half-Heusler Family Research Articles

Vacancy-mediated anomalous phononic and electronic transport in defective half-Heusler ZrNiBi

Studies of vacancy-mediated anomalous transport properties have flourished in diverse fields since these properties endow solid materials with fascinating photoelectric, ferroelectric, and spin-electric behaviors. Although phononic and electronic transport underpin the physical origin of thermoelectrics, vacancy has only played a stereotyped role as a scattering center. Here we reveal the multifunctionality of vacancy in tailoring the transport properties of an emerging thermoelectric material, defective n-type ZrNiBi. The phonon kinetic process is mediated in both propagating velocity and relaxation time: vacancy-induced local soft bonds lower the phonon velocity while acoustic-optical phonon coupling, anisotropic vibrations, and point-defect scattering induced by vacancy shorten the relaxation time. Consequently, defective ZrNiBi exhibits the lowest lattice thermal conductivity among the half-Heusler family. In addition, a vacancy-induced flat band features prominently in its electronic band structure, which is not only desirable for electron-sufficient thermoelectric materials but also interesting for driving other novel physical phenomena. Finally, better thermoelectric performance is established in a ZrNiBi-based compound. Our findings not only demonstrate a promising thermoelectric material but also promote the fascinating vacancy-mediated anomalous transport properties for multidisciplinary explorations.

MgPdSb─An Electron-Deficient Half-Heusler Phase

The half-Heusler family consists of many semiconducting intermetallic compounds, virtually all of them having a valence electron count (VEC) of 18. We have studied an electron-deficient (VEC = 17) phase MgPdSb and its Pd-stuffed variant MgPd1.25Sb. The cubic F4̅3m crystal structure was confirmed by the Rietveld refinement of powder X-ray diffraction (XRD) data. The lattice parameter is a = 6.284 and 6.335 Å for MgPdSb and MgPd1.25Sb, respectively. The Debye temperature and Sommerfeld coefficient for MgPdSb are ΘD = 282 K and γ = 3.3 mJ mol–1 K–2, respectively, and are similar to those obtained for MgPd1.25Sb. There is neither phase transition nor superconductivity observed above 1.8 K. The differences between the electronic structures of Mg-based half-Heusler compounds make them robustly metallic, irrespective of the electron count and the introduction of interstitial transition metal (Pd) atoms.

First principle based investigation of topological insulating phase in half-Heusler family NaYO (Y = Ag, Au, and Cu)

The ternary half-Heusler compounds have shown great potential for realizing new 3D topological insulators. With band gap tuning and spin orbit coupling these compounds may undergo topological phase transitions. In present work, we explore the possibility of realizing a topological insulating phase in half-Heusler family NaYO (Y = Ag, Au, and Cu). We find that for NaAgO, external strain (∼19%) along with spin–orbit coupling (SOC), is required to achieve band-inversion at Γ high-symmetry point and leads to phase transition from trivial to non-trivial topological insulating phase. In case of NaAuO and NaCuO, non-trivial phase appears in their equilibrium lattice constant, hence only SOC is enough to achieve band-inversion leading to non-trivial topology. The non-centrosymmetric nature of crystal geometry leads to the formation of two twofold degenerate point nodes near the Fermi level.

A Systematic Approach for Semiconductor Half-Heusler

The key to designing a half-Heusler begins from the understanding of atomic interactions within the compound. However, this pool of knowledge in half-Heusler compounds is briefly segregated in many papers for specific explanations. The nature of the chemical bonding has been systematically explored for the large transition-metal branch of the half-Heusler family using density-of-states, charge-density, charge transfer, electron-localization-function, and crystal-orbital-Hamilton-population plots. This review aims to simplify the study of a conventional 18-electron configuration half-Heusler by applying rules proposed by renowned scientists to explain concepts such as Zintl-Klemm, hybridization, and valence electron content (VEC). Atomic and molecular orbital diagrams illustrate the electron orbital transitions and provide clarity to the semiconducting behavior (VEC = 18) of half-Heusler. Eighteen-electron half-Heusler usually exhibits good thermoelectric properties owing to favorable electronic structures such as narrow bandgap (<1.1 eV), thermal stability, and robust mechanical properties. The insights derived from this review can be used to design high-performance half-Heusler thermoelectrics.

