STRUCTURAL RELAXATION AROUND SUBSTITUTIONAL CR3+ IONS IN SAPPHIRE

The local structural arrangement of ions around a substitutional Cr3ion in sapphire (ruby) has been studied experimentally using extended X-ray absorption fine structure (EXAFS) in the vicinity of the Cr abso rption edge, The findings are compared with an ionic model (Mott-Littl eton) computation using two sets of pairwise potentials, Both the EXAF S results and the computations reveal that when Cr substitutes for an Al ion in sapphire, the surrounding ions relax to an arrangement simil ar to that for Cr in alpha-Cr2O3. Also, most of the structural relaxat ion is accommodated by the first shell of oxygen and aluminum ions aro und the substituted Cr3+ ion. The computations also indicate that with applied pressure (tensile and compressive) the ionic positions change self-similarly and in proportion to the macroscopic strain.