First principle design of new thermoelectrics from TiNiSn based pentanary alloys based on 18 valence electron rule

TL;DR

This paper explores the design of new thermoelectric materials based on TiNiSn alloys using the 18 valence electron rule, analyzing their electronic, lattice, and transport properties to optimize thermoelectric performance.

Contribution

It introduces a first-principles approach to designing thermoelectrics via aliovalent substitution maintaining 18 VEC, tuning band gaps and reducing thermal conductivity.

Findings

Lattice thermal conductivity decreases with atomic mass fluctuation.

Certain substituted systems achieve ZT values up to 0.77 at 550 K.

Optical-acoustic phonon mixing enhances phonon scattering, lowering thermal conductivity.

Abstract

In this study, we have reported electronic structure, lattice dynamics, and thermoelectric (TE) transport properties of a new family of pentanary substituted TiNiSn systems using the 18 valence electron count (VEC) rule. From our calculated band structures and density of states, we show that by preserving the 18 VEC through aliovalent substitutions at the Ti site of TiNiSn semiconducting behavior can be achieved, and hence one can tune the band structure and band gap to maximize the thermoelectric figure of merit (ZT) value. Two approaches have been used for calculating the lattice thermal conductivity ($\kappa_{L}$), one by fully solving the linearized phonon Boltzmann transport (LBTE) equation from first$-$principles anharmonic lattice dynamics calculations implemented in Phono3py code and other using Slack's equation with calculated Debye temperature and Gr\"{u}neisen parameter using the calculated elastic constant values. The calculated $\kappa_{L}$ values decrease from parent TiNiSn to pentanary substituted TiNiSn systems as expected due to fluctuation in atomic mass. The calculated $\kappa_{L}$ for Hf containing systems La$_{0.25}$Hf$_{0.5}$V$_{0.25}$NiSn and non Hf containing system La$_{0.25}$Zr$_{0.5}$V$_{0.25}$NiSn calculated from Phono3py (Slack's equation) are found to be 0.37 (1.04) and 0.16 (0.95) W/mK, at 550\,K, respectively and the corresponding ZT value are found to be 0.54 (0.4) and 0.77 (0.53). Among the considered systems, the calculated phonon spectra and heat capacity show that La$_{0.25}$Hf$_{0.5}$V$_{0.25}$NiSn has more optical$-$acoustic band mixing which creates more phonon$-$phonon scattering and hence lower the $\kappa_{L}$ value and maximizing the ZT. Based on the calculated results we conclude that one can design high-efficiency thermoelectric materials by considering the 18 VEC rule with aliovalent substitution.