Screening potential topological insulators in half-Heusler compounds via compressed-sensing

Ternary half-Heusler compounds with widely tunable electronic structures, present a new platform to discover topological insulators (TIs). Due to time-consuming computations and synthesis procedures, the identification of new TIs is however a rough task. Here, we adopt a compressed-sensing approach to rapidly screen potential TIs in half-Heusler family, which is realized via a two-dimensional descriptor that only depends on the fundamental properties of the constituent atoms. Beyond the finite training data, the proposed descriptor is employed to screen many new half-Heusler compounds, including those with integer and fractional stoichiometry, and a larger number of possible TIs are predicted.

Band structure of topological insulator Li(Na)AuS

Half-Heusler semiconductors exhibit similar properties: the differences among their properties lie only in the fact that in ternary compositions the zinc-blende binary substructure does not provide the required 18 electrons, but this is improved by adding an extra transition metal, which restores the electronic balance. Half-Heusler ternary compound with 18 valence electrons under an appropriate uniaxial strain is a topological insulating phase. Most importantly, it is proposed that in the half-Heusler family, the topological insulator should allow the incorporating of superconductivity and magnetism. Using the first-principle full-potential linearized augmented wave method we study the band structure of a series of Li(Na)AuS topological insulators. The electronic and magnetic properties of Heusler alloys are investigated by the WIEN2k package. The exchange-correlations are treated within the generalized gradient approximation of PerdeweBurke and Ernzerhof (GGA), the local spin density approximation (LSDA), by using the modified Becke-Johnson exchange potential and the correlation potential of the local-density approximation (MBJ). Spin-orbit coupling is treated by means of the second variational procedure with the scalar-relativistic calculation as basis. We first determine the equilibrium lattice constants by calculating the total energy. The theoretical lattice constant of LiAuS full-potential GGA is 6.02 Å, which is somewhat greater than the result of pseudopotential(5.99 Å). The calculated equilibrium lattice parameter is 5.86 Å for LSDA. Most of the half-Heusler compounds have band inversion, and open the nature band gaps, but the gap of MBJ is not very good. Smaller uniaxial stress damages the cubic structure and also such a natural band gap of topological insulators. By applying uniaxial tensile stress until the equilibrium position is reached in all directions of the structure, the system band gap value is about 0.2 eV, which is consistent with the result obtained from the band gap of cubic structure equilibrium position. When uniaxial tensile stress is 41%, the system turns into a tetragonal structure, the equilibrium lattice constant is <i>a </i>= 5.2477 Å and <i>c</i>/<i>a</i> = 1.41. We use the method of substitution of homologous elements to ensure the properties of topological insulator of materials without changing the cubic structure, and open the bandgap of materials under the equilibrium lattice constant of the system, thereby improving the feasibility of experimental synthesis of topological insulator materials. Our results for the doping suggest that epitaxial strain encountered during experiment can result in electronic topological transition. We hope that the results presented here conduce to further experimental investigation of the electronic topological transition in half-Heusler compounds.

Rare-earth based half-Heusler topological quantum materials: A perspective

Topological insulator (TI) materials which are conductive at the surface but insulating in the bulk have drawn much attention in the past decade due to their fascinating properties and potential application in the field of spintronics, quantum computing, topological superconductivity and next generation electronics. In the search of three-dimensional TIs, half-Heusler compounds are the new entrants. Half-Heusler compounds are equiatomic ternary compounds with cubic symmetry. Due to the availability of a large pool of elements in the half Heusler family, the physical properties of these materials can be tuned by choosing the desired combination of elements. In this perspective, we have briefly discussed the development of structural relations, the quantum hall effect, Landau quantization, and topological properties of a few representative systems in the half-Heusler family, including methods by which they are studied and characterized such as Angle Resolved Photoemission Spectroscopy, Shubnikov-de-Hass Oscillations and Nuclear Magnetic Resonance.

