Half Heusler Research Articles

Evolution of a weak magnetic moment in the FeNbSb based HH materials via Ni doping at Fe site

Heusler based materials have been identified as a potential candidate for their wide range of applications. Most of the Heusler based material’s properties can be scaled by their valence electron count (VEC). Herein, we have synthesized Ni-doped FeNbSb half-Heusler materials by solid-state reaction route to study its magnetic properties. The XRD and microscopic study show the half- Heusler (HH) single phase for Fe1-xNixNbSb (x = 0.025 and 0.05) compositions. Experimental results well corroborated the theoretically estimated magnetic moment by the Slater-Pauling rule. The XRD simulation shows atomic disorder due to swapping of nearly 50% Fe at the Nb site, which reduces the magnetic moment value accordingly. The controlled Ni-doping in the FeNbSb matrix revealed a weak magnetic moment that was not seemed to be saturated even at a high magnetic field. That directly supports the compositions to be hard magnetic materials. Moreover, for both sample’s, magnetization vs. temperature graph shows a competitive behavior between the Ferro-antiferro phase in the 5 K to 300 K temperature range associated with swapping of Fe at Nb site as well as Ni substitution.

An Insight into the Electronic, Optical and Transport Properties of a Half Heusler Alloy: NiVSi

The half-Heusler alloy NiVSi is investigated theoretically by using first-principles calculations based on the density functional theory (DFT). For a better description of the electronic properties, the TB-mBJ potential is used for exchange-correlation potential. The structural, electronic, magnetic, optical and thermoelectric properties was calculated by WIEN2k software. The negative cohesive and formation energies found reveal that the NiVSi is thermodynamically stable. Electronically, the NiVSi is a half-metal with an indirect band gap of 0.73 eV in the spin-down channel whereas the spin up channel is metallic. The total magnetic moment is of 1. Optically, the obtained high absorption coefficient in ultraviolet wavelength range, make the NiVSi useful as effective ultraviolet absorber. Thermoelectrically, a high figure of merit in the p- and n-type region was obtained, what makes this compound very functional for thermoelectric applications. The generation of a fully spin-polarized current make this compound unsuitable for spintronic applications at room temperature, a doping may be a satisfactory solution to improve this property.

A strategic high throughput search for identifying stable Li based half Heusler alloys for spintronics applications

In this work, high throughput DFT calculations are performed on the alkali metal-based half Heusler alloys; LiYpY1−p′S (Y, Y′ = V, Cr, Mn, Fe, Co, Ni and p = 0, 0.25, 0.5, 0.75, 1). Starting with 243 structural replica, systematic filters are designed to select the energetically and vibrationally favorable compositions by considering the contributions stemming from the magnetic alignments of the ions. Thereby, 26 dynamically stable magnetic compositions are identified, of which 10 are found to be ferromagnetic (FM), 4 antiferromagnetic (AFM) and 12 ferrimagnetic (FiM). 4 FM and 8 FiM ones are found to show 100 % spin polarization. Further, tetragonal distortion is found to be present in 4 FM, 3 FiM and 4 AFM compositions, which indicates the possibility of easy-axis magnetocrystalline anisotropy. The ferromagnetic LiFe0.5Mn0.5S and antiferromagnetic LiFeS are found to have the most prominent easy-axis magnetocrystalline anisotropy.

Prediction of some physical properties in new half Heusler alloy NbAgSi

A first principle investigation of structural, dynamical, mechanical, thermodynamic and electronic properties of ‘NbAgSi’ alloy was performed using the density functional theory. Among ‘α’, ‘β’, and ‘γ’ structural phases, ferrimagnetic ‘β’-phase was found to be the most favorable ground state. The structural and dynamical stabilities of the compound were determined by the formation/cohesive energies. The dynamical stability was further strengthened by positive phonon frequencies, observed in the phonon dispersion curves. The mechanical properties of the sample material were explored using the elastic constants to explore its mechanical applications. To include the effect of highly correlated system Nb, the Hubbard parameter (U) was determined and incorporated in the calculations to predict the half-metallic behavior of the sample alloy correctly. The inclusion of the U term had a significant effect on the electronic and magnetic properties of the system. The material was found to possess anisotropic character with ductile behavior. The thermodynamic properties were also measured over the temperature range 0 – 500K under different pressures. The optical phonon modes were found to be a mixed character of Raman and infrared active. The electronic and bonding behavior were analyzed from the Fermi surface, charge density plot and electron localized function. The charge transfers between the atoms were investigated using the Bader charge technique. The results presented in the present manuscript open the possibility of the sample material as a promising candidate for spintronic applications.

