Structural Relaxation around Substitutional $\mathrm{Cr^{3+}}$ Ions in Sapphire

The local structural arrangement of ions around a substitutional Cr3+ ion in sapphire (ruby) has been studied experimentally using extended X-ray absorption fine structure (EXAFS) in the vicinity of the Cr absorption edge. The findings are compared with an ionic model (Mott-Littleton) computation using two sets of pairwise potentials. Both the EXAFS results and the computations reveal that when Cr substitutes for an Al ion in sapphire, the surrounding ions relax to an arrangement similar to that for Cr in α-Cr2O3. Also, most of the structural relaxation is accommodated by the first shell of oxygen and aluminum ions around 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