Half Heusler Research Articles

Augmented thermoelectric performance of LiCaX (X = As, Sb) Half Heusler compounds via carrier concentration optimization

The present study is focussed on the detailed physical insight into the structural, thermal, and dynamical properties of 8-valence electron Half Heusler (HH) compounds LiCaX (X = As, Sb) using Density functional theory. The thermal and dynamic stabilities of the compounds are assessed via ab-initio molecular dynamic simulations and phonon dispersion calculations, respectively. The Tran-Blaha modified Becke Johnson potential is used to accurately predict the band gap of investigated compounds. It is found that they are indirect band gap semiconductors with band gaps of 2.52 eV (LiCaAs) and 2.09 eV (LiCaSb). The transport parameters are obtained for p-type and n-type doping at temperatures ranging from 300 K to 800 K by solving the Boltzmann Transport equation. The deformation potential theory is employed to calculate the temperature dependent relaxation time for both compounds. The results of the various thermoelectric parameters obtained using actual values of time-dependent relaxation time are compared with that calculated under constant relaxation time approximation. The maximum power factor is 10.95 × 1011 (4.99 × 1011) Wm−1K−2s−1 for p-type (n-type) LiCaAs and 12.53 × 1011 (5.30 × 1011) Wm−1K−2s−1 for p-type (n-type) LiCaSb at optimized carrier concentration. The obtained low lattice thermal conductivities for LiCaSb (0.66 Wm−1K−1) and LiCaAs (0.88 Wm−1K−1) are explicated it in terms of different phonon modes. The Figure of Merit at 800 K for p-type (n-type) LiCaAs is as high as 0.90 (0.73) and 0.93 (0.84) for LiCaSb at optimum carrier concentration ∼1020 cm−3 (∼1019 cm−3), which has been experimentally realized in other 8-valence electron Li-based HH compounds. The good thermoelectric performance of p-type LiCaX in comparison to n-type suggests that p-type LiCaX alloys are viable candidates for high temperature energy harvesting applications.

Electronic structure, magnetic and thermodynamic properties of yttrium based half Heusler alloys YXZ (X = Fe, Co, Cr; Z = As, Sb): A first principles study

The structural properties are explored with three different atomic configurations named as α, β and γ phases in AlLiSi type structure and it is found that α phase is the most stable phase for yttrium based half Heusler alloys YXZ (X = Fe, Co, Cr; Z = As, Sb). The total energy values show that these materials are ferromagnetic. From the density of states plot, the energy gap values in the majority spin state for YFeAs, YFeSb, YCoAs, YCoSb, YCrAs and YCrSb are found to be 1.058 eV, 0.875 eV, 1.231 eV, 1.221 eV, 0.520 eV and 0.865 eV and there is no gap in the minority spin state. All these materials are found to exhibit half-metallic characteristics with 100% spin polarization which make them potential candidates for spintronic applications. The phonon calculations indicate that YFeSb and YCoSb are dynamically stable.

Structural, vibrational, electronic, thermodynamic and thermoelectric properties of CaCdSi Half Heusler compound: A first-principles study

The new half-Heusler CaCdSi alloy, as a new crucial nonmagnetic thermoelectric candidate, is desired to be understood in terms of its potential physical properties and stability in both experimental and theoretical studies. Here we investigate the thermal properties of half-Heusler CaCdSi using the plane-wave pseudopotential approach based on the density functional theory within the generalized gradient approximation (GGA). Firstly, dynamical and mechanical stability was assured in its cubic phase. These results also elucidate the stable and brittle nature of the compound. Furthermore, electronic band structure calculation shows that the CaCdSi alloy has semiconductor behavior with a direct band gap of 0.59 eV. Lastly, the Seebeck coefficient, electronic conductivity, electronic thermal conductivity, the power factor, and figure of merit have been investigated using the semi-classical Boltzmann transport theory. The high value of ZT of the CaCdSi alloy makes it a potential candidate material for thermoelectric applications. Our research not only enriches the family of thermoelectric materials but also provides a good platform for further experimental investigations.

Low Lattice Thermal Conductivity in a Wider Temperature Range for Biphasic-Quaternary (Ti,V)CoSb Half-Heusler Alloys.

