gems, corundum, synthetic corundum.7, spectroscopy, FTIR

The atomic-scale structure, relative stability and infrared spectroscopic properties of OH defects in corundum (α-Al₂O₃) are theoretically investigated at the density functional theory level. Comparison with experimental data makes it possible to assign most of the narrow bands observed between 3150 and 3400 cm⁻¹ in natural and Ti- or V-doped synthetic corundum to specific defects. These defects correspond to the association of one OH group with an Al vacancy and M⁴⁺ for Al³⁺ substitutions in neighboring sites. The OH group is located in the large oxygen triangle forming the base of the vacant Al site. Models of interstitial proton associated with a nearby Mg²⁺ for Al³⁺ substitution are consistent with the broad band observed at 3010 cm⁻¹ in Mg-doped corundum. It is also suggested that two weaker OH-stretching bands observed in nominally pure synthetic corundum at 3163 and 3209 cm⁻¹ could be associated with intrinsic defects combining an Al and an O vacancy. These results highlight the importance of defect clustering in the high-temperature incorporation of hydrogen in nominally anhydrous minerals.