MgAgAs-type Structure Research Articles

A new candidate for optoelectronic device applications: CoTiX (X: P, As, Sb) half-heusler compounds

Some theoretical calculations of CoTiX(X:P, As, Sb) Half-Heusler(HH) compounds in MgAgAs-type structure have been analyzed using density functional theory (DFT). These Half-Heusler compounds are stable mechanically, and CoTiAs are entirely isotropic, whereas CoTiP and CoTiSb are anisotropic. Mechanical properties indicate that they have a ductile nature in all three compounds. In addition, elastic properties are presented in three dimensions (3D) for detailed analysis of Young's modulus, linear compressibility, shear modulus, and Poisson's ratio. Electronically, the investigated compounds are semiconductors with an indirect bandgap. The calculated positive phonon vibration frequencies of these Half-Heusler compounds show their dynamic stability in the cubic MgAgAs phase. Optical parameters such as dielectric function, reflectivity, absorption coefficient, refractive index, loss function, and absorption coefficient are given in IR, visible, and ultraviolet regions according to considerable photon energies. Finally, the effects of pressure and temperature on thermodynamic properties are discussed in detail.

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

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

Investigation of thermoelectric material based on Lu1-xZrxNiSb solid solution. I. Experimental results

The effect of doping of half-Heusler phase p-LuNiSb (MgAgAs structure type) by Zr atoms on the structural, kinetic, energetic and magnetic characteristics of the semiconductor solid solution Lu1-xZrxNiSb was studied in the ranges: T = 80–400 K, x = 0–0.10. From experimental studies it has been established that doping of p-LuNiSb compound with Zr atoms simultaneously generates both structural defects of acceptor and donor nature, the concentration of which increases with increasing content of Zr atoms. It was shown that the investigated semiconductor solid solution Lu1-xZrxNiSb is a promising thermoelectric material.

Electronic, Structural, Mechanical, and Thermodynamic Properties of CoYSb (Y = Cr, Mo, W) Half-Heusler Compounds as Potential Spintronic Materials

We used density functional theory (DFT) calculations to investigate the structural, electronic, magnetic, mechanical, and thermodynamic properties of CoYSb (Y = Cr, Mo and W) compounds. These are XYZ type half-Heusler alloys, which also exist in the face centred cubic MgAgAs-type structure and conform to F4¯3m space group. We computed these properties in three different atomic arrangements known as Type-I, Type-II, and Type-III phases. In all these phases, the alloys were found to be in the ferromagnetic state. Furthermore, the calculated electronic band structure and the total electronic density of states indicated a metallic behavior in CoWSb, nearly half-metallic behavior in CoMoSb, and half-metallic behavior in CoCrSb, with a minority-spin band gap of 0.81 eV. Furthermore, the calculated mechanical properties predicted an anisotropic behavior of these alloys in their stable phase. Finally, due to its high Debye temperature value, CoCrSb shows stronger covalent bonding than CoMoSb and CoWSb, respectively.

