Study on the Anharmonic Interaction in Negative Thermal Expansion Compounds Ag₂O and Cu₂O by Three-Phonon Scattering

The anharmonicity of negative thermal expansion (NTE) compounds Ag₂O and Cu₂O is investigated by three-phonon scattering based on density functional theory. It is confirmed that the low-energy phonons are closely associated with the NTE behavior. Moreover, Ag₂O with more remarkable negative Grüneisen parameters has stronger anharmonicity than Cu₂O. The lattice thermal conductivity (LTC) is systematically studied, combining the correlation of LTC with temperature, frequency, and phonon mean free path. The results show that the LTC of Cu₂O has a higher value and faster decrease rate than that of Ag₂O. Phonons below 2.0 THz in Ag₂O and below 10.0 THz in Cu₂O can lead to rapid accumulation of LTC and contribute to the majority of LTC. The phonon lifetime is the only quantity in the expression for LTC that is directly affected by phonon anharmonicity. It is found that the low-frequency phonons of both compounds have an overwhelmingly long phonon lifetime, which leads to a more significant accumulation of LTC at low frequencies. To understand the role of the selection rules and three-phonon scattering phase space to the anharmonicity, we calculate the frequency-dependent joint density of states (JDOS) and weighted joint density of states (w-JDOS) and find that the recombination event of phonons in both compounds tends to occur at low-frequency region where acoustic phonons play an important role.