Determination of the lattice thermal conductivity of the TiO2 polymorphs rutile and anatase by molecular dynamics simulation

Abstract

In this study, the structure of the lattice thermal conductivity of two TiO2 polymorphs, namely rutile and anatase, is determined by using equilibrium molecular dynamics simulations in conjunction with the Green-Kubo formalism. The most reliable potential parameters are used to describe the interatomic potentials. The results are investigated over a wide temperature range, from 200 to 800 K, with a 100 K temperature step. As titanium dioxide is an anisotropic material, all of the parameters are investigated in both the a- and c-directions. The raw data for the heat current autocorrelation function is analysed to determine the structure of the lattice thermal conductivity. It is revealed that the lattice thermal conductivity of the TiO2 polymorphs can be decomposed into three contributions due to the acoustic short-range and long-range phonon and optical phonon modes. These three contributions can be presented in the form of simple kinetic formulae consisting of the products of the heat capacity, the square of the average phonon velocity and the average relaxation time of the acoustic short- and long-range phonon and optical phonon modes. In particular, it is shown that the average phonon velocities of the acoustic short- and long-range phonon and optical phonon modes are approximately equal to each other and can be expressed through the second-order fluctuations of the heat current vector. The effects of different simulation cell sizes at different temperatures on the lattice thermal conductivity are also investigated. Finally, the results from this work are compared with the experimental data and good agreement is found.