Thermal reduction of sillenite and eulite single crystals

Abstract

Thermal treatments in vacuum produce surface modification of Bi 12SiO 20, Bi 12GeO 20 and Bi 4Ge 3O 12 compounds. In this process the loss of oxygen induces the reduction of Bi 3+ 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 Cr 4+ in Bi 12SiO 20, Bi 12GeO 20 and Bi 4Ge 3O 12 single crystals. This behaviour is associated with the reduction of bulk Cr 6+ and Cr 5+ ions to Cr 4+ 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, α r 750 and the total chromium concentration, [Cr]: α 750 r (cm −1) = 1.12 × 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 Cr 6+ and Cr 5+ traps fully inhibits the photochromic effect induced by ultraviolet light.