Ultralow Thermal Conductivity and High-Temperature Thermoelectric Performance in n-Type K_2.5Bi_8.5Se₁₄

We studied the narrow band-gap (0.55 eV) semiconductor K2.5Bi8.5Se14 as a potential thermoelectric material for power generation. Samples of polycrystalline K2.5Bi8.5Se14 prepared by spark plasma sintering exhibit exceptionally low lattice thermal conductivities (κlat) of 0.57–0.33 W m–1 K–1 in the temperature range of 300–873 K. The physical origin of such low κlat in K2.5Bi8.5Se14 is related to the strong anharmonicity and low phonon velocity caused by its complex low symmetry, large unit cell crystal structure, and mixed occupancy of Bi and K atoms in the lattice. High-resolution scanning transmission electron microscopy studies and microanalysis indicate that the K2.5Bi8.5Se14 sample is a single phase without intergrowth of the structurally related K2Bi8Se13 phase. The undoped material exhibits an n-type character and a figure-of-merit (ZT) value of 0.67 at 873 K. Electronic band structure calculations indicate that K2.5Bi8.5Se14 is an indirect band-gap semiconductor with multiple conduction bands close to the Fermi level. Phonon dispersion calculations suggest that K2.5Bi8.5Se14 has low phonon velocities and large Grüneisen parameters that can account for the observed ultralow κlat. The degree of n-type doping can be controlled by introducing Se deficiencies in the structure, providing a simple route to increase the ZT to ∼1 at 873 K.