Thermoelectric Properties of Ta-doped Zr0.6-xTi0.4TaxNiSn n-type Half-Heusler Materials

Half-Heusler (HH) thermoelectric materials are promising for mid- to high-temperature applications, and MNiSn (M = Ti, Zr, Hf) is a representative n-type HH alloy. In general, the M sites are mixed with isoelectronic elements Ti, Zr, and Hf, to lower the lattice thermal conductivity, and the Sn sites are doped with Sb to adjust the electron concentration. However, Hf is a rare element in earth’s crust, and the volatility of Sb makes it difficult to maintain the initial Sb amount during material synthesis. In this study, as an alternative, Ta was added in the M sites along with the host elements Ti and Zr to produce Hf-free Zr0.6-xTi0.4TaxNiSn alloys (0 ≤ x ≤ 0.04), and the effects of Ta doping on the thermoelectric properties were analyzed. The electrical conductivity of Zr0.6-xTi0.4TaxNiSn increased with Ta content, and the electron concentration increased almost linearly, reaching 5.6 × 1020 cm -3 at x = 0.04. By adding Ta, the maximum power factor also increased by approximately 17% to 3.94 × 10-3 W m-1K-2 at x = 0.02. The lowest lattice thermal conductivity (κlat) was observed at x = 0.02, reaching approximately 1.9 W m-1K-1 at 723 K, but overall, a dramatic decrease in κlat was not observed with Ta doping in Zr0.6-xTi0.4TaxNiSn. This is probably due to the existing effect of Zr/Ti mixing at the M sites, which enhances phonon scattering. A maximum figure of merit (zT) of 0.91 was obtained in Zr0.58Ti0.4Ta0.02NiSn at 873 K, which is a high value for ZrNiSn-based Hf-free HH materials. In conclusion, Ta doping is a viable method to replace Sb doping in ZrNiSn-based HH alloys.

Half-metallic Characteristics of the Novel Half Heusler Alloys XCrSb (X=Ti, Zr, Hf )

Ab-initio calculations are performed to examine the structural, mechanical, electronic, magnetic and thermodynamic properties of the half-Heusler ternary alloys XCrSb (X = H f , Ti, Zr). In this study, the spin-polarized density functional theory (DFT) method that is spin-polarized with generalised gradient approximation (GGA) are used to perform ab-initio calculations to investigate the physical properties of a novel half-Heusler ternary alloys XCrSb (X = H f , Ti, Zr). It was confirmed that the alloys are stable mechanically and exhibit ferromagnetic states (FM). The study reveals that the alloys portray half-metallic character with narrow energy gaps. And it also shows that they have a total magnetic moment of approximately 3ub. From the formation energy calculation, it shows that the alloys can be synthesized experimentally. Also, it was observed that they are mechanically stable. The heat capacities and Debye temperatures were also computed and they show high thermodynamic stability.

Role of sintering temperature on electronic and mechanical properties of thermoelectric material: A theoretical and experimental study of TiCoSb half-Heusler alloy

Ternary half-Heusler (HH) alloys are being widely explored for mid temperature range thermoelectric (TE) applications. Here, we have discussed the effect of sintering temperature on electronic, structural and mechanical properties of TiCoSb HH alloy. Four samples of TiCoSb were synthesized employing arc melting followed by spark plasma sintering at different sintering temperatures. Subsequently, the phase purity of all the synthesized samples was confirmed by X-ray diffraction (XRD) analysis. The crystallite size is calculated employing Williamson-Hall method, which increases monotonically with sintering temperature. The morphology coupled with compositional analysis of the samples was studied employing Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive X-Ray Spectroscopy (EDS), respectively. Further, these samples were characterized for their electronic transport properties. The overall transport properties which are represented by power factor (PF) found to be increasing with sintering temperature. A maximum value of power factor ∼0.79 mW/m-K2 at ~625 K is achieved for sample sintered at 1423 K. Furthermore, the mechanical properties (micro-hardness and fracture toughness) were found to be increased monotonically with increasing sintering temperature. To understand the underlying physics, the electronic structure and transport properties of the material have been studied by using first principles-based Density Functional Theory (DFT).

