Constant Relaxation Time Approximation Research Articles

Can Pressure be a Good Strategy for Optimizing Thermoelectric Performance of SnPS3${\rm SnPS}_3$?

AbstractExternal pressure can significantly alter the transport coefficients, power factor, and figure of merit because of its direct influence on the electronic structure, electron–phonon, and phonon–phonon couplings. This study delves into the electronic and thermal transport properties of at external pressures up to 30 GPa using first‐principles calculations and Boltzmann transport theory. The electron–phonon relaxation time is computed within the electron–phonon‐averaged (EPA) approximation, enabling exploration beyond the constant relaxation time approximation. The first‐principles calculations reveal an indirect bandgap of 1.76 (without pressure) and 0.12 eV (30 GPa). The density functional perturbation theory calculations confirm the dynamic stability of at external pressure up to 30 GPa. The electronic transport properties are improved by more than one order of magnitude at 30 GPa, consistent with experimental observations. The Peierls–Boltzmann transport calculations demonstrate the room temperature lattice thermal conductivity of 0.22 (without pressure) and 7.4 (at 30 GPa). The results emanate that exhibits of 0.71 at 900 K at a hole doping of 2 at zero pressure, which decreases with increasing pressure. The findings explore the effect of external pressure on both electronic and thermal transport properties of , warranting further experimental exploration of thermal transport properties at higher pressures.

Investigation of AcXO3 (X = Al, Ga) perovskites for energy harvesting applications: a DFT approach

Full potential linearized augmented plane wave (FP-LAPW) based computational study is presented for AcAlO3 and AcGaO3. Tolerance factor (τG) is 0.93 for AcAlO3 and 0.90 for AcGaO3 which reveal the stability of proposed perovskite oxides in cubic phase. The calculated band structures for both materials reveal the nonmagnetic semiconductive nature with energy band gaps of 3.98 and 2.75 eV for respective Al and Ga based perovskites. Total, and partial density of states (DOS) are computed for meaningful understanding of semiconducting behavior of the proposed perovskites. The partially filled O-2p and Ac-6d states are observed to lie in the vicinity of Fermi level. Furthermore, various parameters of optical spectra have also been computed to study the light matter interaction. Moreover, thermoelectric (TE) properties of both materials have been investigated by using semiclassical Boltzmann theory with constant relaxation time approximation. Calculated figure of merit (ZT) values for AcAlO3 and AcGaO3 are 0.23 and 0.20 at 800 K, respectively. Overall, computational study for titled perovskites indicate that these materials have application in UV sensors, photovoltaic and thermoelectric devices.

Efficient first-principles electronic transport approach to complex band structure materials: the case of n-type Mg3Sb2

We present an efficient method for accurately computing electronic scattering rates and transport properties in materials with complex band structures. Using ab initio simulations, we calculate a limited number of electron–phonon matrix elements, and extract scattering rates for acoustic and optical processes based on deformation potential theory. Polar optical phonon scattering rates are determined using the Fröhlich model, and ionized impurity scattering rates are derived from the Brooks-Herring theory. Subsequently, electronic transport coefficients are computed within the Boltzmann transport theory. We exemplify our approach with n-type Mg3Sb2, a promising thermoelectric material with a challenging large unit cell and low symmetry. Notably, our method attains competitive accuracy, requiring less than 10% of the computational cost compared to state-of-the-art ab initio methods, dropping to 1% for simpler materials. Additionally, our approach provides explicit information on individual scattering processes, offering an alternative that combines efficiency, robustness, and flexibility beyond the commonly employed constant relaxation time approximation with the accuracy of fully first-principles calculations.

Theoretical Investigations of the Structural, Dynamical, Electronic, Magnetic, and Thermoelectric Properties of CoMRhSi (M = Cr, Mn) Quaternary Heusler Alloys

The structural, dynamical, electrical, magnetic, and thermoelectric properties of CoMRhSi (M = Cr, Mn) quaternary Heusler alloys (QHAs) were investigated using density functional theory (DFT). The Y-type-II crystal structure was found to be the most stable configuration for these QHAs. Both CoCrRhSi and CoMnRhSi alloys possess a half-metallic behavior with a 100% spin-polarization as the majority spin channel is metallic. On the other hand, the minority spin channel is semiconducting with narrow indirect band gaps of 0.54 eV and 0.57 eV, respectively, along the Γ−X high symmetry line. In addition, both CoCrRhSi and CoMnRhSi alloys possess a ferromagnetic structure with total magnetic moments of 4 μB, and 5 μB, respectively, which are prominent for spintronics applications. The thermoelectric properties of the subject QHAs were calculated by using Boltzmann transport theory within the constant relaxation time approximation. The lattice thermal conductivities were also evaluated by Slack’s equation. The predicted values of the figure-of-merit (ZT) for CoCrRhSi and CoMnRhSi were found to be 0.84 and 2.04 at 800 K, respectively, making them ideal candidates for thermoelectric applications.