Intrinsically high lattice thermal conductivity has remained a major bottleneck for achieving a high thermoelectric figure of merit (zT) in state-of-the-art ternary half-Heusler (HH) alloys. In this work, we report a stable n-type biphasic-quaternary (Ti,V)CoSb HH alloy with a low lattice thermal conductivity κL ≈ 2 W m-1 K-1 within a wide temperature range (300-873 K), which is comparable to the reported nanostructured HH alloys. A solid-state transformation driven by spinodal decomposition upon annealing is observed in Ti0.5V0.5CoSb HH alloy, which remarkably enhances phonon scattering, while electrical properties correlate well with the altering electronic band structure and valence electron count (VEC). A maximum zT ≈ 0.4 (±0.05) at 873 K was attained by substantial lowering of κL and synergistic enhancement of the power factor. We perform first-principles density functional theory calculations to investigate the structure, stability, electronic structure, and transport properties of the synthesized alloy, which rationalize the reduction in the lattice thermal conductivity to the increase in anharmonicity due to the alloying. This study upholds the new possibilities of finding biphasic-quaternary HH compositions with intrinsically reduced κL for prospective thermoelectric applications.

First-principles investigation on the novel half-Heusler VXTe (X=Cr, Mn, Fe, and Co) alloys for spintronic and thermoelectric applications

Half-Heusler (HH) alloys are a well-known and extensively researched family of thermoelectric (TE), magnetic, and spintronic materials. Doping may significantly increase the thermoelectric conversion efficiency of these materials; nevertheless, practical applications remain far. As a result, the hunt for superior parent TE alloys is critical. Using extensive first-principles density functional calculations, we predicted a novel class of vanadium-based four HH VXTe alloys, where X is one of the four elements: Cr, Mn, Fe, and Co. Their TE properties, as well as their mechanical, magnetic, electrical, and structural stability, have been studied in depth. Their mechanical and thermodynamic stability is confirmed using the predicted elastic constants, formation and cohesive energies, and phonon spectra. A comprehensive analysis of elastic constants and moduli demonstrates that HH VXTe alloys possess elastic anisotropy with reasonably good machinability, higher melting and Debye temperatures, mixed bonding characteristics with ionic and covalent contributions, and brittle nature, except for VCoTe, which is ductile. We find that the ground state of HH VCrTe and VFeTe is ferrimagnetic, while HH VCoTe is ferromagnetic and HH VMnTe is non-magnetic. Three VXTe (X = Cr, Fe, & Co) alloys demonstrated half-metallicity with 100% spin-polarization, whereas HH VMnTe is an 18-electron indirect semiconductor. In the studied HH VXTe alloys, most of the heat flow is caused by the phonon-group velocity of the acoustic phonons. Further studies on the relationship between carrier concentration and temperature dependence of TE properties reveal that the high ZT ∼1.2 at 1000 K of the pristine HH VMnTe alloy is obtained due to its high-power factor of 249.4×1012Wm−1K−2 and this value is greater than the values of some known pristine and doped HH TE materials. Our findings pave the way for further investigation into the HH VXTe alloys in the quest for improved TE and spintronic materials for use in domains that necessitate high thermoelectricity and spintronic performance.

First principle investigation of physical properties of MNiBi: (M = Sc, Y) half-Heusler compounds

We have investigated the half Heusler compounds MNiBi (M=Sc, Y), using the framework of density functional theory DFT within the full potential linearized augmented plane wave (FP-LAPW) method and studied the structural, electronic, optical and elastic properties. The structural properties are predicted using the Generalized Gradient Approximation GGA and Local Density Approximation LDA, the calculations reveal that Lattice constants and other structural parameter are better matched in GGA approximation with experimental and theoretical result than LDA approximation. The calculated band structure and the density of states (DOS) with GGA, LDA and Tran and Blaha modified Becke-Johnson (TB-mBJ) exchange-correlation potentials, indicates a semiconducting nature with indirect narrow band gaps for both compounds ScNiBi and YNiBi, it shown from result that using (TB-mBJ) functionals is much more successful than the LDA and GGA approach in estimating bandgaps for our half Heusler ScNiBi and YNiBi. Optical properties of the compounds under investigation are also reported in this paper, high absorptivity are observed in the visible and ultraviolet region. The bulk modulus, shear modulus, young’s modulus, and other elastic constants are computed to discuss their elastic properties.

