half-Heusler Structure Research Articles

First principles study of structural and optoelectronic properties of semiconductor NaBeP[formula omitted]N[formula omitted] alloys for photovoltaic applications

In this paper, the structural and optoelectronic properties of semiconductor NaBePxN1−x (x=0,0.25,0.50,0.75 and 1) alloys in the half-Heusler structure were investigated using density functional theory with GGA approximation. The lattice parameter a, the formation energy Ef and the cohesive energy Ec were examined. The obtained results show that the lattice parameter as a function of x for NaBePxN1−x alloys obeys Vegard’s Law. The negative values of Ef and Ec indicate that NaBeN and NaBeP alloys have thermodynamic stability and strong chemical bonds, implying that they could be synthesized in future experimental research. The estimated optical properties revealed details for NaBePxN1−x alloys concerning the optoelectronic and photovoltaic applications. The high absorption α(x=0,1)≈1.6×106cm−1 and reflectivity coefficients (R(x=0,1)>55%), indicate their suitability for usage in optoelectronic devices. The promising power conversion efficiency (η≈15%) also predicts that the thermodynamically stable alloys NaBeN and NaBeP are good for solar cell applications.

Investigation of the structural, mechanical, anisotropic, thermal conductivity, electronic, and phonon properties of RhTiZ(Z: As, sb) half heusler compounds under high pressure: A DFT study

This investigation delves into the effects of pressure on the structural, elastic, thermal conductivity, anisotropy, electronic, and phonon properties of the RhTiZ (Z: As, Sb) compound, characterized by a half-Heusler structure, employing the first-principles method. Parameters such as lattice constant, volume, density, bulk modulus, the first derivative of the bulk modulus, and energy were meticulously scrutinized at zero pressure, drawing comparisons with theoretical and experimental data and existing literature. Throughout all pressure values, RhTiZ (Z: As, Sb) compounds exhibit robust structural stability, meeting the stringent criteria outlined by Born, with no discernible signs of instability even under heightened pressure conditions. The compound showcases ductile characteristics based on the Pugh ratio (G/B), Cauchy pressure (C12–C44, C″), and Poisson ratio (λ) criteria. Employing the VESTA program, anisotropic properties were comprehensively assessed in three dimensions, considering the influence of pressure. In terms of electronic properties, the compound maintains its semiconducting nature, resiliently preserving this trait amidst variations in pressure. The absence of negative frequencies in the phonon distribution curve serves as a testament to the dynamic stability inherent in the compound. Finally, a comprehensive evaluation was conducted to delineate the repercussions of pressure on both the physical and electronic attributes of the RhTiZ (Z: As, Sb) compound, which holds promise for applications in thermal insulation materials.

Comparison of the excellent thermoelectric properties of the half-Heusler compounds RbMgSb with a host-guest structure and LiMgSb

In this work, we systematically studied the thermoelectric (TE) transport properties of the half-Heusler compounds AMgSb (A = Li, Rb) based on an incorporation of first-principles calculations with self-consistent phonon theory and Boltzmann transport equations. We noted significant changes in the vibrational behavior of alkali metal atoms with a substantial increase in atomic mass. This alteration led to the formation of a host-guest structure in RbMgSb. In contrast, the stronger harmonic vibrations in LiMgSb results in it maintaining a Half-Heusler structure, without the pronounced anharmonicity found in RbMgSb. The calculation results reveal that RbMgSb has ultra-low lattice thermal conductivity κL, which is ten times lower than LiMgSb. We show that rattlers cause tenfold reductions in the thermal conductivity by increasing the phonon–phonon scattering probability, occurred in a wide frequency range. In addition, after careful consideration of multiple scattering mechanisms, the multiple valleys, high dispersion, flat band edges and band asymmetry near the Fermi level give both materials excellent electron transport properties. At the same time, LiMgSb itself already has a very low electron thermal conductivity κe, but the κe of RbMgSb is ten times smaller than that. As a consequence, remarkable ZT values of 2.51 and 3.45 at 900 K are captured in n-type LiMgSb and p-type RbMgSb, respectively. These findings reveal the promising potential of AMgSb in TE applications.