Structural, electronic, optical, and thermoelectric properties of CsPbI3-yBry (y = 0.5, 1.5, 2.5) compounds: First-principle study

BackgroundAmong photovoltaic materials, halide-based perovskites, such as CsPbI3, have been the subject of many research papers due to their suitable and direct bandgap, very good optical properties, and remarkable thermoelectric properties. Enhancing optoelectronic and thermoelectric properties can be achieved by combining halide atoms intelligently. The purpose of this article is to examine the physical properties of Br substitution at different concentrations in CsPbI3 perovskite. MethodsThe structural, electrical, and optical properties of CsPbI3-yBry (y = 0.5, 1.5, 2.5) compounds have been investigated by using the QUANTUM-ESPRESSO/PWSCF computational package which is based on the density functional theory and the pseudo-potential technique. The thermoelectric properties of CsPbI3-yBry perovskites were calculated by solving the Boltzmann transport equations within the constant relaxation time approximation as employed in the BoltzTraP package. Significant findingsThe structural investigation reveals that when the I atom is substituted by Br, the lattice constant decreases, increasing the bulk modulus and stiffness of compounds. The calculated negative formation energy confirmed the thermodynamic stability of CsPbI3-yBry compounds, and the stability improved with an increase in Br concentration. The electronic investigation indicates the semiconductor nature of compounds with a direct band gap, with a slight increase in electronic band gap as Br concentration increases. The noticeable optical absorption coefficient in the visible range of the electromagnetic spectrum, along with high and tunable optical conductivity, confirms the capability of the compounds for use in optoelectronic devices. The positive value of the Seebeck coefficient revealed the p-type semiconducting nature of CsPbI3-yBry perovskites. Lattice thermal conductivity calculation reveals that this quantity is decreased because of the increase in phonon-phonon interaction due to rising temperature. Thermoelectric investigation indicates that a precise mixture of halide I and Br atoms significantly enhances the electrical and thermal conductivity of perovskites. Also, the substitution of halide I atoms by Br can improve the figure of merit (ZT). The maximum value of ZT at room temperature is equal to 0.99, justifying the brilliant thermoelectric properties of the studied compounds.

Mechanical, magneto-electronic and thermoelectric properties of Ba2MgReO6 and Ba2YMoO6 based cubic double perovskites: an ab initio study

We report an analysis of the structural, electronic, mechanical, and thermoelectric properties of oxide double perovskite structures, specifically the compounds Ba2MgReO6 and Ba2YMoO6. Our study employs first-principles density functional theory (DFT) as the investigative methodology. The electronic attributes of the examined compounds are explained by investigating their energy bands, as well as the total and partial density of states. The computational evaluation of the electronic band structure reveals that both compounds exhibit an indirect band gap semiconductor behavior in the spin-down channel, while demonstrating metallic properties in the spin-up channel. The magnetic attributes indicate a ferromagnetic nature, thus categorizing some double perovskite compounds as materials displaying half-metallic ferromagnetism (HM-FM) in addition to some other properties such as metallic and semiconductor in paramagnetic or antiferromagnetic states. The outcomes derived from the analysis of elastic constants confirm the mechanical robustness of the studied double perovskite compounds. Notably, the computed data for bulk modulus (B), shear modulus (G), and Young’s modulus (E) for Ba2MgReO6 surpass those of Ba2YMoO6. The calculated ratio of Bulk to shear modulus (B/G) indicates that both compounds possess ductile characteristics, rendering them suitable for device fabrication. Furthermore, both compounds display outstanding electronic and elastic properties, positioning them as promising contenders for integration within mechanical and spintronic devices. Finally, we investigate into the thermoelectric potential by evaluating parameters such as the Seebeck coefficient, electrical conductivity, thermal conductivity, figure of merit, and power factor. This assessment is conducted using the semiclassical Boltzmann theory and the constant relaxation time approximation, implemented through the BoltzTraP code. The results indicate that the investigated double perovskite oxides hold promise for utilization in thermoelectric applications.