Half-metallic and thermodynamic properties of new d 0 CsCaZ (Z= Ge, Sn and Pb) half Heusler alloys: A spin-based devices vision for the future

Density functional theory (DFT) was applied to investigate the structural, electronic, elastic, magnetic, thermodynamic and half-metallic properties of the newly d0 Heusler alloys (HAs) CsCaZ (Z= Ge, Sn and Pb). Spin-polarised calculations show that the compounds studied are half-metallic with a magnetic moment of 1.00 μB at the equilibrium lattice parameter, which obeys the well-known Slater–Pauling rule Mtot = 8 – Zt. The half-metallic behavior of the compounds CsCaGe, CsCaSn and CsCaPb is predicted with respect to the equilibrium lattice constants for CsCaGe, CsCaSn and CsCaPb with a narrow band gap in the majority spin channel. Furthermore, the elastic constants (Cij) showed that these materials are ductile and anisotropic. In addition, the negative values of the calculated formation energy and cohesion energy indicate that CsCaZ (Z= Ge, Sn and Pb) are likely to be experimentally synthesized. Non-equilibrium Gibbs function is employed to calculate the thermodynamic properties through the quasi-harmonic Debye model in which the bulk modulus, heat capacity, Debye temperature, thermal expansion coefficient, and entropy are investigated at 0-20 Gpa pressure and 0-1200 K temperature ranges. The significant half-metallic behavior makes the CsCaZ (Z= Ge, Sn and Pb) compounds strong candidates for spintronic applications.

D-Orbital-Driven Low Lattice Thermal Conductivity in TiRhBi: A Root for Potential Thermoelectric and Microelectronic Performance

Half-Heusler (HH) compounds are high-temperature thermoelectric materials with a high power factor upon appropriate doping. However, the efficiency and ZT values are still low due to their high lattice thermal conductivity, κl. It is essential to understand the thermal transport properties to design a potential thermoelectric material such as HH and a microelectronic device in general. At high temperatures, the κl is dominated by intrinsic scattering rates which arise purely from the anharmonic potential of the system. We study theoretically HH compounds, TiRhBi and TiCoBi, with the density functional theory and Boltzmann transport theory for κl calculation. We find that TiRhBi has a much lower κl (2.6 W/mK) than TiCoBi (6.4 W/mK) at 1000 K due to the weaker bond formation capability of diffused Rh 4d-electrons compared to the corresponding narrow band of 3d-electrons of Co in TiCoBi. The diffused Rh 4d-electrons near the valence band maximum participate in the nonbonding and antibonding types of overlap. This leads to an extremely weaker bond strength of TiRhBi, as evident from the COHP analysis. The weaker bond strength corresponds to an anharmonic potential which results in anharmonic effects leading to a lower κl. We discuss in detail several intermediate quantities such as COOP/COHP, heat capacity, phonon entropy, group velocity, Grüneisen parameter, and anharmonic scattering rates to explain the κl magnitudes.

Hf Incorporation in (Ti,Zr)NiSn Half Heusler Solid Solutions via Mechanical Alloying

Half Heusler materials are promising thermoelectric materials with potential application in generators at medium range temperatures. Solid solutions are typically prepared by arc melting, presenting interesting properties. In this work, the effect of Hf incorporation and the formation of solid solutions is discussed. More specifically, Ti1−xHfxNiSn and (Ti0.4Zr0.6)1−yHfyNiSn half Heusler materials were synthesized via mechanical alloying and consolidated via hot press sintering. Hf incorportation in the lattice strongly affected the lattice thermal conductivity due to the large mass fluctuation. The power factor and thermoelectric figure of merit was optimized via Sb doping the values of 34 μW/cmK2 and 38 μW/cmK2; 0.72 and 0.76 at 762 K for Ti0.4Hf0.6NiSn0.985Sb0.015 and (Ti0.4Zr0.6)0.7Hf0.3NiSn0.98Sb0.02, respectively, were reached.