Br4F21]- - a unique molecular tetrahedral interhalogen ion containing a μ4-bridging fluorine atom surrounded by BrF5 molecules.

The reaction of [NMe4][BrF6] with an excess of BrF5 leads to the compound [NMe4][Br4F21]·BrF5. It features molecular [(μ4-F)(BrF5)4]- anions of tetrahedron-like shape containing central μ4-bridging F atoms coordinated by four BrF5 molecules. It is the most BrF5-rich fluoridobromate anion by mass. Quantum-chemical calculations showed that the μ4-F-Br bonds within the anion are essentially ionic in nature. The compound is the first example where F atoms bridge μ4-like neither to metal nor to hydrogen atoms. It was characterized by Raman spectroscopy and by single-crystal X-ray diffraction. The latter showed surprisingly that its crystal structure is related to the intermetallic half-Heusler compound and structure type MgAgAs.

Structure and thermoelectric property evolution of TiFe1.1−xNixSb with the filling of Ni

TiFeSb is a multiphase material with the half-Heusler structure as the main phase. In this work, we discover that TiFeSb can be stabilized by filling Ni to the vacant 4d site of the half-Heusler structure, and a single-phase TiFeNi0.1Sb alloy is obtained. To improve the thermoelectric properties, TiFe1.1−xNixSb (x = 0.1–1.0) samples with more valence electrons are designed and synthesized. Structural analysis reveals that the samples with x = 0–0.3 are single-phase alloys with the half-Heusler-like structure, the samples with x = 0.4–0.7 are two-phase materials with the double half-Heusler as the main phase, and the samples with x = 0.8–1.0 are single-phase alloys with the full-Heusler-like structure. Meanwhile, both promising p-type (x = 0–0.3) and n-type (x = 0.8–1.0) electrical properties are realized in TiFe1.1−xNixSb, which can be well explained by the Slater-Pauling rule. In combination with greatly reduced lattice thermal conductivity owing to the disordered crystal structure, the p-type TiFe0.8Ni0.3Sb and n-type TiFe0.3Ni0.8Sb achieve dimensionless thermoelectric figure of merits zT about 0.6 and 0.4 at 973 K, respectively. We also demonstrate that the thermoelectric properties of these samples can be further optimized by conventional doping or alloying methods. By replacing Ni (or Fe) with Cu, the carrier concentration of p-type TiFe0.8Ni0.2Cu0.1Sb is decreased, and the zT value reaches 0.8 at 973 K, which is the record of vacancy-filled Heusler alloys.

Half-Heusler d0-d gapless semiconductors as strong Z2 topological insulators

The discovery of non-trivial topological materials has attracted a lot of attention in the condensed matter physics. In the following we investigate the mechanical stability, electronic, and topological properties of XAuZ half-Heusler compounds (X=K, Cs; Z=S, Se, Te) using the WIEN2K full-potential electronic structure package, Wannier90, and WannierTools code. It is found that except KAuTe the other compounds are metastable in half-Heusler structure. In addition, the structural phase transition under pressure between stable and metastable phase has been investigated. The band structure without spin–orbit coupling (SOC) indicates a band-inversion and direct gapless semiconducting behavior for all compounds. The SOC effect creates a meaningful band gap in the β-phase compounds KAuS, KAuSe, and CsAuS, thus converting them to topological insulators. The Z2 topological quantum numbers of these compounds are found to be (1; 0 0 0). The Z2 numbers and the surface states spectrum indicate that these compounds are strong topological insulators. The application of modest uniaxial strain is shown to open a gap in γ-phase compounds, and turn CsAuSe and CsAuTe into their topological insulating states. Hence, true metastability together with topology of band structure are promising indicators for potential application of the above compounds as topological spintronic materials, which together with earlier findings of spin-gapless behavior in other d0−d half-Heusler alloys goes to show that these compounds are indeed multifaceted materials for spin-based electronics of the future.