Diffusive-like effects and possible non trivial local topology on the half-Heusler YPdBi compound

The non-ambiguous experimental identification of topological states of matter is one of the main interesting problems regarding this new quantum state of matter. In particular, the half-Heusler family RMT (R = rare-earth, T = Pd, Pt or Au and T = Bi, Sb, Pb or Sn) could be a useful platform to explore these states due to their cubic symmetry and the topological properties tunable via their unit cell volume and/or the nuclear charges of the M and T atoms. In this work, we report electron spin resonance (ESR) and complementary macroscopic measurements in the Nd3 + -doped putative topologically trivial semimetal YPdBi. Following the Nd3 + ESR lineshape as a function of microwave power, size of the particle and temperature, we have been able to observe an evolution from a Dysonian lineshape to a diffusive-like lineshape. Furthermore, the Nd3 + ESR intensity saturation is concentration dependent, which could be due to a phonon-bottleneck process. Comparing these results with the Nd3 + -doped YPtBi, we discuss a possible scenario in which the Nd3 + ions could locally tune the topological properties of the system.

Interpreting the Combustion Process for High-Performance ZrNiSn Thermoelectric Materials.

The ZrNiSn alloy, a member of the half-Heusler family of thermoelectric materials, shows great potential for mid-to-high-temperature power generation applications due to its excellent thermoelectric properties, robust mechanical properties, and good thermal stability. The existing synthesis processes of half-Heusler alloys are, however, rather time and energy intensive. In this study, single-phase ZrNiSn bulk materials were prepared by self-propagating high-temperature synthesis (SHS) combined with spark plasma sintering (SPS) for the first time. The analysis of thermodynamic and kinetic processes shows that the SHS reaction in the ternary ZrNiSn alloy is different from the more usual binary systems. It consists of a series of SHS reactions and mass transfers triggered by the SHS fusion of the binary Ni-Sn system that eventually culminates in the formation of single-phase ternary ZrNiSn in a very short time, which reduced the synthesis period from few days to less than an hour. Moreover, the nonequilibrium feature induces Ni interstitials in the structure, which simultaneously enhances the electrical conductivity and decreases the thermal conductivity, which is favorable for thermoelectric properties. The maximum thermoelectric figure of merit ZT of the SHS + SPS-processed ZrNiSn1-xSbx alloy reached 0.7 at 870 K. This study opens a new avenue for the fast and low-cost fabrication of half-Heusler thermoelectric materials.

Magnetic structures of REPdBi half-Heusler bismuthides (RE = Gd, Tb, Dy, Ho, Er)

We present results of neutron diffraction on single crystals of several equiatomic ternary compounds of rare-earth elements with palladium and bismuth, crystallizing with cubic MgAgAs-type structure (half-Heusler phases). Band structure calculations showed that many members of that family possess electronic band inversion, which may lead to occurrence of topological insulator or topological semimetal. But even for the compounds without intrinsic band inversion another way of topologically non-trivial state realization, through a specific antiferromagnetic order, has been theoretically proposed.Our results show that the antiferromagnetic structures of all studied bismuthides are characterized by the propagation vector, allowing for antiferromagnetic topological insulator state. Therefore, the antiferromagnetic representatives of half-Heusler family are excellent candidates for extended investigations of coexistence of superconductivity, magnetic order and non-trivial topology of electronic states.

Enhancing Thermoelectric Performance of TiNiSn Half-Heusler Compounds via Modulation Doping

