Antisite defect as rule for photorefractive, photochromic and photocatalytic properties of Bi12MO20 (MÂ =Â Ge, Si, Ti) sillenite crystals

Electronic, optical and energetic properties of antisite BiMO4 defect in the Bi12MO20 (M = Ti, Ge, Si) sillenite crystals have been investigated by means of the first-principles calculations on density functional theory level. The defect is studied in neutral, positive and negative charge state, simulating situations of the capture of an electron or a hole. On the basis of obtained results, it is shown that various applicable characteristics of sillenites originate from the presence of antisite defect within the host. It is demonstrated that (1) antisite defect satisfies conditions to be responsible for photorefractive effect, (2) its dominant presence either in neutral or in charged states defines the thermal state of sillenites (coloured or bleached) and explains the reversible transitions between them, (3) its presence causes the photochromic effect, and (4) its presence or absence determines the range of photocatalytic applicability of sillenites. Presented theoretical model of functioning of antisite defect is found to be consistent with all experimental facts.