Ab initio study of absorption and emission properties of BGO and BSO

Bismuth ortho-germanate (BGO) and bismuth ortho-silicate (BSO) crystals are described within an atomic cluster model. Electronic ground and first excited state for both crystal structures are calculated through unrestricted Hartree-Fock and configuration interaction methods, where the associated cluster geometries are optimized. The theoretical transition energies corresponding to the absorption and emission processes are very revealing. The displacement of one of the oxygen ions away from the bismuth in the excited state, together with the distributed spin density clearly shows that the excitation process cannot be understood as a deformed bismuth ion; excitation. Instead, a molecular-like excitation is proposed. (C) 1998 Elsevier Science Ltd. All rights reserved.