MNiSn (M = Ti, Zr, and Hf) half-Heusler (HH) compounds are widely studied n-type thermoelectric (TE) materials for power generation. Most studies focus on Zr- and Hf-based compounds due to their high thermoelectric performance. However, these kinds of compositions are not cost-effective. Herein, the least expensive alloy in this half-Heusler family—TiNiSn—is investigated. Modulation doping of half-metallic MnNiSb in the TiNiSn system is realized by using spark plasma sintering. It is found that MnNiSb dissolves into the TiNiSn matrix and forms a heavily doped Ti1–xMnxNiSn1–xSbx phase, which leads to largely enhanced carrier concentration and also slight increase of carrier mobility. As a result, the electrical conductivity and power factor of the modulation doped compounds are greatly improved. A maximum power factor of 45 × 10–4 W K–2 m–1 is obtained at 750 K for the modulation doping system (TiNiSn)1–x + (MnNiSb)x with x = 0.05, which is one of the highest reported values in literature for TiNiSn systems. Furthermore, the lattice thermal conductivity is also suppressed due to the enhanced phonon scattering. Beneficial from the improved power factor and suppressed lattice thermal conductivity, a peak zT of 0.63 is obtained at 823 K for x = 0.05, which is an ∼70% increase compared to the peak zT of TiNiSn. These results highlight the potential application of inexpensive TiNiSn-based TE materials and the effectiveness of modulation doping in enhancing the TE performance of HH compounds.

Topological insulating characteristic in half-Heusler compounds composed of light elements

The band topology of a new half-Heusler family with 8 valence electrons composed of light elements is investigated by the first-principle calculations. Many of these compounds are predicted to be topological insulators. The new topological insulators exhibit great band inversion strength (up to −2.56 eV). Both the occurrence of the band inversion and the opening topological insulating gap can be performed only by the strain engineering. Based on these results, we also argue that the spin–orbit coupling effect is not an essential factor in the band inversion and the formation of topological insulating gap.

A new class of topological insulators from I-III-IV half-Heusler compounds with strong band inversion strength

In this paper, by first principle calculations, we investigate systematically the band topology of a new half-Heusler family with composition of I(A)-III(A)-IV(A). The results clearly show that many of the I-III-IV half-Heusler compounds are in fact promising to be topological insulator candidates. The characteristic feature of these new topological insulators is the naturally strong band inversion strength (up to −2 eV) without containing heavy elements. Moreover, we found that both the band inversion strength and the bulk insulating gap of the compounds can be tailored through strain engineering, and therefore would be grown epitaxially in the form of thin films, and useful in spintronics and other applications.

Prediction of topological insulating behavior in inverse Heusler compounds from first principles

The topological band structures of the X2YZ inverse Heusler compounds have been investigated by the first-principle calculations. The results of this study clearly indicate that a large number of inverse Heusler compounds naturally exhibit an inverted band structure. We found that, similar to the half-Heusler family, these inverse Heusler compounds can realize the topological insulating state under hydrostatic or uniaxial lattice expansion. Importantly, most of these compounds possess a negative formation energy, which makes them more suitable in material growth and could easily achieve the topological insulating behavior by alloying or proper strain.

Electronic structure and chemical bonding in half-Heusler phases

The electronic structure and chemical bonding in half-Heusler phases have been systematically investigated using first-principles, self-consistent tight-binding linear-muffin-tin-orbital calculations within the atomic-sphere approximation (TB-LMTO-ASA). The density-of-states profiles for the mainstream half-Heusler phases exhibit very similar features originating from the cubic, positional-parameter-free AlLiSi-type structural arrangement and resemblances in composition. The electronic structures of these half-Heusler phases are accordingly very suitable for rigid-band considerations and predictions made on this basis are tested against actually calculated data. The nature of the chemical bonding has been systematically explored for the large transition-metal branch of the half-Heusler family using density-of-states, charge-density, charge-transfer, electron-localization-function, and crystal-orbital-Hamilton-population plots. The study has laid stress on the remarkable consistency in bonding behaviour among the considered intermetallic phases even though the properties range from non-magnetic metals and semiconductors via ferromagnetic metals to half-metallic ferromagnetic metals, brought about by large spread in element combinations and valence-electron content. The typical half-Heusler phase exhibits an appreciable covalent contribution to the bonding independent of the electronic state at the Fermi-level. The empirical rule that the (numerous) half-Heusler phases with valence-electron content of 18 are semiconducting and more stable than those (metallic) with a higher or lower valence-electron content is substantiated.