Thermoelectric properties of M[formula omitted]AgAlBr[formula omitted] (M = K, Rb, Cs) double perovskites: A first principles study

Structural, electronic, and transport properties of aluminum-based double halide perovskite M2AgAlBr6 (M = K, Rb, Cs) are here subject of investigation, using first-principles approaches based on density functional theory, as implemented in the plane-wave pseudopotential method. Thermoelectric performance was evaluated by analyzing the results of first-principles band structure calculations and semiclassical Boltzmann theory within the constant relaxation time approximation. The theoretically predicted values for the electronic contribution for the figure of merit are close to 1.0 at room temperature, suggesting that all compounds studied are promising candidates for applications in thermoelectric devices.

The thermal conductivity of ZrCo alloy under doping effect: First principles study

ZrCo alloy is considered to be one of the most promising materials for hydrogen isotope treatment in the International Thermonuclear Experimental Reactor (ITER) due to its significant advantages in dehydrogenation kinetics, thermodynamics, and safety. The heating during the dehydrogenation process, on the other hand, can easily induce H2 to penetrate the surrounding environment, resulting in hydrogen embrittlement and threatening structural safety. The system's temperature regulation is very critical. The electronic thermal conductivity of ZrCo was computed using the BoltzTraP2 program and the constant relaxation time approximation based on density functional theory (DFT). The phonon thermal conductivity is calculated by lattice dynamics using the finite displacement supercell method. Between 300 K and 800 K, the thermal conductivity and electrical conductivity of ZrCo alloy were effectively predicted for the first time. The effects of Ti and V doping on ZrCo alloy thermal conductivity were investigated. The results show that when temperature rises, phonon-electron and phonon-phonon scattering will be enhanced. Resistance and thermal resistance will also rise, while conductivity and thermal conductivity would decrease. The doping of Ti and V elements will reduce the electronic thermal conductivity of the system. Different concentrations of doping bring different degrees of defects, affecting the average electron-phonon free path, and thereby reducing thermal conductivity.

Transport properties of B and N sites vacancy defects graphene/h-BN heterostructures: First-principles study

First-principles calculations based on spin-polarized DFT-D2 approach have been carried out to study the transport properties of graphene/h-BN (G/h-BN) heterostructure (HS), 1 N vacancy defect in G/h-BN (G/h-BN_1 N), nearest neighbour of 1 N & 1B vacancy defects in G/h-BN (G/h-BN_nBN), and alternate zone of 1 N & 1B vacancy defects in G/h-BN (G/h-BN_aBN) materials. Transport properties like electrical conductivity (σ), electronic contribution of thermal conductivity (K), Seebeck coefficient (S) and thermoelectric power factor (P) are estimated through Boltzmann transport equations (BTE) within constant relaxation time approximation (RTA). Quantum ESPRESSO and BoltzTrap packages are used as a computational tool in the calculations. The considered materials are found to be stable van der Waals (vdWs) HS. The σ and K of graphene, h-BN, G/h-BN, G/h-BN_1 N, G/h-BN_nBN and G/h-BN_aBN are found to be increasing monotonically with temperature. The defected materials have slightly less value of σ than that of G/h-BN due to the effect of their effective mass and carrier concentration. At room temperature, K of G/h-BN, G/h-BN_nBN and G/h-BN_aBN retains similar value, while K of G/h-BN_1 N has increased slightly because the increase in Fermi velocity of electrons near Fermi level due to the reduction in effective mass thereby increasing the mean free path. We have analyzed the temperature dependent (constant value of energy) and energy dependent (constant value of temperature) S and P of above-mentioned materials, and found that S and P of defected HS are higher than that of non-defected HS materials due to combine dependence of n, T and m* on the value of S. The estimated S and P values of considered materials are consistent with reported value. Hence, they have budding employment in the thermoelectric devices.

