half-Heusler Compounds Research Articles

On electrical transport and thermoelectric performance in Half-Heusler phase ScNiSb

Half-Heusler (HH) phases are among the most extensively studied thermoelectric materials. Bipolar thermal conductivity analysis performed for their sub-group based on rare-earth (R) metals, RNiSb, indicated high mobility ratio in favor of electrons. The result motivated studies of ScNiSb[Formula: see text]Tex series in which n-type-doped samples shown higher mobility and significantly improved thermoelectric performance [K. Ciesielski et al., Phys. Rev. Appl. 15, 044047 (2021)]. Recently, Kajikawa attempted alternative interpretation of transport properties in ScNiSb, where multi-parameter fit led to mobility ratio in favor of holes [Y. Kajikawa, Int. J. Mod. Phys. B 2250071 (2022)]. In this work we discuss the details of electrical properties and thermoelectric performance of ScNiSb in the context of structural disorder and the most recent experimental literature on a related chemical system. The paper considers also relevant assumptions regarding band degeneracy and scattering mechanisms for effective mass modeling in ScNiSb. Last, technical difficulties of the model proposed by Kajikawa are addressed. We believe that the provided insight will be useful for understating electrical transport in HH compounds, which can contribute to further improvement of their thermoelectric performance.

First-principles calculations to investigate optical and electrical properties of the half-Heusler materials TiXSn (X = Ni, Pt)

ABSTRACT Thermoelectric compounds have the potential to produce electricity from waste heat, these materials are environmentally-friendly, which can play a key role on sustainable energy solution. This paper reports an investigation of electronic, optical and thermoelectric properties of two Half Heusler compounds TiNiSn and TiPtSn by employing density functional theory (DFT). Both compounds i.e. TiNiSn and TiPtSn exhibit an overestimated band gap with respective values 0.40 and 0.95 eV obtained by Tran Blaha modified Becke–Johnson functional (TB-mBJ) approximation, while, we found 0.113 and 0.84 eV within GGA + U (U = 6.13 eV) approximation, which are around other theoretical and experimental results. A detailed investigation of the obtained optical properties particularly, dielectric function, absorption, refractive index, and reflectivity of these compounds are given and discussed. Additionally, the transport properties of these compounds have been examined within Boltzmann statistics through BoltzTrap code, as well as certain thermoelectric parameters such as electrical conductivity, Seebeck coefficient and figure of merits are also determined. The obtained results reveal that the studied compounds are quite promising materials for thermoelectric applications.

The role of interstitial Cu on thermoelectric properties of ZrNiSn half-Heusler compounds

The density functional theory (DFT) calculations and experiments have confirmed that in the ABC-type half-Heusler compounds, the 3d elements occupying the B position are natural over-stoichiometry. These additional atoms are able to synergistically optimize the electrical and thermal transport properties of half-Heusler compounds. In this work, Cu (3d104s1) is intentionally introduced into the ZrNiSn compound to form Cu interstitial defects. The correlations between the phase structure, microstructure, and thermoelectric properties of ZrNiCuxSn (x = 0–0.20) are investigated with X-ray and neutron diffraction, transmission electron microscopy, atom probe tomography, and band structure and phonon spectra calculations. The diffraction results reveal that Ni/Cu atoms partially occupy the 4d position (3/4, 3/4, 3/4) of the half-Heusler crystal structure, forming interstitial defects. The interstitial Cu defects force the conduction band minimum to gradually move close to the valence band maximum and reduce the bandgap, rather than induce in-gap states as typical Ni interstitials. Besides the interstitial defects, a full-Heusler phase is also formed in the half-Heusler matrix with increasing Cu content. Due to the interstitial defects and interface engineering, the thermal conductivity is suppressed. As a result, a higher figure of merit (ZT) value is achieved (∼1.1 at 950 K) in the ZrNiCu0.05Sn sample. This work analyses the possibility of interstitial defects from a thermodynamic point of view and highlights the defect engineering to positively tune the thermoelectric properties in half-Heusler compounds.

Structural parameters, electronic structure and linear optical functions of LuXCo2Sb2 (X = V, Nb and Ta) double half Heusler alloys

Ab initio calculations based on density functional theory were performed to determine the structural parameters of the LuXCo2Sb2 (X = V, Nb and Ta) double half Heusler compounds and verify their thermodynamic stability in an orthorhombic structure as well as to predict their electronic and optical properties. The predicted electronic band structures show that LuVCo2Sb2, LuNbCo2Sb2, and LuTaCo2Sb2 are semiconductors with indirect bandgaps of 0.728, 0.845, and 0.898 eV, respectively. The effective masses calculated at the valence band maximum at the conduction band minimum show a strong anisotropy. It can be seen that electrons are lighter than holes. The nature of the electronic states involved in the formation of the energy bands has been determined thanks to the density of states calculations. The character of chemical bonds was analyzed through the charge density distribution map. Linear optical functions, namely complex dielectric function, absorption coefficient, optical reflectivity and refractive index, were calculated for all compounds in an energy range up to 20 eV. The calculated optical spectra exhibit a noticeable anisotropy. The compounds under consideration are characterized by strong absorption of incident electromagnetic radiation in a wide energy range.

