Multiple Converged Conduction Bands in K₂Bi₈Se₁₃: A Promising Thermoelectric Material with Extremely Low Thermal Conductivity

We report that K₂Bi₈Se₁₃ 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 K₂Bi₈Se₁₃, ranging between 0.42 and 0.20 W m^-1 K^-1 in the temperature interval 300-873 K. Experimentally, we further support the low thermal conductivity of K₂Bi₈Se₁₃ using phonon velocity measurements; the results show a low average phonon velocity (1605 ms^-1), 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 K_2.5Bi_8.5Se₁₄ minor phase intergrown along the side of the K₂Bi₈Se₁₃ 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 K₂Bi₈Se₁₃ material.