A machine learning approach to predict the structural and magnetic properties of Heusler alloy families

Random forest (RF) regression model is used to predict the lattice constant, magnetic moment and formation energies of full Heusler alloys, half Heusler alloys, inverse Heusler alloys and quaternary Heusler alloys based on existing as well as indigenously prepared databases. Prior analysis was carried out to check the distribution of the data points of the response variables and found that in most of the cases, the data is not normally distributed. The outcome of the RF model performance is sufficiently accurate to predict the response variables on the test data and also shows its robustness against overfitting, outliers, multicollinearity and distribution of data points. The parity plots between the machine learning predicted values against the computed values using density functional theory (DFT) shows linear behavior with adjusted R2 values majorly lying in the range of 0.80 to 0.94 for all the predicted properties for different types of Heusler alloys. Feature importance analysis shows that the valence electron numbers plays an important feature role in the prediction for most of the predicted outcomes. Case studies with one full Heusler alloy and one quaternary Heusler alloy were also mentioned comparing the machine learning predicted results with our earlier theoretical calculated values and experimentally measured results, suggesting high accuracy of the model predicted results.

Electronic Structure-, Phonon Spectrum-, and Effective Mass- Related Thermoelectric Properties of PdXSn (X = Zr, Hf) Half Heuslers.

We hereby discuss the thermoelectric properties of PdXSn(X = Zr, Hf) half Heuslers in relation to lattice thermal conductivity probed under effective mass (hole/electrons) calculations and deformation potential theory. In addition, we report the structural, electronic, mechanical, and lattice dynamics of these materials as well. Both alloys are indirect band gap semiconductors with a gap of 0.91 eV and 0.82 eV for PdZrSn and PdHfSn, respectively. Both half Heusler materials are mechanically and dynamically stable. The effective mass of electrons/holes is (0.13/1.23) for Zr-type and (0.12/1.12) for Hf-kind alloys, which is inversely proportional to the relaxation time and directly decides the electrical/thermal conductivity of these materials. At 300K, the magnitude of lattice thermal conductivity observed for PdZrSn is 15.16 W/mK and 9.53 W/mK for PdHfSn. The highest observed ZT value for PdZrSn and PdHfSn is 0.32 and 0.4, respectively.

Theoretical investigations on electronic and optical properties of half heusler alloy,FeNbSb for opto-electronic applications

Theoretical investigations on electronic and optical properties of half heusler alloy,FeNbSb for opto-electronic applications

Magnetic properties of Heusler compounds Co1+xCrAl (x = 0, 0.25, 0.5, 0.75, 1)

The compounds Co1+xCrAl (x = 0, 0.25, 0.5, 0.75 and 1) have been successfully prepared using the arc melting technique. Room temperature XRD patterns confirm that all compounds have Heusler compounds like fcc structure. Magnetic properties of all as-cast compounds have been studied and it is observed that all compounds are ferromagnetic below their respective PM-FM transition temperature (Tc). For half Heusler compound CoCrAl, the Tc is around 14 K but H/M as a function of temperature (T) curves at high magnetic field follows the modified Curie-Weiss law above 200 K. CoCrAl and Co1.25CrAl compounds violate the universal nature of paramagnetism (above Tc) for low magnetic field values due to the presence of magnetic clusters within the paramagnetic matrix. For the other three compounds Co1.5CrAl, Co1.75CrAl and Co2CrAl universal nature of paramagnetism and Curie-Weiss law have been satisfied properly above Tc. Except for the full Heusler compound Co2CrAl, the value of Tc increases with increasing Co concentration because lattice parameter and distance between magnetic ions decrease with increasing Co concentration. The saturation magnetization (MS) of the compounds increases with increasing Co concentration within the compounds. The saturation magnetization (MS) does not follow the well-known Slater-Pauling rule for any compounds except for CoCrAl. M−H curves for all compounds below Tc suggest the soft magnetic nature of the compounds.

