Ultralow Thermal Conductivity and High-Temperature Thermoelectric Performance in n-Type K2.5Bi8.5Se14

We studied the narrow band-gap (0.55 eV) semiconductor K 2.5 Bi 8.5 Se 14 as a potential thermoelectric material for power generation. Samples of polycrystalline K 2.5 Bi 8.5 Se 14 prepared by spark plasma sintering exhibit exceptionally low lattice thermal conductivities (κ lat ) of 0.57–0.33 W m –1 K –1 in the temperature range of 300–873 K. The physical origin of such low κ lat in K 2.5 Bi 8.5 Se 14 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 K 2.5 Bi 8.5 Se 14 sample is a single phase without intergrowth of the structurally related K 2 Bi 8 Se 13 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 K 2.5 Bi 8.5 Se 14 is an indirect band-gap semiconductor with multiple conduction bands close to the Fermi level. Phonon dispersion calculations suggest that K 2.5 Bi 8.5 Se 14 has low phonon velocities and large Grüneisen parameters that can account for the observed ultralow κ lat . 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.