Cr+3 Distribution in Al1 and Al2 Sites of Alexandrite (BeAl2O4: Cr3+) Induced by Annealing, Investigated by Optical Spectroscopy

Abstract

In order to investigate optical properties of alexandrite, the present work deals with the influence of thermal annealing on optical absorption and luminescence spectra of natural samples. The exposure time to heat treatment at 1000oC is taken into account. Possible migration of Cr3+ ions from Al1 (inversion site) to Al2 (reflection site) is detected. Sample composition is obtained through Scanning Electron Microscopy (SEM) measurements and points to a rearrangement of Cr+3 and Fe3+ ions in the alexandrite crystalline structure, un-der thermal annealing influence. This feature may be used to control the optical properties of natural alexan-drite, which can be associated to the observed laser emission effect.