Temperature dependence of the refractive index of diamond up to925 K

We have measured the refractive index n of diamond for temperatures up to 925 K from Fabry-Perot resonances in far-infrared transmission spectra. For the highest temperatures the data approach the regime where n varies almost linearly with T. In contrast to other investigations, this makes a precise extrapolation to $T=0 \mathrm{K}$ possible and allows us to determine the refractive-index renormalization $\ensuremath{\Delta}n$ due to electron-phonon interaction and thermal expansion as well as an estimate of isotope effects on n. These data and values obtained for the thermo-optic coefficient $[(1/n)(\ensuremath{\partial}n/\ensuremath{\partial}T)]$ are compared to those of previous measurements and results from different theoretical approaches, such as empirical pseudopotential and ab initio calculations.