Anharmonic vibration of Ag atom in low lattice thermal conductivity chain structure phosphide Ag3SnP7

Abstract

This study investigates the phonon-transport properties of transition metal phosphide Ag3SnP7, experimentally and theoretically. Polycrystalline Ag3SnP7 is synthesized using a chemical vapor transport technique with iodine as the transport agent. The experimental lattice thermal conductivity κlat of Ag3SnP7 is low, at approximately 1.5 W K−1 m−1. The phonon properties of Ag3SnP7 are calculated using self-consistent phonon (SCPH) calculation with a fourth-order interatomic force constant (IFC). The value of κlat from SCPH calculation corresponds with that of experimentally obtained κlat, demonstrating that using a fourth-order IFC is important for phonon transport in the Ag3SnP7 system. The shape of the energy potential of Ag at a 4f site is distinctly non-parabolic, suggesting that atomic-Ag vibration originates from the anharmonicity of phonon modes in the Ag3SnP7 system. The estimated phonon lifetime in Ag3SnP7 at 300 K using a logarithmic plot of κlat vs Cpvm2 (the specific heat is Cp, the mean speed of sound is vm) is very small at 0.24 ps, beyond that of Bi2Te3 and other phosphides. Results show that the low value of κlat originates from a short phonon lifetime caused by the anharmonic vibration of Ag at the 4f site.