Machine learned synthesizability predictions aided by density functional theory

A grand challenge of materials science is predicting synthesis pathways for novel compounds. Data-driven approaches have made significant progress in predicting a compound’s synthesizability; however, some recent attempts ignore phase stability information. Here, we combine thermodynamic stability calculated using density functional theory with composition-based features to train a machine learning model that predicts a material’s synthesizability. Our model predicts the synthesizability of ternary 1:1:1 compositions in the half-Heusler structure, achieving a cross-validated precision of 0.82 and recall of 0.82. Our model shows improvement in predicting non-half-Heuslers compared to a previous study’s model, and identifies 121 synthesizable candidates out of 4141 unreported ternary compositions. More notably, 39 stable compositions are predicted unsynthesizable while 62 unstable compositions are predicted synthesizable; these findings otherwise cannot be made using density functional theory stability alone. This study presents a new approach for accurately predicting synthesizability, and identifies new half-Heuslers for experimental synthesis.

Insights into the Prediction Structural, Electronic, Optic, Elastic, and Phonon Properties of Half-Heusler Compound LiAgSe via Density Functional Theory

The structural, electronic, optic, elastic and dynamic features of LiAgSe half-Heusler structure are studied by using first principle calculations. LiAgSe half-Heusler compound is examined with the Generalized Gradient Approximation using the Density Functional Theory. The Quantum Espresso simulation program is preferred to investigate its structural, electronic and dynamic features. The ABINIT simulation program is preferred to investigate its elastic and optic properties. The electronic band structure graph of the LiAgSe crystal formed as a result of the calculation shows that this crystal has a semi-metallic structure. Optic properties such as, complex dielectric constant, extinction coefficient, reflectivity, for the volume of LiAgSe are calculated and plotted. In this study, elastic constants, Poisson's ratio and Debye Temperature values of LiAgSe half-Heusler crystal are determined. Apart from these, phonon dispersion curve graph is obtained. It has been calculated that the LiAgSe half-Heusler crystal is not dynamically stable in the ground state. However, when applied a pressure under nearly 16.396 GPa the crystal becomes stable.

Formula omitted]–[formula omitted] half-Heusler compounds as a potential class of three-dimensional Chern insulators

The high-Tc three-dimensional (3D) Chern insulators are in much demand for the future spin-polarized, massless and dissipationless current applications. Employing ab initio electronic calculations, a novel class of stoichiometric d0–d Dirac and parabolic half-metals in half-Heusler C1b structure with a good chance of application as quantum anomalous Hall materials is proposed including half-Heusler KMnP, KMnSb, RbMnP, RbMnSb, KMnN, and RbMnN. The robust Γ1–Γ5 band inversion across the Fermi level in the semi-metallic spin channel, which results from the s–d hybridization of Mn states together with the maximum d-shell exchange splitting, gives rise to a non-trivial topology with a Chern number C=1 in all the above compounds. With the intrinsic spin–orbit coupling included, a small gap appears in the semi-metallic spin channel that can be further magnified via uniaxial strain, thus converting the above compounds to genuine 3D Chern insulators. The above theoretical findings in d0–d half-Heusler structures together with the characteristic high Curie temperatures and dynamical stability, open an intriguing pathway to realization of quantum anomalous Hall effect in stoichiometric 3D materials.

Low thermal conductivity and high thermoelectric performance via Cd underbonding in half-Heusler PCdNa

Half-Heusler compounds are a very large class of materials that have received attention as promising high-temperature thermoelectric materials due to their favorable electrical transport behavior and other properties beneficial for device fabrication. A particular challenge is that while half-Heusler compounds show an exceptionally large range of thermal conductivities, these are still generally higher than other state-of-the-art thermoelectric materials. Here, we investigate PCdNa, which was reported to be a material that may form in the half-Heusler structure and have an unusually low thermal conductivity. This is in order to understand its very low thermal conductivity and elucidate its electronic properties in relation to thermoelectricity. We find that the low thermal conductivity of 2.6 W/mK at 300 K is a consequence of underbonding of the Cd ions within the network formed by the P. This underbonding leads to anharmonicity and relatively low frequency phonons and may be regarded as a generalized effective rattling phenomenon. The electronic structure shows a high valence band degeneracy, which we find leads to favorable good electrical transport properties. The combination of the low lattice thermal conductivity and favorable electrical transport properties results in high $p$-type thermoelectric performance. The figure of merit is predicted to reach $ZT=3.3$ at 900 K for optimum $p$-type doping. This suggests that PCdNa is a promising $p$-type high-temperature half-Heusler thermoelectric material.

