First-principles study of the BiMO4antisite defect in the Bi12MO20(M=Si, Ge, Ti) sillenite compounds

Structural, electronic and optical properties of the antisite BiMO4 defect in Bi12MO20 sillenites (BMO, M=Si, Ge, Ti) were investigated using density functional theory. The defect is studied in neutral, positively and negatively charged states. It is demonstrated that within the neutral defect the Bi 6s(2) lone pair is broken and the valence state of the Bi is 4+ (6s(1)). Within the charged defects, the Bi 6s orbital is found to be either full (Bi(3+): 6s(2)) or empty (Bi(5+): 6s(0)). All three charged states introduce energy bands within the BMO gap. By analyzing possible transitions between them we deduced a simple model of functioning of the BiMO4 defect that is able to explain the photochromic and photorefractive effect in sillenites and that reproduces almost all known experimental facts.