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

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 κ_lat of Ag₃SnP₇ is low, at approximately 1.5 W K⁻¹m⁻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 κ_lat from SCPH calculation corresponds with that of experimentally obtained κ_lat, 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 κ_lat vs C_pv_m² (the specific heat is C_p, the mean speed of sound is v_m) is very small at 0.24 ps, beyond that of Bi₂Te₃ and other phosphides. Results show that the low value of κ_lat originates from a short phonon lifetime caused by the anharmonic vibration of Ag at the 4f site.