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). As a nominally anhydrous mineral, corundum is known to incorporate some amount of hydrogen under the form of structural OH groups, which are readily observed using infrared spectroscopy (e.g., Eigenmann and Günthard, 1971; Volynets et al., 1972; Beran, 1991; Smith, 1995; Wöhlecke and Kovács, 2001; Libowitzky and Beran, 2006). The hydrogen concentration is however very low, typically below 0.5 wt. ppm of H2O in natural corundum (Beran and Rossman, 2006). Corundum is also 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).
The infrared spectra of synthetic or natural corundum often display a combination of well-resolved OH-stretching bands between 3150 and 3400 cm−1 (Eigenmann and Günthard, 1971; Volynets et al., 1972; Beran, 1991; Moon and Phillips, 1991, 1994; Kronenberg et al., 2000; Ramírez et al., 1997a, b, 2004; Libowitzky and Beran, 2006). All these bands are pleochroic with a dominant polarization parallel to the (001) plane (Engstrom et al., 1980; Moon and Phillips, 1994; Ramírez et al., 2004). Experimental studies of natural and synthetic corundum have suggested that the prominent bands at 3310, 3230 and 3185 cm−1 are associated with titanium ions occurring in the corundum structure as a tetravalent chemical impurity (Moon and Phillips, 1991, 1994). Protons bound to oxygen atoms and structural tetravalent cations substituted in nearby Al sites are expected to locally compensate for the electrostatic charge deficit due to an aluminum vacancy. Importantly, the clustering scheme of tetravalent impurities around the Al vacancy is expected to modify the OH-stretching frequency. Following Moon and Phillips (1991, 1994), the 3310 cm−1 band would be related to OH defects associated with two Ti4+ ions while the 3230 and 3185 cm−1 bands would correspond to association with a single Ti4+ ion. Relative OH band intensities have thus been used to infer Ti diffusion properties and clustering equilibria as a function of temperature in α-Al2O3. Weaker bands are also observed at 3209, 3296 and 3366 cm−1 (Kronenberg et al., 2000; Ramírez et al., 2004). In a V-doped synthetic sample, prominent bands are observed at 3183.9, 3229.4, 3278.3 and 3291.5 cm−1, and a weaker band is observed at 3382 cm−1 (Ramírez et al., 2004). The relative intensity of the OH-stretching bands in Ti- and V-doped samples experimentally treated at high temperature (1400 K) depends on their cooling rate (Ramírez et al., 2004), which is consistent with the record of a temperature-dependent distribution of OH defects displaying different clustering schemes. In addition, a broader band extending from 2900 to 3100 cm−1 with a dominant polarization in the direction perpendicular to the (001) plane is observed in Mg-doped samples (Volynets et al., 1972; Ramírez et al., 1997b). Broad bands with similar properties have been reported at 3025 and 2972 cm−1 in Co- and Ni-doped samples, respectively (Eigenmann and Günthard, 1971).
Although these experimental studies brought strong constraints for the interpretation of OH-stretching infrared spectra of corundum, a full picture in terms of local geometry of the defects has not been obtained. In addition, some of the above-listed bands are observed in nominally “pure” synthetic samples (e.g., Turner and Crawford, 1975; Engstrom, 1980; Ramírez et al., 2004), and their relation to specific impurities or defects occurring at very low concentration levels may prove to be uncertain. To this end, a theoretical approach linking the microscopic structure of the defects to their spectroscopic properties could be useful, as attested by theoretical investigations of OH groups in nominally anhydrous silicates (e.g., Wright, 2006; Balan et al., 2017, 2020; Blanchard et al., 2017; Jollands et al., 2020). Previous theoretical studies have focused on the association of hydrogen with intrinsic defects in corundum (Zhang et al., 2014; T-Thienprasert et al., 2017). Based on theoretical vibrational frequencies, T-Thienprasert et al. (2017) concluded that the OH-stretching bands observed at ∼3200-3300 cm−1 in corundum were consistent with the association of H and Al vacancies.
In the present study, the microscopic structure of a series of defect models associating H atoms with clustered Al vacancies and Ti4+ or V4+ cations is theoretically determined, and their spectroscopic properties are compared to available experimental data. The results confirm the dominance of this type of defects in the infrared spectra of natural and synthetic corundum and shed light on the nature of some debated bands.