First-Principle Calculations to Investigate the Elastic, Thermoelectric, and Electronic Performances of XRhSn (X = V, Nb, Ta) Half-Heusler Compounds

First-Principle Calculations to Investigate the Elastic, Thermoelectric, and Electronic Performances of XRhSn (X = V, Nb, Ta) Half-Heusler Compounds

Coupling of electronic transport and defect engineering substantially enhances the thermoelectric performance of p-type TiCoSb HH alloy

Further advancements in thermoelectric technology rely on the capacity to control both electrical and thermal transport properties simultaneously. Although TiCoSb-based half-Heusler compounds are promising for mid-range-temperature thermoelectric applications owing to their high Seebeck coefficient and good electrical conductivity, their high thermal conductivity has been so far the main issue to overcome. Here, we show that a combined approach of tuning the electronic properties and defect engineering enhances the thermoelectric performance of p-type TiCoSb-based compounds. By alloying on the Co and Ti sites with Fe and Zr, respectively, an overall increase in the peak ZT value of up to ∼90% at 823 K is achieved in Ti0.8Zr0.2Co0.85Fe0.15Sb. This enhancement is directly tied to the more pronounced metallic nature of transport upon Fe alloying combined with a significant reduction in thermal conductivity due to mass and strain field fluctuations driven by the substitution of Zr for Ti, as evidenced by the Debye-Callaway model. Further adjusting the hole concentration with aliovalent Sn doping leads to an additional increase in ZT, eventually leading to a peak value of ∼0.54 at 823 K in Ti0.8Zr0.2Co0.85Fe0.15Sb0.96Sn0.04, which is 224% higher than in TiCo0.85Fe0.15Sb, and the highest value reported so far in Hf-free p-type TiCoSb based HH alloys.

Tunable surface magnetism by gate voltage in a slab of nonmagnetic half-Heusler compound CoTiSb.

The electrical manipulation of magnetization is appealing to the relevant experiment and spintronic device. In this paper, we focus on the electrical and magnetic properties of a thin film cleaved from the nonmagnetic half-Heusler compound CoTiSb. By means of the first-principles calculations, we find that the surface of TiSb termination possesses ferrimagnetism with a magnetic moment of 0.35 (0.49) μB per unit cell without (with) Hubbard U, which can persist below the Curie temperature of 48 (54) K. What is more, such a surface magnetism can be tuned to nonmagnetism by gate-induced hole doping with a concentration of 2.83 × 1014 (3.55 × 1014) cm-2. This magnetic tunability of the CoTiSb slab provides a platform to realize the pseudo-spin valve with both the magnetic electrodes and nonmagnetic space layer made of the same material without hetero-interfaces.

Molecular beam epitaxy of a half-Heusler topological superconductor candidate YPtBi

The search for topological superconductivity has motivated investigations into materials that combine topological and superconducting properties. The half-Heusler compound YPtBi appears to be such a material; however, experiments have thus far been limited to bulk single crystals, drastically limiting the scope of available experiments. This has made it impossible to investigate the potential topological nature of the superconductivity in this material. Experiments to access details about the superconducting state require sophisticated lithographic structures, typically based on thin films. Here we report on the establishment of high-crystalline-quality epitaxial thin films of YPtBi(111), grown using molecular beam epitaxy on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$(0001) substrates. A robust superconducting state is observed, with both critical temperature and critical field consistent with that previously reported for bulk crystals. Moreover, we find that $\mathrm{Al}{\mathrm{O}}_{x}$ capping sufficiently protects the sample surface from degradation to allow for proper lithography. Our results pave a path towards the development of advanced lithographic structures, which will allow the exploration of the potentially topological nature of superconductivity in YPtBi.