A family of two-dimensional semiconductors with transition metal Kagome lattice, large power factor and ultralow lattice thermal conductivity

Two-dimensional (2D) materials hold promising application potential in the future, so it is worthy to explore new 2D materials with fascinating structures and functionals. Here we predict a series of 15 stable 2D semiconductors, A2B3C4 (A = K, Rb, Cs; B = Ni, Pd, Pt; C = S, Se), using first-principles calculations. The feasibility of mechanical exfoliation from their bulk phases has been confirmed by low cleavage energies. Interestingly, a perfect Kagome lattice formed by transition metal Ni/Pd/Pt atoms have been revealed in these fifteen monolayers. All A2B3C4 monolayers exhibit indirect band features (1.82 eV to 2.76 eV). More encouragingly, and it is theoretically proven, based on constant relaxation time (CRT) approximation, that the A2B3C4 monolayers possess large n-type power factors (1.77 ∼ 2.10 × 103 μWm-1K−2), which is mainly contributed from the loop formed by the conduction band minima in different directions in energy band structure. Furthermore, ultralow lattice thermal conductivity (0.20 Wm-1K−1 to 3.04 Wm-1K−1) has been confirmed in 2D A2B3C4 at room temperature. Finally, the thermoelectric figure of merit (ZT) of A2B3C4 monolayers has been evaluated based on CRT approximation. These interesting findings make A2B3C4 monolayers promising candidates for low-dimensional electronic devices.

First-principles-aided evaluation of the Nernst coefficient beyond the constant relaxation time approximation

The solution of the Boltzmann Transport Equation (BTE) under open voltage conditions and in the presence of simultaneously applied magnetic field and temperature gradient results in the so-called Nernst response of the electrons. The calculation of the Nernst coefficient using first-principles calculations as an extension of the BoltzWann code and under Jones–Zener expansion valid for weak magnetic fields has been the subject of our previous work which provided a general framework for the calculation of the Nernst effect but was not able to capture the experimental results due to the constant relaxation time approximation used. In Contrast to the Seebeck coefficient which is not sensitive to the details of the relaxation times, the Nernst coefficient is proportional to the carrier mobility and hence is greatly affected by the relaxation times. In this work, we focus on the inclusion of the energy-dependent electron–phonon and the electron–impurity relaxation times in our formalism. Using the developed formalism, we successfully reproduced the experimental data of several samples. In this paper, we report our results on Ge, Si, and InSb samples. However, the code is not limited to the reported samples and supports a wide range of materials.

First-Principles Studies on the Physical Properties of the Half Heusler RbNbCd and RbNbZn Compounds: A Promising Material for Thermoelectric Applications

This work focuses on study of the structural, electronic, thermodynamic and thermoelectric properties of RbNbCd and RbNbZn Half Heusler (HH), utilizing a full-potential linearized augmented plane wave (FP-LAPW) approach and the Boltzmann transport equation using a constant relaxation time approximation within the context of density functional theory (DFT) as embedded in the WIEN2k code. The structural analysis employed the generalized gradient approximation (GGA) and considered the Birch Murnaghan equation of state (EOS), which results in the stable phase for RbNbCd and RbNbZn. The positive phonon spectra indicate the dynamical stability of the studied RbNbCd and RbNbZn. The compounds under investigation that have no bandgap are metallic, as evidenced by their electronic properties. Their mechanical and thermal stability as well as their anisotropic and ductile character are confirmed by the various elastic and thermodynamic parameters. The lattice thermal conductivity has been calculated. This thorough analysis demonstrates the applicability of the studied RbNbCd and RbNbZn for thermoelectric applications.

Ab initio calculation of thermoelectric properties in 3d ferromagnets based on spin-dependent electron–phonon coupling

Crossed magneto-thermo-electric coefficients are central to novel sensors and spin(calori)tronic devices. Within the framework of Boltzmann’s transport theory, we calculate the resistivity and Seebeck coefficients of the most common 3d ferromagnetic metals: Fe, Co, and Ni. We use a fully first-principles variational approach, explicitly taking electron-phonon scattering into account. The electronic band structures, phonon dispersion curves, phonon linewidths, and transport spectral functions are reported, comparing with experimental data. Successive levels of approximation are discussed: constant relaxation time approximation, scattering for a non-magnetic configuration, then spin polarized calculations with and without spin–orbit coupling (enabling spin-flips). Spin polarization and explicit electron–phonon coupling are found to be necessary to reach a correct qualitative picture: the effect of spin flipping is substantial for resistivity and very delicate for the Seebeck coefficient. The spin-dependent Seebeck effect is also predicted.

