Role of Optical Modes in Thermal Conduction

In contrast to acoustic modes, optical lattice modes have non-zero frequencies in the long wave limit. The cases considered here are when the optical modes have low group velocities and are well separated in frequency from the acoustic modes. The optical modes can contribute to the lattice thermal conductivity, but they can also interact anharmonically with acoustic modes, and thus reduce the acoustic mode conductivity. Point defects can strongly reduce the optical mean free path because the low group velocity implies a high mode density. Approximate expressions for these effects will be given, and their magnitude estimated in the case of TiO 2 . Point defects in stabilized ZrO 2 scatter optical modes strongly, so that they have a minimum mean free path, and their contribution to thermal conduction is small. The anharmonic interaction between optical and acoustic modes has also been studied through the widths of optical absorption lines and of Raman lines due to optical lattice modes at the zone center (long wave limit). These processes will be discussed . This information may be of use in the study of lattice thermal conductivity of crystals and ceramics at high temperatures.