Investigation of semiconductive Lu1-xScxNiSb solid solution

The electrokinetic and energetic characteristics of the semiconductor solid solution Lu 1- x Sc x NiSb were investigated in the ranges Т =80–400 K, х =0–0.10 . Samples were prepared using an electric arc furnace by direct arc melting of the constituent elements under a purified argon atmosphere (porous Ti was used as a getter). The alloys were annealed at 1 073 K for 720 hours and quenched in cold water. Phase analysis was performed using X-ray powder diffraction patterns of the synthesized samples. The elemental and phase compositions of the synthesized samples were examined by Scanning Electron Microscopy (SEM) using Tescan Vega 3 LMU scanning microscope. Microprobe analysis of the concentration of atoms on the surface of Lu 1- x Sc x NiSb samples, including x = 0–0.10, established their correspondence to the initial compositions of the alloys, and X-ray phase and structural analyzes showed no traces of extraneous phases on the diffractograms except for the main phase indexed in the MgAgAs structure type. The temperature dependencies of electrical resistivity ( r ( T )) were measured employing two-probe method on millimeter-scale, well-shaped pieces cut by spark erosion from the polycrystalline samples in the temperature range 80–400 K. Thermopower coefficient ( α ) was measured in relation to the pure copper in the temperature range 80–400 K. It has been experimentally established that in the Lu 1‑ x Sc x NiSb solid solution both structural defects of neutral and donor nature are simultaneously generated, the concentration of which increases with increasing content of Sc atoms. The observed high-temperature activation parts on the temperature dependences of the resistivity ln( ρ (1/ T )) for all Lu 1- x Sc x NiSb samples indicates the location of the Fermi level ε F in the band gap of the semiconductor, and positive values of the thermopower coefficient α ( T ) specify its position near the valence band. The main carriers of electric conductivity are free holes. From the high-temperature parts of the dependence α (1/ T ) the values of the activation energy ε 1 α are calculated, which are proportional to the amplitude of large-scale fluctuations of continuous energy bands. The calculated from the low-temperature parts values of activation energy ε 3 α are proportional to the modulation amplitude of small-scale fluctuations of strongly doped and compensated semiconductor. It was shown that the investigated semiconductor solid solution Lu 1- x Sc x NiSb is a promising thermoelectric material. Keywords : electric conductivity, thermopower coefficient, Fermi level, structural defect.

Thermodynamic, mechanical stabilities and thermoelectric behavior of the XVSi (X = Co, Rh) half-Heuslers

Utilizing Boltzmann transport equations, structural, electronic, elastic, and thermoelectric properties, as well as thermodynamic stability of RhVSi and CoVSi half-Heusler compounds were investigated using the first-principle calculations in the framework of the density functional theory. The results of structural and elastic calculations and the thermodynamic phase diagrams indicate that these compounds are stable in the MgAgAs-type cubic structure with the F $$\overline{4}$$ 3m space group. The electronic structure was calculated using the Generalized Gradient Approximation with Tran–Blaha potential. It was found that both RhVSi and CoVSi are non-magnetic semiconductors with indirect gaps along X–W direction with values of 1.20 eV and 0.92 eV, respectively. High ZT values at 1000 K of 0.79 and 0.78 were obtained for RhVSi and CoVSi. The maximum value of the Seebeck coefficient (S) value for RhVSi and CoVSi is about $$244\,\frac{{\upmu {\text{V}}}}{{\text{K}}}$$ and $$226\,\frac{{\upmu {\text{V}}}}{{\text{K}}}$$ , respectively. The phonon dispersions reveal that the structures of these compounds are stable.

Structural, elastic, electronic and optical properties of the half-Heusler ScPtSb and YPtSb compounds under pressure

First-principles calculations using the plane-wave pseudopotential method within the generalized gradient approximation method were performed to study the pressure dependence of the structural, elastic, electronic and optical properties for the half-Heusler compounds ScPtSb and YPtSb in a cubic MgAgAs-type structure. The calculations were performed with the inclusion of spin-orbit coupling. The calculated equilibrium lattice parameters are in good agreement with the available experimental and theoretical values. The crystal rigidity and mechanical stability were discussed using the elastic constants and related parameters, namely bulk modulus, shear modulus, Debye temperature, Poisson's coefficient, Young's modulus and isotropic sound velocities. The calculated electronic band structures show that ScPtSb has an indirect gap of Γ-X type, whereas YPtSb has a direct band gap of Γ-Γ type. Furthermore, the effect of pressure on the optical properties, namely the dielectric function, absorption spectrum, refractive index, extinction coefficient, reflectivity and energy-loss spectrum is investigated for both compounds ScPtSb and YPtSb.