Promising high temperature thermoelectric performance of layered oxypnictide YZnAsO

Thermoelectric materials can convert heat into electricity directly via the Seebeck effect, providing an alternative to global energy demands. Here we present mechanical, dynamical, and thermoelectric properties of YZnAsO, a quaternary oxypnictide, in the framework of Density Functional Theory. The low value of Debye temperature (θD=308.29K) and high value of the Grüneisen parameter (∼1.77) hint for low lattice thermal conductivity (kl), and this is supported by the computed value of ∼0.4 W/mK at 800 K. The electronic transport coefficients are computed using Boltzmann transport equation beyond the constant relaxation time approximation (CRTA) including different scattering mechanisms. While the power factor for hole doping is higher along ‘a’, the kl is extremely low along ‘c’, resulting in a high figure of merit (ZT) along ‘c’. Hence, we predict YZnAsO as a promising n-type oxide thermoelectric owing to its low kl with a high ZT =1.07 at 800 K with 15% maximum thermoelectric efficiency.

Efficiently predicting directional carrier mobilities in organic materials with the Boltzmann transport equation.

Describing charge carrier anisotropy in crystalline organic semiconductors with ab initio methods is challenging because of the weak intermolecular interactions that lead to both localized and delocalized charge transport mechanisms. Small polaron hopping models (localized) are generally used to describe materials with small charge carrier mobilities, while periodic band models (delocalized) are used to describe materials with high charge carrier mobilities. Here, we prove the advantage of applying the constant relaxation time approximation of the Boltzmann transport equation (BTE) to efficiently predict the anisotropic hole mobilities of several unsubstituted (anthracene, tetracene, pentacene, and hexacene) and substituted (2,6-diphenylanthracene, rubrene, and TIPS-pentacene) high-mobility n-acene single crystals. Several density functionals are used to optimize the crystals, and the composite density functional PBEsol0-3c/sol-def2-mSVP predicts the most experimentally similar geometries, adequate indirect bandgaps, and the theoretically consistent n-acene charge transport mobility trend. Similarities between BTE and Marcus mobilities are presented for each crystal. BTE and Marcus charge carrier mobilities computed at the same geometry result in similar mobility trends, differing mostly in materials with more substitutions or structurally complex substituents. By using a reduced number of calculations, BTE is able to predict anisotropic carrier mobilities efficiently and effectively for a range of high-mobility organic semiconductors.

ElecTra code: Full-band electronic transport properties of materials

This paper introduces ElecTra, an open-source code which solves the linearized Boltzmann transport equation in the relaxation time approximation for charge carriers in a full-band electronic structure of arbitrary complexity, including their energy, momentum, and band-index dependence. ElecTra stands for ‘ELECtronic TRAnsport’ and computes the electronic and thermoelectric transport coefficients electrical conductivity, Seebeck coefficient, electronic thermal conductivity, and mobility, for semiconductor materials, for both unipolar and bipolar (small bandgap) materials. The code uses computed full-bands and relevant scattering parameters as inputs and considers single crystal materials in 3D and 2D. The present version of the code (v1) considers: i) elastic scattering with acoustic phonons and inelastic scattering with non-polar optical phonons in the deformation potential approximation, ii) inelastic scattering with polar phonons, iii) scattering with ionized dopants, and iv) alloy scattering. The user is given the option of intra- and inter-band scattering considerations. The simulation output also includes relevant relaxation times and mean-free-paths. The transport quantities are computed as a function of Fermi level position, doping density, and temperature. ElecTra can interface with any DFT code which saves the electronic structure in the ‘.bxsf’ format. In this paper ElecTra is validated against ideal electronic transport situations of known analytical solutions, existing codes employing the constant relaxation time approximation, as well as experimentally well-assessed materials such as Si, Ge, SiGe, and GaAs. Program summaryProgram title: ElecTra – Electronic Transport simulation labCPC Library link to program files:https://doi.org/10.17632/ycgx2fjzb6.1Licensing provisions: GPLv3Programming Language: MATLAB®Nature of the problem: computing the electronic and thermoelectric charge transport coefficients of materials with arbitrary complex full-band electronic structures, considering the carrier energy-, momentum-, and band-dependence of the scattering rates.Solution method: Semiclassical Linearized Boltzmann transport equation, with electronic structures (DFT or analytical) as input, formed into constant-energy surfaces, with scattering rates evaluated using Fermi's Golden Rule.Additional comments including restrictions and unusual features:•Programming interface: any DFT code which saves data in the ‘.bxsf’ format.•RAM: a case study for a half-Heusler bandstructure on a 51×51×51k-mesh, 2 Gb per processor is needed•Running time: for the example above, depending on the number and complexity of the scattering mechanisms and the number of simulated Fermi levels and temperatures considered, the time needed varies from ∼ 1 hour on a desktop PC or laptop (light simulations), to 5-10 hours on an HPC with 30-45 cores (heavy simulations). Using the constant relaxation time and constant mean-free-path approximations on a desktop PC or laptop, the running time is of the order of minutes.

