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

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.