr, Hf) half-Heusler alloys

High-resolution synchrotron radiation X-ray powder diffraction was used for the structural characterization of half-Heusler alloys with compositions MNiSn, M0.5M′0.5NiSn, and M0.5M′0.25M”0.25NiSn ((M, M′, M”) = Ti, Zr, Hf). These alloys were synthesized by arc melting and subsequent thermal annealing. Rietveld refinements of the room temperature patterns revealed that the unit cell parameter is larger for ZrNiSn and smaller for TiNiSn, in accordance with the respective atomic radii. The site occupation of each atomic species exhibited similar trends to those reported in the literature. The values of the linear coefficient of thermal expansion (α) for all the investigated phases within the alloys are < 10−5 K−1. In addition, the value of the Debye temperature (θD) is estimated by analyzing the temperature dependence of the isotropic atomic displacement parameters. In the case of these half-Heusler alloys, it is observed that a lower θD corresponds to a lower α. The good agreement between the temperature dependence of the Debye temperature and the calculated values using the Thirring-Stern expansion enables us to demonstrate the significant role of anharmonicity: the lighter the compound, the lower the anharmonicity. Furthermore, we also discuss the implications of anharmonicity on the thermal conductivity of this family of half-Heusler compounds at room temperature.

Experimental and computational study of the role of defects and secondary phases on the thermoelectric properties of TiNi1+x Sn (0 ≤ x ≤ 0.12) half Heusler compounds

The half Heusler TiNiSn compound is a model system for understanding the relationship among structural, electronic, microstructural and thermoelectric properties. However, the role of defects that deviate from the ideal crystal structure is far from being fully described. In this work, TiNi1+x Sn alloys (x = 0, 0.03, 0.06, 0.12) were synthesized by arc melting elemental metals and annealed to achieve equilibrium conditions. Experimental values of the Seebeck coefficient and electrical resistivity, obtained from this work and from the literature, scale with the measured carrier concentration, due to different amounts of secondary phases and interstitial nickel. Density functional theory calculations showed that the presence of both interstitial Ni defects and composition conserving defects narrows the band gap with respect to the defect free structure, affecting the transport properties. Accordingly, results of experimental investigations have been explained confirming that interstitial Ni defects, as well as secondary phases, promote a metallic behavior, raising the electrical conductivity and lowering the absolute values of the Seebeck coefficient.

Insight to structural, electronic, optical and thermoelectric propertiesof NaCaSb and KCaSb half Heusler compounds: a DFT approach

Insight to structural, electronic, optical and thermoelectric propertiesof NaCaSb and KCaSb half Heusler compounds: a DFT approach

Insight to the Structural, Electronic, Optical, and Thermoelectric Properties of NaCaSb and KCaSb Half Heusler Compounds: A DFT Approach

Insight to the Structural, Electronic, Optical, and Thermoelectric Properties of NaCaSb and KCaSb Half Heusler Compounds: A DFT Approach

Electronic properties of half-Heusler compounds XCrSb (X = Fe, Ru, Os): Potential applications as spintronics and high-performance thermoelectric materials

Electronic properties of half-Heusler compounds XCrSb (X = Fe, Ru, Os): Potential applications as spintronics and high-performance thermoelectric materials

First-principles research on the thermoelectric properties of NbCoGe based on the scattering mechanisms

ABSTRACT Half-Heusler compounds have excellent power generation performance at high temperatures. The scattering mechanism is an essential factor affecting the electrical transport properties of thermoelectric materials. In computational simulations, only the role of acoustic phonon scattering on the thermoelectric properties of materials is considered, whereas other scattering mechanisms are neglected. In this work, we investigate the thermoelectric properties of NbCoGe compounds with different combinations of acoustic deformation potential, polar optical phonon, and ionised impurity scattering mechanisms. The calculated results show that the ZT values of n-type and p-type NbCoGe compounds reach 8 and 2.8, respectively, when only acoustic deformation potential scattering is considered, indicating that this sole scattering is insufficient. The calculated ZT values of p-type NbCoGe compounds reach 1.8 at 1200 K and more than 1 at 800 K for p- and n-type NbCoGe compounds under the combined effect of the three scattering mechanisms due to their high–power factor. This provides solid theoretical guidance for the search for potentially high–temperature half-Heusler thermoelectric materials.

Spin and current transport in the robust half-metallic magnet c-CoFeGe

Spintronics is an emerging form of electronics based on the electrons’ spin degree of freedom for which materials with robust half-metallic ferromagnet character are very attractive. Here we determine the structural stability, electronic, magnetic, and mechanical properties of the half-Heusler (hH) compound CoFeGe, in particular also in its cubic form. The first-principles calculations suggest that the electronic structure is robust with 100% spin polarization at the Fermi level under hydrostatic pressure and uni-axial strain. Both the longitudinal and Hall current polarization are calculated and the longitudinal current polarization () is found to be and extremely robust under uniform pressure and uni-axial strain. The anomalous Hall conductivity and spin Hall conductivity of hH cubic CoFeGe (c-CoFeGe) are found to be S cm−1 and S cm−1, respectively. Moreover, the Curie temperature of the alloy is calculated to be ∼524 K with a 3 magnetic moment. Lastly, the calculated mechanical properties indicate that c-CoFeGe is ductile and mechanically stable with a bulk modulus of ≈154 GPa. Overall, this analysis reveals that cubic CoFeGe is a robust half-metallic ferromagnet and an interesting material for spintronic applications.

First-principles studies on the structural, electronic and thermal transport characteristics of half-Heusler compounds LiXN (X=Mg, Zn)

The electronic structures and thermal transport properties of the half-Heusler (HH) compounds LiMgN and LiZnN are calculated in detail by combining first-principles calculations with Boltzmann transport theory. Our phonon calculations showed that LiMgN and LiZnN are dynamic stability. The calculated bulk moduli and equilibrium lattice constants are in good agreement with the experimental and available theoretical data. The electronic band structures obtained by PBE (HSE06) method indicate that both LiMgN and LiZnN are direct bandgap semiconductors with band gaps of 2.33 eV (3.23 eV) and 0.53 eV (1.42 eV), respectively. It is shown that the Li–N，Li–Mg and Li–Zn are ionic bond via ELF analysis. At room temperature, the calculated lattice thermal conductivities of LiMgN and LiZnN compounds are 19.31 Wm−1K−1 and 18.74 Wm−1K−1, respectively; But with the external temperature increases to 1100 K, their lattice thermal conductivities are decrease rapidly to 4.96 Wm−1K−1 and 4.32 Wm−1K−1, respectively. Moreover, we analyzed the mean free paths, phase volume spaces (P3), Gruneisen parameters (γ), scattering rates and group velocities in detail in order to further understand the origins of their relatively low lattice thermal conductivities and corresponding microscopic mechanism. We also investigated thermoelectric performances of LiMgN and LiZnN under different temperatures, and our results showed that with the external temperature increasing from 300 K to 1100 K, the maximum ZT values can increase from 0.05 (0.03) and 1.83 (1.32) for LiMgN (LiZnN), indicating that they are potential high-temperature thermoelectric materials. Our work can offer some theoretical references for improving thermoelectric performance and for future experimental studies.

Mechanical Stability, Electronic, and Magnetic Properties of XZrAs (X = Cr, Mn, V) Half-Heusler Compounds

Mechanical Stability, Electronic, and Magnetic Properties of XZrAs (X = Cr, Mn, V) Half-Heusler Compounds