Electronegativity and magnetic correlation in Cr-, Mn-, Co- and Ni-doped LiMgP half-Heusler as promised ferromagnetism material: A DFT study

This work reports first-principle calculations for LiMgP half-Heusler compound doped by the transition metal elements Cr, Mn, Co and Ni motivated by present findings, in which the ferromagnetism conduct is predicted. The studied LiMg[Formula: see text]P alloy ([Formula: see text], Mn, Co and Ni) showed the ferromagnetic behavior. The calculations revealed that the main contributions to the net magnetization come from Cr, Mn, Co and Ni. The Cr[Formula: see text] will have four electrons, in which 2 electrons are in [Formula: see text] and other 2 occupy the [Formula: see text]. Then, this orbital is set on the Fermi level. For LiMg[Formula: see text]Co[Formula: see text]P alloy, the half-metallic conduct is estimated with 100% polarized on the downside of the Fermi level. Also, LiMg[Formula: see text]Ni[Formula: see text]P alloy exhibits the half-metallic conduct on the downside of the Fermi level which is occupied by [Formula: see text] minority state. This study stated that electronegativity and magnetic properties have correlation with regard to Cr-, Mn-, Co- and Ni-doped LiMgP, in which the trends of partial moments, electronegativity and total moments are [Formula: see text]; [Formula: see text] and [Formula: see text].

Revealing the decisive factors of the lattice thermal conductivity reduction by electron-phonon interactions in half-Heusler semiconductors

The role of electron-phonon (EP) scattering in lattice thermal conductivity (κL) of thermoelectric material has not been fully studied until now, especially for the decisive factors that influence the reduction of κL. To address this issue, we report the effect of EP interactions on κL at a series of temperatures and carrier concentrations for 18 half-Heusler compounds. Among all the compounds investigated, the hole-doped TiCoSb and the electron-doped ZrIrSb have the largest κL reductions (32% & 20%) by EP interactions at 300 K, under the carrier concentration of 1021 cm−3. Detailed analyses reveal that the system with strong EP coupling strength and high electronic density of states at the Fermi level (N (EF)) favor the κL reduction, because these two factors are beneficial for EP scattering rates. And a high N (EF) can be caused by a high carrier concentration and/or a large effective mass. Temperature is another factor that affects the reduction of κL by EP interactions due to its imbalance influence on EP and phonon-phonon (PP) scatterings. Furthermore, after considering the influences from the aliovalent doping and grain boundary, the EP interactions still play a non-negligible role on κL reduction, especially at low temperatures and high carrier concentrations. Our work provides a complete picture for understanding the mechanism of EP interactions in material's thermal transport.

High-temperature oxidation mechanism of ZrCoSb-based half-Heusler thermoelectric compounds

ZrCoSb-based half-Heusler (HH) compounds are among the most promising thermoelectric (TE) materials for high-temperature power generation. Oxidation resistance is one of the key issues for realizing the practical application of TE materials for long-term service in the ambient working environment. In this work, the oxidation behavior of Zr0.5Hf0.5CoSb0.8Sn0.2 (ZHCSS) half-Heusler is systematically studied in the service temperature range from 873 to 1073 K. It is revealed that three typical layers of oxidation products tend to form on the surface of HH sample, namely, the dense oxide layer (DOL) composed of (Zr,Hf)O2 and CoSb, the alternate oxide layer (AOL) composed of repeated (Zr,Hf)O2 and CoSb2O6/Co3O4, and the CoSb layer between the DOL and AOL. The mass gain during oxidation is mainly caused by the rapid growth of AOL, which is controlled by the outward diffusion of Zr/Hf and the inward diffusion of oxygen. The formation of a continuous CoSb layer and DOL is found beneficial to block the outward diffusion of Zr/Hf. Based on the analysis of the dominant factors on the outward and inward diffusions as well as the reaction activation energy, a simple approach is proposed to improve the oxidation resistance of Zr0.5Hf0.5CoSb0.8Sn0.2 by pre-oxidizing the sample in low oxygen pressure to form the dense (Zr,Hf)O2 and CoSb layers as oxidation protecting and/or diffusion blocking layers. The oxidation test shows the effectiveness of such pre-oxidation on the formation and growth of the AOL and therefore on improving the service stability of Zr0.5Hf0.5CoSb0.8Sn0.2 at high temperatures in the air.

Topological properties of half Heusler compounds: Tuned by the spin-orbit coupling and p-d hybridization

The topological properties of Half Heusler compounds XYZ (X=Na, K; Y=Cu, Ag, Au; Z=S, Se, Te) are studied by the density functional theory (DFT) and tight-binding model Hamiltonian. The Y-s, Y-d and Z-p states form the bands around the Fermi level and determine their topological properties. When the energy of Y-s state is much larger than the other states, the compounds are normal insulators, just like NaCuS and NaCuS. On the contrary, when the Y-s state energy is only a little higher than the other states, the compounds are topological insulators (TI) or Dirac semi-metal (DSM) depended on the effective spin orbital coupling (SOC), which can be tuned by the relation between positive SOC Z-p and Y-d states. The compounds with negative effective SOC are TI such as NaAgS, NaAuS, while the compounds possessing positive SOC are DSM, just like NaAgTe and NaAuTe. Our simple tight-binding model can be used to design topological materials before the time consuming DFT calculations.