YARI-HEUSLER NaYSi BİLEŞİĞİNİN BAZI FİZİKSEL ÖZELLİKLERİ : İLK İLKELER ÇALIŞMASI

The structural, electronic, elastic and optic properties of NaYSi formed in the half-Heusler structure are studied by using ab-initio density-functional theory. The calculated equilibrium lattice constants are compaired with the available experimental and theoretical data. The elastic parameters have calculated. Computed elastic results prove that this compound were mechanically stable. The band gap of this compound predicted to be semiconductor. Further the phonon spectra and optical analysis have been obtained for the energy range of 0–40 eV.

Mechanical Stability, Electronic And Magnetic Properties Of Half- Heusler FeCrAs Alloy For Spintronics Application

The search for functional materials in spintronic devices has become a major component of material research in recent times. The structural, elastic, mechanical, electronic and magnetic properties of half-Heusler FeCrAs alloy (HHFCA) have been examined adopting spin-polarized density functional theory calculations. Our result shows that the hexagonal structure is the high pressure phase of the FeCrAs alloy while the half-Heusler structure is the more stable phase at ambient pressure. Also, the HHFCA is mechanically stable and exhibits half-metallic ferromagnetism besides an indirect band gap in the minority spin channel. The total magnetic moment in one formula unit of the alloy is 1.00 μB, in agreement with the Slater-Pauling rule and the bulk of the magnetic moment contributed by the Cr atoms. Furthermore, high Curie temperature of ~ 1000 K has been obtained for the HHFCA which suggests that it is a promising material for spintronic applications.

Enhanced thermoelectric performance of n-type Zr0.66Hf0.34Ni1+xSn Heusler nanocomposites

By exploiting the benefit of the half-Heusler (HH) structure, the nanocomposite strategy has been an effective mechanism for most HH-based thermoelectric materials. We have successfully developed HH-based nanocomposites by economically feasible solid-state reaction (SSR) route followed by Spark Plasma Sintering (SPS). X-ray diffraction (XRD) analysis confirms the single HH phase for Zr0.66Hf0.34NiSn composition, and full-Heusler (FH) as an inclusion phase in HH matrix for Zr0.66Hf0.34Ni1+xSn (x = 0.01, 0.03, 0.05, 0.10) alloys. Energy Dispersive Spectroscopy (EDS) also reveals the nominal stoichiometric compositions to be in close matching with experimental compositions. Thermoelectric measurements have been performed for all the compositions Zr0.66Hf0.34Ni1+xSn (x = 0, 0.01, 0.03, 0.05, 0.10) from 323 to 773 K. Interestingly, increasing FH concentration in the HH matrix (up to Zr0.66Hf0.34Ni1.03Sn) leads to the simultaneous increase in Seebeck coefficient due to optimization of carrier concentration and increase in electrical conductivity due to increased mobility of carriers. Therefore, an optimized power factor of 35.31 μW K−2 cm−1 at 773 K was obtained for Zr0.66Hf0.34Ni1.03Sn alloy. In addition to this, a reduced lattice thermal conductivity of 1.26 Wm−1 K−1 was optimized for Zr0.66Hf0.34Ni1.1Sn composition at 773 K. Thus, enhanced power factor and reduced thermal conductivity have finally resulted in an increased figure of merit of 0.93 at 773 K for the optimized composition Zr0.66Hf0.34Ni1.03Sn, which is nearly 260% more as compared to the pristine HH composition Zr0.66Hf0.34NiSn.

