Anharmonicity in negative thermal expansion materials ZrW₂O₈ and ZrV₂O₇: Three-phonon interactions

This work investigates the anharmonic properties of ZrW₂O₈ and ZrV₂O₇, which are typical negative thermal expansion (NTE) materials, based on three-phonon interactions. The ultralow lattice thermal conductivity (LTC) is found, and the boundary mean free path (MFP) dependent scattering rate is explored. It is obtained that the scattering rate of both compounds decreases with increased boundary MFP. In comparison, the low-frequency phonons (< 2.0 THz) of ZrW₂O₈ have a higher scattering rate than that of ZrV₂O₇, while the scattering rate at other frequencies of ZrW₂O₈ is lower than that of ZrV₂O₇. Additionally, it is found that the phonon lifetime of both compounds decreases with elevated temperature due to stronger phonon couplings. The low-frequency phonons with significant negative mode-Grüneisen parameters contribute most to their NTE. Furthermore, the phonon scattering processes show that ZrW₂O₈ has a larger w-JDOS value than ZrV₂O₇, therefore ZrW₂O₈ has more available three-phonon scattering channels.