Improved Thermoelectric Performance of NbCoSb with Intrinsic Nb Vacancies and Ni-Doping-Induced Band Degeneracy

Lately, the nominal 19-electron half-Heusler compound NbCoSb, historically viewed as a metal, has attracted reacquaintance and widespread attention because of its unexpected high thermoelectric (TE) performance. Here, the electronic structures of NbxCo1–yNiySb (x = 0.8, 1; y = 0, 0.1) have been systematically investigated by using the first-principles method and semiclassical Boltzmann transport theory. We demonstrate that Ni doping at Co sites in NbCoSb with 20% intrinsic Nb vacancies (Nb0.8CoSb) leads to a large band degeneracy, and the conduction band edge are mainly provided by the d-orbitals of the Ni, Co, and Nb atoms. The relative localization of the d orbitals not only remarkably increased the density-of-states effective mass near the Fermi energy but also retained a high mobility, which resulted in an optimal electrical conductivity without significant reduction of the Seebeck coefficient (∼−411.08 μV K –1 at 800 K) and thereby a large power factor. Meanwhile, Ni doping still preserved a low lattice thermal conductivity. As a result, a peak zT of 1.18 was achieved at 1100 K in the compound Nb0.8Co0.9Ni0.1Sb with a carrier concentration of 4.35 × 1020 cm–3. The present work identifies that Ni doping is an effective method to improve the TE properties of nominally 19-electron NbCoSb compounds with Nb vacancies and demonstrates the great potential for searching of nonstoichiometric 19-electron half-Heusler compounds with vacancies.

Investigation of the electronic structure, mechanical, and thermoelectric properties of novel semiconductor compounds: XYTe (X = Ti/Sc; Y = Fe/Co).

We report the structural, mechanical, electronic, phonon, and thermoelectric properties of new XYTe (X= Ti/Sc; Y = Fe/Co) half-Heusler compounds by employing first principles based DFT computation and Boltzmann transport equations. At their equilibrium lattice constants, these alloys exhibit a crystal structure with a space group (#216) of F4̄3m and adhere to the Slater Pauling (SP) rule, while being non-magnetic semiconductors. The Pugh's ratio of TiFeTe shows that it is a ductile material, which makes it suitable for use in thermoelectric applications. On the other hand, ScCoTe's brittleness or fragility makes it less promising as a potential thermoelectric material. The dynamical stability of the system is investigated using the phonon dispersion curves obtained from its lattice vibrations. The band gaps of TiFeTe and ScCoTe are 0.93 eV and 0.88 eV, respectively. The electrical conductivity (σ), Seebeck coefficient (S), thermoelectric power factor (PF), and electronic thermal conductivity are calculated at various temperatures ranging from 300 K to 1200 K. At 300 K, TiFeTe has a Seebeck coefficient of 1.9 mV K-1 and a power factor of 136.1 mW m-1 K-2. The highest S value for this material is obtained through n-type doping. The optimal carrier concentration for achieving the highest Seebeck coefficient in TiFeTe is 0.2 × 1020 cm-3. Our study indicates that the XYTe Heusler compounds exhibit n-type semiconductor behavior.

Thermoelectric transport properties of single-crystalline ZrCoBi half-Heusler

Half-Heusler compounds are one of most promising thermoelectric materials for power generation at high temperatures. Recent studies focus on fine-grained polycrystalline samples because of their lower thermal conductivity, and the induced defects are found to play an important role in the thermoelectric transport properties. Here, we report the thermoelectric transport properties of single-crystalline ZrCoBi. Two samples from different batches clarify the same charge carrier concentration of ~1020 cm-3, denoting the robust Fermi level position in the ZrCoBi single crystals. The high electron density is attributed to the Zr interstitial point defects. Moreover, a high power factor of over 3.3 mWm-1K-2 is achieved in the single-crystalline ZrCoBi. By comparing the thermoelectric properties of single-crystalline and fine-grained polycrystalline samples, we reveal the role of grain boundary scattering in reducing the thermal conductivity from ~11.5 Wm-1K-1 to ~9 W/mK at 300 K. The present work declares the significance of defects in tuning the transport properties of ZrCoBi half-Heusler compound.

First-principle computation of some physical properties of half-Heusler compounds for possible thermoelectric applications.