Energetic and electrokinetic characteristics of semiconducting solid solution TiСo1-xMnxSb

The samples of TiСo 1- x Mn x Sb solid solution ( x = 0.01, 0.02.03, 0.04, 0.05 0.07 and 0.10) were synthesized in an electric arc furnace. Homogenizing annealing was carried out at 1070 K for 720 hours. X-ray diffraction analysis was performed using the diffractometer DRON-2.0m (Fe K α -radiation). The temperature and concentration dependences of the electrical resistivity and thermopower relative to copper of the samples were measured in the temperature range 80–400 K. According to X-ray diffraction analysis, the synthesized samples belong to the MgAgAs structure type (space group F 3 m ). Previous researches of the half-Heusler phase TiCoSb and the solid solutions based on it TiCo 1- x Ni x Sb, TiCo 1- x Cu x Sb, Ti 1- x V x CoSb, Ti 1- x Mo x CoSb, Ti 1- x Sc x CoSb, allowed to establish both the defectivity in the structure of the TiCoSb compound and the mechanisms of the introduction of impurity atoms into its structure. It was found that in the structure of the compound TiCoSb there are simultaneously ~ 1% vacancies in the position 4a of Ti atoms and ~ 1% of additional Co * atoms in the tetrahedral voids of the structure, which can be described by the formula (Ті 0.99 Va с 0.01 )Co(Co * 0.01 )Sb. In this case, vacancies in the position of Ti atoms generate structural defects of acceptor nature, and additional Co* atoms in the tetrahedral voids of the structure generate defects of donor nature. The calculation of the density of electronic states of TiСo 1- x Mn x Sb solid solution, for an ordered variant of the structure in which the Co atoms are replaced by Mn atoms in 4 c position, showed that the Fermi level is in the band gap and shifts toward the valence band as the concentration of Mn increases. Such behavior of the Fermi level would have to lead to maximum values of resistivity and change the sign of the thermopower coefficient of TiСo 1- x Mn x Sb from negative to positive, and the intersection of the Fermi level and the valence band should change the conduction from semiconducting to metallic type. The obtained experimental results of the electrokinetic characteristics for TiСo 1- x Mn x Sb indicate that more complicated structural changes occur in the crystal than the linear substitution of Co atoms by Mn ones. At low concentrations of Mn atoms ( x = 0.01 and 0.02), the temperature dependence of resistivity has an activating character, at x = 0.03 and 0.04 it has metallic type and at concentrations x = 0.07 and 0.10 again activations take place. In turn, negative values of the thermopower coefficient of TiСo 1- x Mn x Sb for all investigated samples indicate that electrons are the majority charge carriers. Thus, based on the study of electronic structure, electrokinetic and energetic characteristics of TiСo 1- x Mn x Sb, it was established that both structural defects of acceptor and donor nature (the effective charge of which is opposite) are generated, the concentration of which increases with increasing Mn atoms. Keywords: electrical conductivity, thermopower coefficient, Fermi level, structural defect.

Thermoelectric and Piezoelectric Properties in Half-Heusler Compounds TaXSn (X = Co, Rh and Ir) Based on Ab Initio Calculations

In this paper, we report a theoretical study of the structural, electronic, thermoelectric and piezoelectric properties of TaXSn (X = Co, Rh and Ir) half-Heusler compounds crystallizing with cubic MgAgAs-type structure. We have made a quantitative evaluation of thermoelectric figure of merit (ZT) and the electromechanical coupling coefficient (k14) of these compounds. Accordingly, we intend to combine the first-principles band structure calculations using (DFT)-based FP-LAPW approach and the semi-classical Boltzmann transport theory within constant scattering time approximation (CSTA) to interpret and predict the thermoelectric performance (ZTe) without the lattice thermal conductivity as a function of the chemical potential at various temperatures. Further, to obtain a reasonable estimate for (ZT) with the intrinsic lattice thermal conductivity, we have calculated the relaxation time (τ) at various temperatures using Bardeen–Shockley theory. Finally, for predicting piezoelectric coefficients, we have employed the modern theory of polarization as provided by density-functional perturbation theory (DFPT) based on plane waves and pseudo-potentials (PP-PW). Our key result is that these half-Heusler semiconductors are attractive for practical applications in energy-harvesting technology, which has a high (ZT) and (k14) of 0.89 and 0.25, respectively, at room temperature.

