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 oxyge n induces the reduction of Bi3+ to metallic bismuth as well as the seg regation of BXO (X = Ge,Si) phases with a Bi/X ratio lower than the in itial one. Moreover, the thermal treatments in vacuum induce a bulk ef fect reflected in the growth of the optical absorption bands centred a t 750 nm and associated with Cr4+ in Bi12SiO20, Bi12GeO20 and Bi4Ge3O1 2 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 cr ystals exhibit a linear relation between the room temperature optical absorption at 750 nm, alpha(750)(r) and the total chromium concentrati on, [Cr]: alpha(750)(r) (cm(-1)) = 1.12 X 10(-17)[Cr] (cm(-3)). This i ndicates 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 f ully inhibits the photochromic effect induced by ultraviolet light. (C ) 1997 Elsevier Science Ltd. All rights reserved.