High-pressure studies of a biphasic NiTiSn/Ni2TiSn Heusler alloy by in situ X-ray diffraction and first principle calculations.

The present work studies the effect of hydrostatic pressure on a biphasic Heusler alloy using in situ X-ray diffraction. A nanostructured biphasic NiTiSn/Ni2TiSn Heusler alloy was synthesized by four hours of mechanical alloying using a vibrational high-energy mill and characterized by x-ray diffraction measurements and Rietveld method. The sample was submitted to high-pressure conditions (up to 11.9 GPa) using a diamond anvil cell (DAC) loaded with He, and the structural evolutions were followed by in situ synchrotron x-ray diffraction. The results validate the stability of the half and full-Heusler alloys, as no pressure-induced structural phase transition was observed over the entire pressure range. The behavior of the crystallographic parameters, equation of state and the compressibility factors for both the Heusler phases were obtained from the experimental results as well from the density functional theory (DFT) calculations. The second-order Birch–Murnaghan equation of state determined from our experimental results yields bulk moduli of 143(9) GPa and 156(6) GPa for the Half and Full Heusler phases, respectively. According to the DFT calculations, the corresponding values are 131.0(9) GPa and 161.8(8) GPa.

DFT studies on electronic, magnetic and thermoelectric properties of half Heusler alloys XCaB (X = Li, Na, K and Rb)

In this work, the results of first principles calculations of structural, electronics, magnetic and thermoelectric properties of half Heusler alloys XCaB (X = Li, Na, K and Rb) are presented. The pseudopotential method as implemented in the Quantum Espresso package has been used with the exchange correlation functional of Perdew, Burke and Ernzerhof (PBE). The total energy calculations show that the proposed alloys are in stable ferromagnetic phase and the calculated equilibrium lattice constant are in good agreement with the earlier reported results. All the alloys show strong spin polarisation of energy states near the Fermi level confirming ferromagnetic nature and also semiconducting for majority spin channel and metallic for down spin channel exhibiting half metallic property. The calculated magnetic moment of all the alloys XCaB (X = Li, Na, K and Rb) is around 2.00 µB mainly arises from the np electrons of B atom with partial involvement of nd electrons of Ca atom. The thermoelectric properties of the materials are calculated using the BoltzTrap code. The calculated transport properties like electrical conductivity, thermal conductivity and power factor increases with temperature which is a perfect fit for thermoelectric applications. The half metallic gap, strong spin polarization and high figure of merit (ZT) values shows that these studied materials can be used for both spintronics and thermoelectric device applications.

The Role of Magnetic Interaction on the Thermoelectric Performance of ZrNiSn Half‐Heusler Alloys

The half‐Heusler (HH)‐based materials are appealing for mid‐high temperature thermoelectric (TE) applications. Herein, the effect of Mn doping on the TE properties of ZrNiSn HH alloy is explored. The samples of ZrNi1−xMnxSn (x = 0.01–0.03) are prepared via the fast‐processing route comprising arc melting along with spark plasma sintering and then characterized for structural as well as TE properties. The Seebeck coefficient at room temperature with Mn doping is realized to be increased as a result of increased carrier effective mass, which might be due to the charge carrier–magnetic moment interaction. The improved power factor of ≈3.8 × 10−3 Wm−1 K−2 at ≈873 K is noticed for the composition ZrNi0.97Mn0.03Sn. The figure of merit is observed to be more or less unaltered with Mn concentration due to the simultaneously increased thermal conductivity.

