Structural relaxation around substitutional Cr sup / 3+ / ions in sapphire

The local structure arrangement of ions around a substitutional Cr sup / 3+ / ion in sapphire (ruby) is studied experimentally using EXAFS near the Cr absorption edge. Results reveal that when Cr substitutes for an Al ion in sapphire, the surrounding ions relax to an arrangement similar to that for Cr in alpha -Cr sub 2 O sub 3 . Most of the structural relaxation is accommodated by the first shell of O and Al ions around the substituted Cr sup / 3+ / ion. With applied pressure (tensile and compressive), the ionic positions change self-similarly and in proportion to the macroscopic strain. 38 refs.