Mechanical Stability, Electronic And Magnetic Properties Of Half- Heusler FeCrAs Alloy For Spintronics Application

The search for functional materials in spintronic devices has become a major component of material research in recent times. The structural, elastic, mechanical, electronic and magnetic properties of half-Heusler FeCrAs alloy (HHFCA) have been examined adopting spin-polarized density functional theory calculations. Our result shows that the hexagonal structure is the high pressure phase of the FeCrAs alloy while the half-Heusler structure is the more stable phase at ambient pressure. Also, the HHFCA is mechanically stable and exhibits half-metallic ferromagnetism besides an indirect band gap in the minority spin channel. The total magnetic moment in one formula unit of the alloy is 1.00 μB, in agreement with the Slater-Pauling rule and the bulk of the magnetic moment contributed by the Cr atoms. Furthermore, high Curie temperature of ~ 1000 K has been obtained for the HHFCA which suggests that it is a promising material for spintronic applications.

The investigation of the half-metallic properties of half-Heusler KXP (X = Cr & Mo) compounds: A first-principles study

In this article, the structural phase stability of the KXP (X = Cr & Mo) compounds in the half-Heusler structure were investigated. The stability of the KXP compound was taken into account in the three phases α, β, and γ of the half-Heusler structure within various ferromagnetic (FM), antiferromagnetic (AFM), and nonmagnetic (NM) states. Total energy calculations, as well as cohesive energy calculations, indicated that for each KXP compound the β phase of the half-Heusler structure in the FM state has the lowest energy compared to the other phases α and γ. For all structural phases of the KMoP compound, the positive amount of the formation energy predicts that the compound cannot be grown in the hypothetical half-Heusler structure and prefers to decompose into stable phases of the constituent elements. The hull distance calculations justify that, unlike KMoP, the β and γ phases of the half-Heusler KCrP compound can be considered to be thermodynamically stable and suitable for experimental synthesis. Based on the elastic results, it can be argued that, unlike α phase, both β and γ phases of the KCrP compound in the FM state satisfy the conditions of mechanical stability. Moreover, γ phase of the KCrP compound is stiffer than β phase, while β phase has a more ductile nature compared to γ phase. In agreement with the elastic results, the phonon scattering calculations showed that both β and γ phases of the half-Heusler KCrP compound are dynamically stable and can be synthesized, while the α phase is regarded to be an unstable phase. The electronic calculations indicated that the KCrP compound in the different phases β and γ possess a half-metallic property with the highly desired Curie temperature, making it a potential candidate for use in spintronic devices.

Chemical Bonding Origin of the Thermoelectric Power Factor in Half-Heusler Semiconductors

Intermetallic semiconductors with the cubic Half-Heusler structure (XYZ) have excellent thermoelectric properties. This has been attributed to the high degeneracy of the carrier pockets in the band structure, but large differences are found between different material compositions. Half-Heuslers are often interpreted within Zintl chemistry, making a clear distinction between an electropositive cation ($X^{n+}$) and an extended polyanion ($YZ^{n-}$). Based on quantitative real space chemical bonding analysis, we unravel large degrees of covalent bonding between the formal cation and anion, making the Zintl distinction clearly invalid. This covalence is shown to strongly affect the band structure, thermoelectric properties and response properties in the materials, with improved thermoelectric properties observed for those materials that least follow the Zintl concept. This expands our knowledge of the chemical bonding motifs governing physical properties, and gives a critical view on the simplistic chemical concepts too often applied for design of complex materials.

Machine learning based prediction of lattice thermal conductivity for half-Heusler compounds using atomic information

Half-Heusler compound has drawn attention in a variety of fields as a candidate material for thermoelectric energy conversion and spintronics technology. When the half-Heusler compound is incorporated into the device, the control of high lattice thermal conductivity owing to high crystal symmetry is a challenge for the thermal manager of the device. The calculation for the prediction of lattice thermal conductivity is an important physical parameter for controlling the thermal management of the device. We examined whether lattice thermal conductivity prediction by machine learning was possible on the basis of only the atomic information of constituent elements for thermal conductivity calculated by the density functional theory in various half-Heusler compounds. Consequently, we constructed a machine learning model, which can predict the lattice thermal conductivity with high accuracy from the information of only atomic radius and atomic mass of each site in the half-Heusler type crystal structure. Applying our results, the lattice thermal conductivity for an unknown half-Heusler compound can be immediately predicted. In the future, low-cost and short-time development of new functional materials can be realized, leading to breakthroughs in the search of novel functional materials.

