SITES AND VALENCIES OF CHROMIUM IN BISMUTH GERMANATES - A MAGNETIC CIRCULAR-DICHROISM AND ABSORPTION STUDY

Bismuth germanate exists as Bi4Ge3O12 (BGO 2:3) which is used as a scintillator material and Bi12GeO20 (BGO 6:1) which is photorefractive. We present a magnetic circular dichroism (MCD) and absorption study of the two varieties doped with Cr. The site and valency of Cr in the 2:3 crystal is found to depend upon whether the doping is done with a piece of highly Cr-doped BGO 6:1 or with chromium oxide. In the first case, we provide very clear evidence that Cr3+ lies at the Bi site in a high field environment. In the second, Cr enters the lattice as Cr4+, at both the Ge and the Bi sites. The MCD of Cr-doped BGO 6:1 also depends upon the history of the sample investigated. We suggest that Cr enters this lattice as Cr5+ (Ge site) and Cr4+.