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Many crystals, such as lanthanum zirconate (La2Zr2O7), exhibit a flatteningtrend in thermal conductivity at elevated temperatures. This phenomenon hasrecently been attributed to the inter-band phonon tunneling (or diffuson)contribution using different formalisms. However, the contributions offinite-temperature corrections (e.g., higher-order phonon-scattering, phononrenormalization, and phonon scattering cross-section softening effects) remainunclear. In this work, we predict and compare the thermal conductivity ofLa2Zr2O7 using three distinct first-principles methods. The first method isGreen-Kubo molecular dynamics (MD) based on temperature-dependent machinelearning interatomic potentials (MLIPs) trained from ab initio MD simulations,which successfully predict the flattening trend at ultra-high temperatures. Thesecond method is the Peierls Boltzmann transport equation (BTE), within thephonon particle framework, using phonon lifetime that includes all thefinite-temperature corrections. Four-phonon scattering is found large but iscancelled by the phonon scattering cross-section softening effect. As a result,BTE with temperature corrections does not reproduce the flattening of thermalconductivity. The third method is Wigner formalism, which includes both phononparticle and wave contributions, which successfully reproduce the flattening ofthermal conductivity. Diffuson and phonon contribute about 67% and 28% ofthermal conductivity at 1800 K, respectively. The radiation contribution tothermal conductivity is around 5%. The scaling laws of the phonon, diffuson,radiation, and total thermal conductivity are found to be ~T^-1, ~T^0.4, ~T^2,and ~T^-0.4, respectively. This work clarifies the thermal transport mechanismsin La2Zr2O7 at ultra-high temperatures from different aspects.

On the flattening trend of thermal conductivity of La2Zr2O7 at ultra-high temperatures