Prediction of the structural, electronic, and piezoelectric properties of narrow-bandgap compounds FeVX (X = P, As, Sb)

A systematic theoretical investigation is carried out on the possible piezoelectric effect in the half-Heusler FeVX (X = P, As, Sb) compounds crystallizing in the cubic MgAgAs-type structure C1b by combining density functional theory (DFT) and perturbation theory (DFPT) based on the pseudopotential plane wave (PP-PW) method implemented in the ABINIT code. The ground-state properties such as the lattice constants, and the bulk modulus and its pressure derivative are obtained using both the local density approximation and the generalized gradient approximation for the exchange–correlation functional, and the results are compared with other studies. The results for the electronic properties reveal that all the compounds exhibit semiconducting behavior with a narrow indirect bandgap. In addition, the elastic, dielectric, and piezoelectric constants are computed using density functional perturbation theory. The title compounds are found to exhibit a good electromechanical coupling coefficient (k14). No experimental or theoretical data are available for their piezoelectric properties. Thus, the results of this study can be considered to represent theoretical predictions of the properties of new piezoelectric half-Heusler compounds that can be selected based on the values of their piezoelectric coefficient (e14), which are greater than the value of 0.16 C/m2 measured empirically for GaAs. Finally, the results presented herein shed light on the use of such piezoelectricity as a possible effect in various applications such as micromechanical actuators, sensors, and self-powered devices.

Investigation of Ti1-xMoxCoSb Semiconducting Solid Solution

The effect of doping of the TiCoSb compound (MgAgAs structure type) by Mo atoms on the features of the structural characteristics and behavior of the electrokinetic, energetic and magnetic properties of the Ti1-xMoxCoSb semiconducting solid solution (х = 0 - 0.06) in the temperature interval 80 - 400 K was studied. It was shown that including of Mo atoms (rМо= 0.140 nm) in the ToCoSb structure by substitution of Ti atoms (rТі= 0.146 нм) in 4a position is accompanied with non-monotonous variation of the lattice parameter values а(х), indicating unpredictable structural changes. Based on analysis of the variation of the electric resistivity values, thermopower coefficient, magnetic susceptibility and energetic characteristics, it was concluded that simultaneous generation in the crystal of the structural defects of the donor and acceptor nature (donor-acceptor pairs), which generate corresponding energy levels in the band gap of semiconductor and determine its electrical conductivity.

