Optical properties of BN in cubic and layered hexagonal phases

Linear optical functions of cubic and hexagonal BN have been studied within first principles density functional theory in the local density approximation. Calculated energy-loss functions show reasonable agreement with experiments and previous theoretical results both for h-BN and for c-BN. Discrepancies arise between theoretical results and experiments in the imaginary part of the dielectric function for c-BN. Possible explanations of this mismatch are proposed and evaluated: lattice constant variations, h-BN contamination in c-BN samples, and self-energy effects.