Topological phase transition associated with structural phase transition in ternary half Heusler compound LiAuBi

In this article, we report detailed theoretical investigations of topological phases in non-centrosymmetric half Heusler compound LiAuBi up to a pressure of 30 GPa. It is found that the compound forms into a dynamically stable face-centered cubic (FCC) lattice structure of space group (216) at ambient pressure. The compound is topologically non-trivial at ambient pressure, but undergoes a quantum phase transition to trivial topological phase at 23.4 GPa. However, the detailed investigations show a structural phase transition from FCC lattice (space group 216) to a honeycomb lattice (space group 194) at 13 GPa, which is also associated with a non-trivial to trivial topological phase transition. Further investigations show that the compound also carries appreciable thermoelectric properties at ambient pressure. The figure of merit (ZT) increases from 0.21 at room temperature to a maximum value of 0.22 at 500 K. The theoretical findings show its potential for practical applications in spintronics as well as thermoelectricity, therefore LiAuBi needs to be synthesized and investigated experimentally for its applications.

A Comparative Study of Spin Magnetic Moment, Spin Polarization and Curie Temperature of Heusler type Compounds (A<sub>2-X-Y</sub>Z<sub>y</sub>)Mnz (A = Ni, Pd, Pt And Z = Sb, Sn)

The Heusler type compounds (A2-x-yZy)MnZ (A = Ni, Pd, Pt; Z = Sb, Sn; x = 1, 0; and y = 0, antisite defects of Z atoms) are investigated to understand the induced magnetic properties. The structural phases C1b type half Heusler (x = 1, y = 0) compounds AMnZ and the corresponding L21 type full Heusler (x = y = 0) compounds A2MnZ and some disordered compounds (A1-yZy)MnZ and (A2-yZy)MnZ (y = 0.01, 0.05) were calculated using the Korringa-Kohn-Rostoker Green’s function method. The antisite doping of 1% (5%) Sb retained (suppressed) the half metallicity at (Ni1-ySby)MnSb, whereas the other compounds were found to be gapless in both spin directions. The density of states at the Fermi level exhibits the explicit spin polarization in the compounds. The net magnetic moments of the compounds are closer in their values, wherein the manganese atom is responsible for the major contribution to the magnetic moments, which is much larger than the partial moment of the other constituents. The Curie temperature (TC) of the ordered ferromagnetic compounds was estimated using the mean-field approximation. The TC higher than the room temperature was found for the ordered compounds, except for the cases of Pt2MnSb and Pt2MnSn. No literature value of TC has been reported yet for these two compounds. Calculated spin moments and TC agree well with the experimental results, where available. J. Bangladesh Acad. Sci. 45(2); 217-229: December 2021

Narrow gap electronic structure and thermoelectric performance of p-type ErMSb (M = Ni, Pd) half Heusler compounds

The structural, electronic and thermoelectric properties of narrow gap ErMSb (M = Ni, Pd) half Heusler (HH) compounds are determined using first-principle calculations, based on density functional theory combined with Boltzmann transport theory. We employed Coulomb corrected generalized gradient approximation (GGA + U) which is the most competent method to treat rare earth compounds. We found spin up/down Ni/Pd-d and spin down Er-f states are mainly contributing to the total density of states (DOS) near Fermi energy (EF). Both the investigated compounds exhibit the indirect narrow energy gaps ∼0.25 eV that are in agreement with the corresponding experimental values. We obtained fairly high values of the Seebeck coefficient (S) in agreement with available experimental data. The calculated value of total S, using two current model, is 141 μV/K and 159 μV/K at 300 K in ErNiSb and ErPdSb HH compounds, respectively. We also find the monotonically increasing value of ZT and reaches 0.76 at 800 K in ErNiSb compounds, whereas 0.51 is the maximum ZT for the ErPdSb compound at 300 K. We obtained favorable and stable ZT values (ZT > 0.7) for a wide temperature range in ErNiSb compound in agreement with the experiment. It suggests ErNiSb as a promising candidate for thermoelectric applications.