First principles calculations to investigate electronic, magnetic anisotropy energy and optical properties of MnCrP half-Heusler alloy

In this current research work, the electronic, magnetic and optical properties of half Heusler alloy MnCrP are calculated and discussed within DFT framework. These calculations are done by using Wien2k code with Perdew–Burke–Ernzerhof Generalized Gradient Approximation and Tran-Blaha modified Becke-Johnson (TB-mBJ) exchange and correlation potentials. Band gap study along with density of states analysis divulge the magnetic nature of MnCrP. The magnetic moments with PBE-GGA and TB-mBJ are 4.426μB and 4.4542μB respectively. It shows the suitability of this material in spintronic device fabrications. The magnetic anisotropy energy of this half Heusler is also determined by the difference of energies in easy and hard magnetization directions. Several optical parameters are calculated, including dielectric constants, absorption parameter, refractive indices, reflectivity, extinction coefficient, energy loss function and conductivity. The imaginary part of the dielectric constant also predicts that the compound MnCrP is optically metallic and suitable for sensor application and optoelectronic device.

Study of thermoelectric performance and intrinsic defect of promising n-type half-Heusler FeGeW

Half-Heusler (HH) compounds are considered promising thermoelectric materials for high-temperature power generation due to their good electrical properties and thermal stability. Some new HHs with excellent thermoelectric properties are found to be p-type, the discovery of competitive n-type HH materials has been extremely challenging. Here, we report a new HH compound FeGeW through first-principles calculation, which exhibits a high ZT of 2.36 at 1000 K. Systematically studied its thermoelectric performance indicates that a large dispersion or small band effective mass of conduction band can efficiently improve the electrical conductivity of n-type FeGeW. From the calculated formation energy of intrinsic point defects, we find that positive charged Fe interstitial are found to be the dominant defect at Fe-rich/Ge-poor condition, which account for the n-type conduction. Moreover the transition levels of Fe interstitial defect is shallow, which means that this donor defect does not damage electrical conductivity and thermoelectric performance. These results not only find a new n-type HH compound FeGeW, but also are helpful for understanding the roles of point defects in FeGeW, which is expected to encourage more experimental and theoretical investigations to study this kind of n-type HH thermoelectric material and seek out strategies to optimize thermoelectric performance using intrinsic point defect.

Atom Probe Tomography of a Cu-Doped TiNiSn Thermoelectric Material: Nanoscale Structure and Optimization of Analysis Conditions.

Cu-doping and crystallographic site occupations within the half-Heusler (HH) TiNiSn, a promising thermoelectric material, have been examined by atom probe tomography. In particular, this investigation aims to better understand the influence of atom probe analysis conditions on the measured chemical composition. Under a voltage-pulsing mode, atomic planes are clearly resolved and suggest an arrangement of elements in-line with the expected HH (F-43m space group) crystal structure. The Cu dopant is also distributed uniformly throughout the bulk material. For operation under laser-pulsed modes, the returned composition is highly dependent on the selected laser energy, with high energies resulting in the measurement of excessively high absolute Ti counts at the expense of Sn and in particular Ni. High laser energies also appear to be correlated with the detection of a high fraction of partial hits, indicating nonideal evaporation behavior. The possible mechanisms for these trends are discussed, along with suggestions for optimal analysis conditions for these and similar thermoelectric materials.

Electronic structure, magnetism and disorder effect in double half-Heusler alloy Mn2FeCoSi2

• A new type of double half Heusler alloy Mn 2 FeCoSi 2 is designed. • The study of phonon diffusion and molecular dynamics proves its dynamic stability. • Half metallicity can be preserved when c / a ranges from 1.8 to 2.2. • The Mn 2 FeCoSi 2 possesses a robust half-metallicity against the atomic disorder. The electronic structure and magnetism of a new type of half-Heusler alloy, i.e., double half-Heusler (DHH) alloy Mn 2 FeCoSi 2 are investigated. Our calculations reveal that Mn 2 FeCoSi 2 is a ferrimagnet with a magnetic moment of 3 μ B , it has a stable magnetism and its half metallicity can be preserved when c/a ranges from 1.8 to 2.2. Furthermore, 6 kinds of swap disorders and 12 kinds of antisite disorders are simulated to study the influence of atomic disorder on half metallicity of Mn 2 FeCoSi 2 . Half metallicity is maintained in Co-Fe disordered structure and spin polarization of Mn-Si, Mn-Fe and Si-Co disordered structure also reaches up to 94%, 98% and 97%, respectively. With regard to various antisite disorders, 100% spin polarization could be preserved in Co(Fe), Fe(Co), Fe(Mn), Mn(Co), Mn(Fe) and Si(Co) disordered structures. Therefore, Mn 2 FeCoSi 2 possesses a robust half-metallicity against the atomic disorder.

