Ab initio temperature dependence of the thermal expansion of diamond and the frequency shift of optical phonons

The temperature dependences of the linear thermal expansion coefficient and the frequency shift of a triply degenerate optical phonon at the center of the Brillouin zone have been calculated in the density functional theory approximation and compared with the experimental data. Application of the static quasi-harmonic approximation for finding the temperature dependence of the unit cell volume and thermal expansion of a crystal has been discussed. It has been found that the contribution from the thermal shift of the phonon due to the thermal expansion is greater than that due to the anharmonic interaction of phonons.