NiMnSn half Heusler alloy: Critical phenomena at the ferromagnetic to paramagnetic phase transition

The critical magnetic analysis and magnetocaloric effect of NiMnSn half Heusler alloy have been explored in this article. This alloy undergoes ferromagnetic to paramagnetic second order phase transition above room temperature at around 388 K. To understand the exchange interaction, magnetic ordering and magnetic phase transition, the critical magnetic analysis has been carried out around the transition temperature. Critical exponents are turned out to be β = 0.44, γ = 1.1 and δ = 3.1. These values imply that the exchange interaction is following towards mean field theory i.e., long-range magnetic ordering is present in the system. The absence of one Ni atom from full Heusler Ni2MnSn alloy does not affect much the magnetic ordering of half Heusler NiMnSn. For environment-friendly solid state cooling applications, the magnetocaloric effect of NiMnSn has been studied. The magnetic entropy change is calculated to be high as 0.81 J/kg-K near the transition temperature for a moderate field change of 30 kOe.

Structural, electronic and optical properties of ternary rare-earth based GdPtSb half Heusler compound

We present a density functional theory (DFT) calculation for structural, electronic and optical properties of rare earth based GdPtSb half Heusler (HH) compound by using GGA + U approximation. Our calculated total density of states (DOS) and partial density of states (PDOS) of GdPtSb show that near Fermi energy (EF), the maximum contribution is due to the Gd-5d and Pt-5d states. The electronic band structure calculation shows the half-metallic nature of GdPtSb compound. We found a narrow energy band gap (86 meV) in majority spin, while in minority spin with electronic states of valence band crosses the Fermi level (EF). Optical response of GdPtSb compound is investigated by complex dielectric function, extinction coefficient, absorption coefficient, refractive index and reflectivity. From the best of our literature survey the electronic and optical properties are less explored in theoretical work. Our calculated structural and electronic properties are in agreement with other theoretical and experimental work, while the optical properties of GdPtSb compound are not explored experimentally till date.

Influence of carbon nanotubes on thermoelectric properties of p- and n-type Heusler alloys

This paper presents the results of studying the effect of carbon nanotubes on thermoelectric properties of p-type (Nb0.6Ta0.4)0.8Ti0.2FeSb and n-type Ti0.5Zr0.25Hf0.25NiSn half Heusler alloys. The experimental data obtained indicate a strong effect of the carbon nanotubes on electrical conductivity and Seebeck coefficient of the n-type compound, while the changes in these properties in the p-type compound were significantly less. It is suggested that a possible reason for this difference is the formation of a conducting cluster of carbon nanotubes in the sample of the n-type Heusler alloy. Keywords: carbon nanotubes, thermoelectric properties, Heusler alloys, electrical conductivity, Seebeck coefficient.

Pursuit of Stability, Electronic and Thermoelectric Properties of Novel Pdvga Half Heusler Compound

Pursuit of Stability, Electronic and Thermoelectric Properties of Novel Pdvga Half Heusler Compound

Influence of the Chemical Environments of Magnetic Ions on the Fully Compensated Ferrimagnetic Behavior of Half Heusler Mn2si Alloys

Influence of the Chemical Environments of Magnetic Ions on the Fully Compensated Ferrimagnetic Behavior of Half Heusler Mn2si Alloys

Влияние углеродных нанотрубок на термоэлектрические свойства сплавов Гейслера p- и n-типа

This paper presents the results of studying the effect of carbon nanotubes on thermoelectric properties of p-type (Nb0.6Ta0.4)0.8Ti0.2FeSb and n-type Ti0.5Zr0.25Hf0.25NiSn half Heusler alloys. The experimental data obtained indicate a strong effect of the carbon nanotubes on electrical conductivity and Seebeck coefficient of the n-type compound, while the changes in these properties in the p-type compound were significantly less. It is suggested that a possible reason for this difference is the formation of a conducting cluster of carbon nanotubes in the sample of the n-type Heusler alloy.

Topological Properties of Half Heusler Compounds: Tuned by the Spin-Orbit Coupling and P-D Hybridization

Topological Properties of Half Heusler Compounds: Tuned by the Spin-Orbit Coupling and P-D Hybridization