A simple formula for calculating the carrier relaxation time

An empirical formula to calculate the carrier relaxation time as a function of temperature (τT) has been presented. The method mainly depends on the Seebeck coefficient and the temperature. The formula was tested on metals and several types of semiconductors (Clathrate, Half Heusler Alloy, Zintl phases…). The obtained values are in good agreement with the available experimental results, especially in poor metals and heavily doped semiconductors at low temperatures especially room temperature. On the other hand, the calculated values of electrical conductivity σ(T) and electronic thermal conductivity ke(T) under the constant relaxation time approximation (CRTA) are given as fractions (σ/τorke/τ). Treating τ as an empirical parameter (τ=cte) by fitting to one value at a specific carrier concentration is a common method utilized by researchers. However, it cannot explain the behaviour of conductivity as temperature rises. Whereas, in this case, using our formula is useful to investigate σ(T) and the obtained curves show a similar behaviour to the experimental ones when the temperature increases.

Enhancement of thermoelectric properties at room temperature through isoelectronic ruthenium-substitution in [formula omitted]-type half-Heusler VFeSb compounds

The effects of ruthenium (Ru) substitution on the thermoelectric properties of half-Heusler (HH) V(Fe1−xRux)Sb compounds have been theoretically and experimentally investigated. All V(Fe1−xRux)Sb samples with deficient HH structures reveal deficiencies in V 4a and Fe 4c sites, interstitial Fe occupied 4d sites, and Ru substituted for Fe 4c sites, by Rietveld analysis. Based on the refined crystal structures and electronic structure analysis, the Ru content x with a maximum power factor (PF) is predicted to be around x=0.02. A small amount of Ru-substitution improves the PF by 26% at room temperature for the V(Fe0.98Ru0.02)Sb sample, which is coincident with the predicted range of Ru content for maximum PF. The lattice thermal conductivity κL of the V(Fe1−xRux)Sb samples decreases with an increase in x. The atomic mass and radius of Ru are substantially larger than those of Fe; therefore, κL of the V(Fe1−xRux)Sb samples decreases mainly due to phonon scattering strengthened by the introduction of Ru at the Fe 4c sites. A significantly reduced κL of 5.8 W/Km at room temperature is obtained for the V(Fe0.92Ru0.08)Sb sample, which is a ca. 40% decrease compared with the pristine HH-VFeSb sample. The dimensionless thermoelectric figure-of-merit reaches close to 0.2 at 300 K for the V(Fe0.98Ru0.02)Sb sample.

Thermoelectric power generation in the core of a nuclear reactor

Thermoelectric energy converters offer a promising solution to generate electrical power using heat in the nuclear reactor core. Despite significant improvements in thermoelectric efficiency of nanostructured materials, the performance of these advanced materials has yet to be demonstrated in the harsh radiation environment of a reactor core. Herein, we demonstrate a thermoelectric generator (TEG) made from nanostructured bulk half-Heusler (HH) materials generating stable electrical power density > 1140 W/m2 after 30 days in the MIT Nuclear Research Reactor under an unprecedented fast-neutron (>1 MeV) fluence of 1.5 × 1020 n/cm2. Despite an initial degradation due to irradiation damage when operating under relatively low temperatures, our TEG showed a 20-fold increase in power output when operating under high temperature due to in-situ annealing and resulting thermoelectric property recovery. First-principles modeling indicates that a chemically disordered metallic phase was formed under irradiation at lower temperatures, resulting in a drastic degradation in thermoelectric properties, while at sufficiently high temperatures the system returned to the initial chemically ordered HH phase and the thermoelectric properties recovered. Transmission electron microscopy and electron diffraction demonstrated that the chemically disordered phase was formed upon ion irradiation, confirming the prediction from first-principles simulations. The results suggest that with proper control over the TEG operating temperatures, the nanostructured bulk TEGs could produce stable electrical power and operate indefinitely in the core of a nuclear reactor.