Temperature-dependent FTIR spectroscopy of OH defects in Verneuil-grown corundum (α-Al2O3)

The temperature dependence of the infrared absorption spectra of two Verneuil-grown corundum samples is investigated in the OH stretching range. The spectra display three main bands at 3184, 3232 and 3309 cm^{− 1}, belonging to the so-called "3309 cm^{− 1} series", as well as two additional bands at 3163 and 3278 cm^{− 1} previously reported in some synthetic corundum samples. The anharmonic behavior of the observed bands is analyzed using the pure dephasing model of Persson and Ryberg and depends on the local geometry of the OH defects, which are all associated with Al vacancies. A weak band at 3209 cm^{− 1} displays anomalous intensity changes with temperature which support a revised interpretation of both the 3209 and 3232 cm^{− 1} bands. The two bands are interpreted as resulting from the low-temperature equilibrium between two Ti-associated OH defects, enabled by the possibility of hydrogen quantum tunneling within the Al vacancy. The temperature-dependent properties of the 3278 cm^{− 1} band are similar to those of the other Al-vacancy related defects and a comparison with the theoretical properties of selected OH defects suggests that this band corresponds to the association of the H atom with a non-dissociated Al Frenkel pair. Finally, the properties of the band at 3163 cm^{− 1} are consistent with its previously proposed association with Si for Al substitution in corundum.