Contribution of optical phonons to lattice thermal conductivity in complex structural thermal insulation materials

It has generally been believed that the lattice thermal conductivity (kappa(L)) is mainly contributed by acoustic phonons and the contribution of optical phonons can be neglected in crystalline materials. However, in this work, we demonstrate that the contribution of optical phonons play an important role in the complex structural low-kappa(L) oxides. The mode-wised kappa(L) of three representative oxides, MgO, BaHfO3 and La2Zr2O7, with different structural complexity was calculated using first-principles phonon Boltzmann transport equation (BTE) method. It is found that the contribution of optical phonons become significant as the structural complexity increases. Especially, for La2Zr2O7, the kappa(L) is dominated by optical phonons due to the large density of states. Optical phonons exhibit amorphous-like phonon-phonon scattering characteristics that are different from those of acoustic phonons. Furthermore, calculations for materials other than the three representative oxides were conducted and the results confirmed the validity of our conclusions.

Automated summary

The contribution of optical phonons in complex structural low thermal conductivity oxides, especially in La2Zr2O7, was found to be significant. As structural complexity increases, the contribution of optical phonons becomes more pronounced. This highlights the importance of considering optical phonon contributions in understanding lattice thermal conductivity in crystalline materials.