Investigation of the intracrystalline Cr3+ distribution in natural and synthetic alexandrites

The chromium distribution over octahedrally coordinated Al-I and Al-II site s in three synthetic and five natural alexandrites was examined by electron paramagnetic resonance measurements. In all samples Cr3+ predominantly sub stitutes the larger Al-II sites whereas a minor fraction of Cr3+ occupies t he smaller Al-I sites. The distribution ratio of Cr3+ over Al-II and Al-I s ites is approximately 3:1 and very similar for low Cr doping levels of Czoc hralski-grown alexandrites. The flux-grown alexandrite exhibits a ratio of approximately 2:1. Additional dopants like iron do not lead to a redistribu tion of Cr over the Al sites in the samples examined. With increasing chrom ium contents the Cr3+ percentage at Al-II sites increases in natural sample s. The intensity of the colour change (alexandrite effect) correlates with the total amount of chromium in a sample. An influence of Cr3+ at Al-I site s on the intensity of the colour change was not observed.