Effect of Bi³⁺, Ti⁴⁺ and Ta⁵⁺ doping on the thermoelectric properties of KSr₂Nb₅O₁₅: A first-principles investigation

The enhancement of thermoelectric characteristics in tungsten bronze structural materials is contingent upon the attainment of a high figure of merit (ZT) value. The first-principles calculations are used to investigate the thermoelectric characteristics of KSN doped with Bi³⁺, Ti⁴⁺ and Ta⁵⁺. The findings indicate that the electrical conductivity of KSN can be enhanced more effectively by doping Bi³⁺ that replacing K⁺ due to the larger disparity in valence electrons between Bi³⁺ and K⁺. When KSN is doped with Ti⁴⁺ and Ta⁵⁺, the thermal conductivity of KSN is decreased, especially for KSN-Ti, it is shown to fall from 1.643 W/m/K to 0.906 W/m/K at a temperature of 1200 K. That is due to the presence of Ti⁴⁺ and Ta⁵⁺ in the framework site B of the KSN structure results in significant lattice distortion, and the distortion is particularly pronounced when Ti⁴⁺ is introduced as dopants. Consequently, stronger phonon scattering occurs, the structure becomes more unstable due to the presence of defects. Simultaneously, the presence of Ti⁴⁺ leads to the generation of oxygen vacancies, resulting in a notable increase in the scattering interactions between phonons and defects, thus reducing the average free path of phonons, thereby greatly reducing the thermal conductivity of the material as a result. Following the process of cation doping, the electrical conductivity of KSN is enhanced while its thermal conductivity is lowered, resulting in a notable increase in the ZT. Notably, the ZT of KSN experienced a significant rise from 0.434 to 0.586 (1200 K) with the introduction of Ti⁴⁺ doping. This study offers theoretical insights on enhancing the thermoelectric efficiency of KSN.