Thermal reduction of sillenite and eulite single crystals

Thermal treatments in vacuum produce surface modification of Bi12SiO20, Bi12GeO20 and Bi4Ge3O12 compounds. In this process the loss of oxygen induces the reduction of Bi3+ to metallic bismuth as well as the segregation of BXO (X = Ge,Si) phases with a Bi/X ratio lower than the initial one. Moreover, the thermal treatments in vacuum induce a bulk effect reflected in the growth of the optical absorption bands centred at 750 nm and associated with Cr4+ in Bi12SiO20, Bi12GeO20 and Bi4Ge3O12 single crystals. This behaviour is associated with the reduction of bulk Cr6+ and Cr5+ ions to Cr4+ by trapping the electrons released in the lattice after molecular oxygen loss. After reduction, sillenite crystals exhibit a linear relation between the room temperature optical absorption at 750 nm, alpha(750)(r) and the total chromium concentration, [Cr]: alpha(750)(r) (cm(-1)) = 1.12 X 10(-17)[Cr] (cm(-3)). This indicates that in sillenites after reduction most of the bulk chromium ions may be reduced to the 4+ state. The loss of Cr6+ and Cr5+ traps fully inhibits the photochromic effect induced by ultraviolet light. (C) 1997 Elsevier Science Ltd. All rights reserved.