Multiple Converged Conduction Bands in KBiSe: A Promising Thermoelectric Material with Extremely Low Thermal Conductivity.

We report that KBiSe exhibits multiple conduction bands that lie close in energy and can be activated through doping, leading to a highly enhanced Seebeck coefficient and a high power factor with elevated temperature. Meanwhile, the large unit cell, complex low symmetry crystal structure, and nondirectional bonding lead to the very low lattice thermal conductivity of KBiSe, ranging between 0.42 and 0.20 W m K in the temperature interval 300-873 K. Experimentally, we further support the low thermal conductivity of KBiSe using phonon velocity measurements; the results show a low average phonon velocity (1605 ms), small Young's modulus (37.1 GPa), large Grüneisen parameter (1.71), and low Debye temperature (154 K). A detailed investigation of the microstructure and defects was carried out using electron diffraction and transmission microscopy which reveal the presence of a KBiSe minor phase intergrown along the side of the KBiSe phase. The combination of enhanced power factor and low thermal conductivity results in a high ZT value of ∼1.3 at 873 K in electron doped KBiSe material.