Mechanistic insights into the weakly temperature-dependent thermal conductivity of La₂Zr₂O₇ crystals through quartic...

Crystals with intrinsically low thermal conductivity (κ) are essential for thermoelectric, thermal barrier, and thermal management applications. Here, taking La₂Zr₂O₇ crystals as a representative system, we investigate the microscopic origin of the weakly temperature-dependent thermal conductivity via advanced anharmonic lattice dynamics driven by a machine-learning potential. By incorporating temperature-dependent quartic anharmonicity to renormalize the phonon spectrum, we quantitatively evaluate its effect on phonon group velocities and relaxation times. Contrary to previous ground-state anharmonic lattice dynamics studies which attributed the flat high-temperature κ of La₂Zr₂O₇ to wave-like phonon tunneling, our results demonstrate that particle-like phonon propagation within the Peierls-Boltzmann framework plays the leading role. It is also found that fully renormalized quartic anharmonicity enhances particle-like phonon propagation through limiting three-phonon and four-phonon scattering while suppressing wave-like phonon tunneling by reducing the contribution of off-diagonal diffusons, synergistically reproducing the experimental flat low κ of La₂Zr₂O₇ in both magnitude and temperature dependence. This work reconciles the discrepancy between experimental and predicted flat low κ in crystals, providing a distinct physical picture for phonon thermal transport and critical insights into tailoring the thermal transport properties of complex oxides through lattice anharmonicity.