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

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. We have modeled the pentanary substituted TiNiSn by supercell approach with the aliovalent substitution, inspite of the traditional isoelectronic substitution. We have performed the detailed analysis of electronic structure, lattice dynamics, and TE transport properties of selected systems from this family. From our calculated band structures and density of states we show that by preserving the 18 VEC through aliovalent substitutions at 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 (κL), 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 Grüneisen parameter using the calculated elastic constant values. At high temperatures the calculated κL and ZT values from both these methods show very good agreement. The calculated κL values decreases from parent TiNiSn to pentanary substituted TiNiSn systems as expected due to fluctuation in atomic mass. The substitution of atoms with different mass creates more phonon scattering centers and hence lower the κL value. The calculated κL for Hf containing systems La0.25Hf0.5V0.25NiSn and non Hf containing system La0.25Zr0.5V0.25NiSn calculated from Phono3py (Slack’s equation) are found to be 0.37 (1.04) and 0.16 (0.95) W/mK, at 550K, 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 La0.25Hf0.5V0.25NiSn has more optical-acoustic band mixing which creates more phonon–phonon scattering and hence lower the κL value and maximizing the ZT. Based on the calculated results we conclude that one can design high efficiency thermoelectric materials by considering 18 VEC rule with aliovalent substitution.