Defects associated with Sn (±Ti) and H in corundum (Al2O3)

The mechanisms by which H is incorporated into corundum crystals containing elevated concentrations of Sn is studied using a combination of experimental infrared spectra and density functional theory (DFT) calculations. The spectra (both bulk and spatially resolved), along with trace element concentrations, were recorded from samples passing through the laboratories of the Gemological Institute of America. We propose that bands in the FTIR spectra of corundum at 3197, 3235, 3249, 3262 and 3285 cm^-1 can all be attributed to OH stretching associated with defects including one or two Sn⁴⁺, H⁺ and Al-site vacancies. Some of the defects also include Ti⁴⁺, i.e. are mixed Sn-Ti defects. Specifically, we attribute the following bands to these defects: 3197 and 3235 cm^-1: ${\left({\mathrm{Sn}}_{\mathrm{Al}}^{•}{V}_{\mathrm{Al}}^{″\prime }{\mathrm{OH}}_o^{•}\right)}^{\prime }$ (two configurations); 3249 cm^-1: ${\left({\mathrm{Sn}}_{\mathrm{Al}}^{•}{\mathrm{Sn}}_{\mathrm{Al}}^{•}{V}_{\mathrm{Al}}^{″\prime }{\mathrm{OH}}_o^{•}\right)}^{\times }$; 3262 and 3285 cm^-1: ${\left({\mathrm{Sn}}_{\mathrm{Al}}^{•}{\mathrm{Ti}}_{\mathrm{Al}}^{•}{V}_{\mathrm{Al}}^{″\prime }{\mathrm{OH}}_o^{•}\right)}^{\times }$ (two configurations). All bands show similar polarization behavior, with the strongest absorption when the electric vector is perpendicular to the c-axis.