Insight into elastic anisotropy, mechanical and dynamical stability, electronic properties, bonding and weak interactions analysis of LuAuSn Half-Heusler

Despite their great interest showing that LuAuSn half-Heusler is a topologically trivial semimetal, most of information about its physical behaviors seem rather remote from completion for the fact that several of its physical properties remain still unknown. Therefore, we try to outline, going into much specific details, a study on elastic anisotropy, mechanical and dynamical stability, electronic properties as well as the analysis of bonding and weak interactions. The lattice parameter value obtained by GGA-WC is very close to the experimental one, therefore, this functional has been chosen to carry out the rest of this work. The detailed study of the elastic behavior has revealed the mechanical stability of this compound, it results that its structure resists to slight strains. The obtained results also indicate a large elastic anisotropy of LuAuSn. Electronic results obtained by TB-mBJ(SOC) have confirmed its semimetallic behavior as well as being topologically trivial. This last property has been confirmed not only by the absence of band overlap but also by the absence of band inversion by the analysis of fat bands with and without SOC effect. The analysis of bonding and weak interactions has shown that the covalent character is predominant with the presence of weak repulsive interactions in cage regions and strong repulsive interactions between Lu and Sn. The vibrational study has revealed the dynamical stability of LuAuSn in the ground state, which indicates that this compound keeps its half-Heusler structure even at very low temperatures. Also, its zone center phonon modes have been determined and analyzed in detail by showing their frequencies and polarizations.

Zintl chemistry leading to ultralow thermal conductivity, semiconducting behavior, and high thermoelectric performance of hexagonal KBaBi

We identify the ground state structure of the phase KBaBi using ab initio evolutionary structure search methods and demonstrate that the bonding of this compound leads to a combination of very low thermal conductivity and electronic properties that are favorable for thermoelectric performance. The structure is the hexagonal ZrBeSi-type structure, with the cubic half-Heusler structure as a higher-energy competing phase. The bonding of the hexagonal structure leads to strongly anharmonic vibrational properties, which underlie the low thermal conductivity. The calculated figure of merit is approximately $ZT=3$ at 1000 K with optimized doping. This work underscores the use of chemical control of bonding to obtain low thermal conductivity and identifies a chemistry that achieves this.

A simple model for vacancy order and disorder in defective half-Heusler systems.

Defective half-Heusler systems X 1-x YZ with large amounts of intrinsic vacancies, such as Nb1-x CoSb, Ti1-x NiSb and V1-x CoSb, are a group of promising thermoelectric materials. Even with high vacancy concentrations they maintain the average half-Heusler crystal structure. These systems show high electrical conductivity but low thermal conductivity arising from an ordered YZ substructure, which conducts electrons, while the large amounts of vacancies in the X substructure effectively scatters phonons. Using electron scattering, it was recently observed that, in addition to Bragg diffraction from the average cubic half-Heusler structure, some of these samples show broad diffuse scattering indicating short-range vacancy order, while other samples show sharp additional peaks indicating long-range vacancy ordering. Here it is shown that both the short- and long-range ordering can be explained using the same simple model, which assumes that vacancies in the X substructure avoid each other. The samples showing long-range vacancy order are in agreement with the predicted ground state of the model, while short-range order samples are quenched high-temperature states of the system. A previous study showed that changes in sample stoichiometry affect whether the short- or long-range vacancy structure is obtained, but the present model suggests that thermal treatment of samples should allow controlling the degree of vacancy order, and thereby the thermal conductivity, without changes in composition. This is important as the composition also dictates the amount of electrical carriers. Independent control of electrical carrier concentration and degree of vacancy order should allow further improvements in the thermoelectric properties of these systems.