Using the density functional theory (DFT) method, we investigate the properties of LaXSi (X = Pt, Pd) half-Heusler compounds. To ensure the stability of both compounds, we employed two criteria: the Birch-Murnaghan equation of state and the negative formation energy. The evaluation of elastic constants (ECs) plays a crucial role in determining the mechanical stability of both compounds. Specifically, we ensure that the conditions C11 - C12 > 0, C11 > 0, C11 + 2C12 > 0, and B > 0 are satisfied and exhibit mechanical anisotropy and ductility. The analysis of electronic properties clearly indicates that LaPtSi displays metallic behavior in both the spin-up and spin-down states. In the spin-up state of LaPdSi, a band gap is observed, which indicates its characteristic of being a half-metal. A comprehensive investigation of optical properties revealed that these compounds display notable absorption and optical conductivity at higher energy levels. Conversely, they exhibit transparency to incident photons at lower energy levels. Based on the findings, it can be concluded that these compounds are highly suitable for application in high-frequency UV devices. The thermoelectric properties clearly indicate that both materials exhibit high power factors, electrical conductivity, and figures of merit (ZT), suggesting their potential as exceptional thermoelectric materials. The simulations conducted in this study consider the effect of on-site Coulomb interactions by incorporating the Hubbard U term within the GGA + U. Our findings contribute valuable insights that can facilitate further experimental investigations and provide comprehensive validation.

Exploring structural, mechanical, and thermoelectric properties of half-Heusler compounds RhBiX (X = Ti, Zr, Hf): A first-principles investigation.

In this study, the full potential linearization enhanced plane wave method in density functional theory is used. Additionally, the structure, mechanical, and thermoelectric properties of half-Heusler compounds RhBiX (X = Ti, Zr, Hf) are investigated for the first time. The indirect semiconductors RhBiTi and RhBiZr have 0.89 and 1.06 eV bandgap energies, respectively. In contrast, RhBiHf is a direct bandgap semiconductor with a bandgap energy of 0.33 eV. The thermoelectric parameters such as Seebeck coefficient, power factor, electronic conductivity, lattice thermal conductivity, electronic thermal conductivity, and figure of merit ZT, are studied with the semi-classical Boltzmann transport theory. When T = 300 K, RhBiTi, RhBiZr, and RhBiHf show small lattice thermal conductivities, i.e., 10.60, 10.15, and 7.71 W mK-1, respectively, which are consistent with related other studies. The maximum ZT values of RhBiTi, RhBiZr, and RhBiXHf are 0.91, 0.94, and 0.79 at 900 K, respectively. Furthermore, we observed that RhBiX (X = Ti, Zr, Hf) alloy is a thermoelectric material with great potential.

Simultaneous optimization of the electrical and thermal transport properties of LuNiSb via aliovalent doping

With the introduction of Sn or Co into half-Heusler compound LuNiSb, it is discovered that aliovalent doping facilitates the simultaneous optimization of electrical and thermal transport properties.

Ultralow lattice thermal conductivity and improved thermoelectric performance in a Hf-free half-Heusler compound modulated by entropy engineering

A great improvement in the thermoelectric and mechanical properties of a TiNiSn-based compound has been achieved by entropy engineering.

Correlation of rattlers with thermal transport and thermoelectric performance.

The presence of rattlers in the host-guest structure has sparked great interest in the field of thermoelectrics, as it allows for the suppression of thermal transport in materials through vigorous anharmonic vibrations. This work predicts a ternary half-Heusler compound, LiAgTe, with good thermoelectric properties and high-temperature stability, which possesses a host-guest structure. Furthermore, it provides a detailed analysis of the role of rattlers in the transport process. By microscopically exploring rattlers, we have revealed that rattlers (Ag atoms), while suppressing the thermal transport properties of the host framework, provide a significant enhancement of the electronic transport capability through the provision of nearly free weakly bound electrons. Using self-consistent phonon theory combined with compressive sensing lattice dynamics method, we captured the exact lattice thermal conductivity considering quartic anharmonicity and four-phonon scattering, and obtained the electronic transport parameters through the calculation of τe, which includes full anisotropic acoustic deformation potential scattering, polar optical phonon scattering, and ionized impurity scattering. We systematically dissected the role of rattlers in the host-guest structure by combining methods such as electron local function, Bader charge density, and Vibration visualization. The anharmonic vibrations of rattlers enhance the temperature response of scattering, resulting in rapid deterioration of thermal transport at high temperatures. Moreover, the extended d-orbital electrons of the rattlers, together with the p-orbital electrons of the Te atom in the host framework, result in the coexistence of maximum degeneracy and high dispersion bands in the valence band, which greatly enhances the electronic transport properties.