Vibration uncoupling of germanium with different valence states lowers thermal conductivity of Cs2Ge3Ga6Se14

Abstract

The thermal phonon transport is a key matter for heat managing in materials science which is crucial for device miniaturization and power density increase. Herein, we report the synthesis, structure and characterization of a new compound, Cs2Ge3Ga6Se14, with a unique anisotropic structure simultaneously containing Ge3+ and Ge2+ that adopt (Ge1)3+2Se6 dimer or (Ge2)2+Se6 octahedron, respectively. The thermal conductivity was measured to be 0.57–0.48 W m−1 K−1 from 323 to 773 K, the lowest value among all the known Ge containing compounds, approaching its glass limit according to the Cahill’s formulation. More importantly, we discover for the first time that the vibration uncoupling of Ge with different valence states hinders the effective thermal energy transport between the (Ge1)3+2Se6 dimer and (Ge2)2+Se6 octahedron, and consequently lowers the thermal conductivity. In addition, we propose a structure factor fi = sin(180 − β) × dGe−Q/li (i = A, B), with which a structure map of the Cs2Ge3M6Q14 family is given.