FTIR Spectroscopy to Monitor Impurities and Impurity-Interactions in Sillenite Crystals

High resolution (0.04 cm^–1) absorption spectroscopy is applied at 9 K to study the role played on the vibrational spectra by impurities (Im), as Si⁴⁺, Ge⁴⁺, P⁵⁺, V⁵⁺, Mn⁵⁺, and S⁶⁺, in Bi₁₂SiO₂₀ (BSO), Bi₁₂GeO₂₀ (BGO), and Bi₁₂TiO₂₀ (BTO) sillenite single crystals. Up to three-phonon transitions (overtones of the F mode and combination of F and A modes) are detected for the impurities in ImO₄ tetrahedra. Weak anharmonicity effects are monitored. The presence of additional Im impurities modify the lines due to a) the stretching of OH, typical of the undoped material, and b) the multi-phonon transitions either of the intrinsic MeO₄ (Me = Si, Ge, and Ti) and of the ImO₄ tetrahedra. New bands are monitored due to the Im-OH interaction.