The electronic structures of crystals with interstitial O atoms were studied, within the framework of fully relativistic self-consistent Dirac-Slater theory, using a numerically discrete variation (DV-Xα) method. It was concluded from the calculations that interstitial O atoms combined with 2 formal lattice O ions to form O molecular ions which exhibited donor energy levels in the forbidden bands. The optical transition energy was 3.85eV, which correspond to the 324nm absorption bands. It was predicted that 324nm absorption bands originated from interstitial O in CaWO4 crystals.

First-Principles Study on Electronic Structures and Absorption Bands of CaWO4 Crystals with Interstitial Oxygen Atoms. Z.X.Shao, Q.R.Zhang, T.Y.Liu, J.Y.Chen: Physica Status Solidi A, 2007, 204[9], 3159-64