Abstract
Abstract. 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. Its 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.
Highlights
Corundum (α-Al2O3) is an important refractory mineral which forms in a large variety of natural environments ranging from the primitive solar system to the Earth lithosphere (e.g., Bowles et al, 2011)
The defect modeling was performed within the density functional theory (DFT) framework using the generalized gradient approximation (GGA) to the exchange-correlation functional as proposed by Perdew, Burke and Ernzerhof (PBE functional; Perdew et al, 1996)
As usually observed in DFT modeling performed at the GGA level, the theoretical cell lengths are overestimated with respect to their experimental counterparts (a = b = 4.7589 Å, c = 12.991 Å; Newnham and de Haan, 1962)
Summary
Corundum (α-Al2O3) is an important refractory mineral which forms in a large variety of natural environments ranging from the primitive solar system to the Earth lithosphere (e.g., Bowles et al, 2011). Natural gem-quality ruby and sapphire, whose color is related to Cr or Fe and Ti impurities, are emblematic corundum varieties of cultural and trading importance (e.g., Muhlmeister et al, 1998; Smith, 1995; Rossman, 2009). Corundum is an oxide ceramic with important industrial uses related to its mechanic, dielectric and optic properties. To this respect, occurrence of trace quantities of hydrogen in alumina deserves particular attention because it can affect the transport properties, mechanical strength and response to irradiation of the material (e.g., Engstrom et al, 1980; Ramírez et al, 1997a, b, 2004; Kronenberg et al, 2000)
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