The TbNiIn1-xMx (M = Al, Ge, Sb) systems

Interaction of the components in TbNiIn 1- x Al x , TbNiIn 1- x Ge x and TbNiIn 1- x Sb x systems at 873 K was investigated by means of X-ray phase and partially local analysis in full concentration range. The samples for the investigation were synthesized by arc-melting purity metals with subsequent annealing at 873 K for a month. Phase analysis was carried out by X-ray powder diffraction (DRON-2.0M, Fe K α-radiation, PANalytical X’Pert Pro, Cu K a- radiation, Guinier FR552, Cu K a-radiation). The EDX analysis of the samples and single crystal was carried out by REMMA-102-02 and Zeiss EVO MA10. The influence of Indium substitution by other p -element on the nature of interaction and solubility type was determined. Solubility ranges for solid solutions were found and changes of unit cell parameters were calculated: TbNiIn 1-0 Al 0-1 (ZrNiAl-type structure, space group P - 62 m , a = 0.7449–0.6997(2), c = 0.378–0.3889(1) nm); TbNiIn 1-0 . 6 Ge 0-0 . 4 (ZrNiAl-type structure, space group P - 62 m , а = 0.7449–0.74348(6), с = 0.378–0.38063(4) nm); TbNiGe 1-0 . 5 In 0-0 . 5 (TiNiSi-type structure, space group Pnma , а = 0.69499–0.69598(7), b = 0.42306–0.42475(4), с = 0.72813–0.72996(7) nm); TbNiSb 1 . 0-0 . 6 In 0-0 . 4 (MgAgAs-type structure, space group F -43 m , a = 0.62662(2)–0.62011(5) nm). The crystal structures of the TbNiIn 0 . 28 Al 0 . 72 compound were investigated by single crystal X-ray analysis (Stoe IPDS II diffractometer, Mo K α-radiation). The refined compositions are confirmed by the results of the EDX analysis (Zeiss EVO MA10 scanning electron microscope). The structures were solved and refined using JANA2006 package: ZrNiAl-type structure, space group P - 62 m, Pearson symbol hP 9, a = 0.71452(6), c = 0.38394(3) nm, R 1 = 0.0152, 307 F 2 values, 15 variables). The formation of solid solutions and the character of the unit cell parameters variation in the studied and related systems briefly discussed. Keywords: indium, solid solution, powder data, single crystal, crystal structure.

Synthesis, Structural, Electrical Transport and Energetic Characteristics of ZrNi1-хVxSn Solid Solution

The samples of ZrNi1-хVxSn solid solution (x = 0 – 0.10) based on the ZrNiSn half-Heusler phase (MgAgAs structure type) were synthesized by direct arc-melting with homogenous annealing at 1073 K. The electrokinetic and energy state characteristics of the ZrNi1-хVxSn semiconducting solid solution were investigated in the temperature range T = 80 - 400 K. An analysis of behavior of the electrokinetic and energetic characteristics, in particular, the motion rate of the Fermi level, ΔεF/Δx for ZrNi1-хVxSn, allows to assume about the simultaneous generation of the structural defects of donor and acceptor nature in the crystal. The additional researches are required to establish the mechanisms of donor generation.

First principles study on the structural, electronic, mechanical and lattice dynamical properties of XRhSb (X = Ti and Zr) paramagnet half-Heusler antimonides

The half-Heusler TiRhSb and ZrRhSb alloys in the formation of face-centered cubic MgAgAs-type structure, which conforms to the space group with 216 as the space number, have been investigated using Generalized Gradient Approximation (GGA) implemented in Density Functional Theory (DFT). The calculated formation enthalpies and the plotted energy-volume curves of different types of structural phases (α, β, and γ) in these alloys indicate that the γ-phase structure is the best energetically suitable structure. In addition, TiRhSb and ZrRhSb alloys have been found as paramagnetic (PM) with the investigation of antiferromagnetic (AFM), ferromagnetic (FM), and paramagnetic (PM) orders in the most stable γ-phase structure. Therefore, their electronic, mechanical, and dynamical properties have been examined in the γ structural phase and paramagnetic order. These alloys have semiconducting nature due to the observed same band gaps in both the majority and minority spin channels of the calculated spin-polarised electronic band structure. These calculated band gaps are 0.75 eV for γ-TiRhSb and 1.18 eV for γ-ZrRhSb. The predicted elastic constants indicate that the alloys in this study are mechanically stable and show nearly isotropic behavior in the γ structural phase. Also, the minimum and the diffuson thermal conductivites have been determined for these alloys. Finally, the calculated phonon dispersion spectras for the γ-TiRhSb and γ-ZrRhSb half-Heusler antimonide alloys show the dynamic stability of these systems.