Thermal and electrical transport conductivities of novel ordered double two-dimensional MXenes via density functional theory

In this paper, we examined the effects of transition metal replacement and ordering in 2D M3C2O2 MXene on the electronic structure and thermal and electrical conductivities using the density functional theory and also Boltzmann’s transport theory within the constant relaxation time approximation. We derived the following oxygen-functionalized ordered double-metal MXenes Ti2YC2O2, Y2TiC2O2, Sc2YC2O2, and Y2ScC2O2 through the surface and sub-layer tuning. The results revealed that the ordered MXenes give rise to different electronic structure and transport properties. The electronic thermal conductivities obtained in terms of the relaxation time increase with increase in absolute temperature, while the electrical conductivities decrease with increase in temperature. The respective transport properties of Y2TiC2O2 and Sc2YC2O2 were found to be enhanced due to the increased squared band velocity, density of state, and low Seebeck coefficient compared to Ti2YC2O2 and Y2ScC2O2 MXenes. Generally, the MXene materials investigated in this work were found to possess metallic properties with the existence of energy overlap around the Fermi level. The computed formation energies support chemical stability in all the materials.

First-principles based computational framework for the thermal conductivity of complex intermetallics: The case study of MgZn2 and Mg4Zn7

Complex intermetallics usually exist as second phases in metal alloys. How these second phases can affect the thermal conductivity of alloys is generally unknown because the intrinsic thermal transport properties of these complex intermetallic compounds are quite less explored. In this work, we propose a computational framework based on first-principles calculations to study the electron and phonon thermal transport in complex intermetallics. Two typical intermetallics, i.e., MgZn2 and Mg4Zn7, are studied as prototypes. The rigorous mode-level first-principles calculations are first carried out to study the thermal transport of MgZn2. The calculations not only provide accurate thermal conductivity results, but also allow to prove that the constant relaxation time approximation and the Slack model work quite well in complex intermetallics. Then these two models are combined with first-principles calculations to predict the thermal transport properties for Mg4Zn7. Our results show that the directional average thermal conductivities for MgZn2 and Mg4Zn7 are 53.9 and 21.9 W/mK, significantly smaller than those of their elemental counterparts. Electrons are found to be the main heat carriers in these compounds, leading to a nearly temperature-independent thermal conductivity. Phonon thermal conductivity is negligible due to large unit cells and weak metallic bondings. Our work provides reliable thermal conductivity values for MgZn2 and Mg4Zn7. The computational framework developed in this work can also be further extended to study the electrical and thermal transport of other complex intermetallics.

Electronic and phonon contributions to the Thermoelectric properties of newly discovered half-Heusler alloys XHfPb (X= Ni, Pd, and Pt)

In this work we calculate the thermoelectric figure of merit of XHfPb (X= Ni, Pd, and Pt) by computing the both the power factor and the lattice thermal conductivity by first principles. We make reasonable approximations: we use the Constant Relaxation Time Approximation (CRTA) to compute the electron transport contribution and the modified Debye–Callaway model to calculate the thermal lattice conductivity. We also report the dielectric properties of these semiconductors and the mode Grüneisen parameters. Not surprisingly we find that the average Grüneisen coefficient correlates with the thermal conductivity. Next, we consider a realistic relaxation time τ and carrier concentration n from experimental data on ZrHfPb and obtain the figure of merit ZT as a function of temperature. Our main finding is that despite the Pt is isoelectronic with Ni and Pd, the ZT of PtHfPb is larger and behaves differently from the other two materials, suggesting that PtHfPb is better suited for high temperature thermoelectric generators.

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.