Vibrational properties of OH groups associated with divalent cations in corundum (α-Al2O3)

Abstract

The infrared spectra of synthetic corundum (α-Al2O3) samples either doped directly with divalent cations (Mg2+) or containing divalent cations formed by reduction of trivalent cations in H2 gas (Co2+, Ni2+) may display broad OH stretching bands at ∼3000 cm-1 due to the structural incorporation of trace amounts of hydrogen. Experimental spectra recorded from some natural sapphires display a similar absorption band associated with a dominant absorption at 3161 cm-1, and some beryllium-diffused corundum crystals show a band at 3060 cm-1. All of these also display smaller and generally narrower bands between 1900 and 2700 cm-1, whose natures are poorly defined. In this work, the atomic-scale structure, relative stability and infrared spectroscopic properties of a series of OH defects in corundum (α-Al2O3) are theoretically investigated at the density-functional-theory level. The investigated defects consist of interstitial H+ ions forming OH groups and compensating for the charge imbalance related to the presence of divalent cations (Be2+, Mg2+, Cr2+, Mn2+, Fe2+, Co2+, Ni2+) substituted for Al3+ at nearby octahedral sites. Bands occurring at ∼3000 cm-1 in experimental spectra are assigned to the OH stretching modes of some of these defects, with bands observed around 1900 and 2700 cm-1 being assigned to overtones of corresponding OH bending modes. The results also support the assignment of the so-called "3161 cm-1 series", observed in experimental spectra of some rubies and yellow sapphires, to structural OH groups in association with Fe2+ ions, rather than Si4+, as has been previously proposed. These inferences are also supported by analysis of correlations between band areas in experimental infrared spectra extracted from a database of corundum gemstones. A qualitative explanation relating the anomalous intensity and the polarisation properties of the OH bending overtone bands to the electrical anharmonicity of OH groups involved in medium-strength H bonds is proposed.