Synthesis, electrical transport, magnetic properties and electronic structure of Ti1-xScxCoSb semiconducting solid solution

The effect of Sc doping on the electric transport, magnetic properties and electronic structure of the half-Heusler TiCoSb semiconductor was studied. Samples of the Ti1-xScxCoSb solid solution (0.005 ≤ x ≤ 0.15) were synthesized by arc-melting, annealed at 1070 K and examined by X-ray powder diffraction and scanning electron microscopy with energy-dispersive X-ray analysis. All alloys of the solid solution crystallize in the MgAgAs structure type (space group F4¯3m) and stability range reaches the Ti0.85Sc0.15CoSb composition. Electrical resistivity and Seebeck coefficient were measured in the 80–380 K temperature range. It was found that doping TiCoSb by Sc atoms results in transition from semiconducting behavior to metallic. Increasing of Sc acceptor impurity in the Ti1-xScxCoSb solid solution changes the Seebeck coefficient from negative to positive values. The mechanisms of electrical conductivity of the Ti1-xScxCoSb solid solution were analyzed and compared with the results of the electronic band structure calculations.

Effect of High Pressure and Temperature on Magneto-Electronic, Thermodynamic, and Transport Properties of Antiferromagnetic HoPdX (X = As, Ge) Alloys

We present electronic, magnetic, thermoelectric, and thermodynamic properties of half-Heusler HoPdAs and HoPdGe materials in stable α-phase. They crystallize in cubic MgAgAs-type structure. The stable phase is found to be antiferromagnetic in comparison to the other magnetic and nonmagnetic states. The electronic structure calculations display the metallic behavior in both spin states. The effect of pressure and temperature on unit cell volume, thermal expansion coefficient, heat capacity, and Gruneisen parameter have also been analyzed using the quasi-harmonic Debye model. The alloys present a high absolute Seebeck coefficient with a value of 203.26 μV/K for HoPdAs and 53.19 μV/K for HoPdGe at room temperature.

The effect of pressure on structural, electronic, elastic, vibration and optical properties of ScXSb (X=Ni, Pd, Pt) compounds

The pressure dependence of the electronic, elastic, vibration and optical properties for ScXSb (X = Ni, Pd, Pt) half-Heusler compounds in cubic MgAgAs-type structure was systematically performed using the density functional theory with PAW method. The computed lattice parameters are in reasonable accord with available experimental data. Using the elastic constants and related parameters, the crystal rigidity and mechanical stability were discussed. The calculated electronic band structures revealed that ScNiSb and ScPdSb have an indirect gap in Γ-X Brillouin zone, whereas ScPtSb has a direct band gap in Γ-Γ Brillouin zone. We also presented phonon dispersion relation and density of phonon states. It is found that the considered structure is stable for three compounds between 0 and 50 GPa pressure. The optical properties were also predicted at various pressures.

Experimental and DFT study of the V–Co–Sb ternary system

The phase diagram of the V–Co–Sb ternary system was constructed at 870 K over the whole concentration range using X-ray powder diffraction and electron probe microanalysis. The formation of one ternary compound VCoSb with MgAgAs type structure (half-Heusler phase, space group F-43m, a = 0.57838(2) nm) was confirmed. The influence of V doping on electrical transport property behavior of the binary skutterudite CoSb3 (CoAs3-type, space group Im-3) was studied by measurements of electrical resistivity and Seebeck coefficient as well as ab initio DFT calculations.

Electronic structure and superconducting behaviour of LuPtBi half-Heusler compound: A first principle study

The electronic, phonon and superconducting properties of LuPtBi half-Heusler compound in MgAgAs-type cubic structure have been studied by using first principles density functional theory and linear response technique. Electronic and bonding properties have been analysed from electronic band structure, density of states, Fermi surfaces and charge density plot. Positive phonon frequencies confirm the stability of LuPtBi in cubic MgAgAs phase. Superconducting transition temperature (TC) is calculated using Eliashberg spectral function (α2F(ω)). For calculation of TC we have used screened Coulomb repulsion constant (μ*) as 0.15 and found TC = 1.1 K, which agree well with the experimental value (TC = 1.0 K). About 10% error in TC from its experimental value is observed for μ* = 0.15 which increases by 40% when the value of μ* = 0.13 (TC = 1.4 K).

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.