Vibrational spectroscopy of impurity-OH defects in insulating crystals

Abstract

Hydroxyl ion, a common impurity in insulating crystals, interacts with cation impurities, giving rise to new complexes. High resolution (0.04cm−1) FTIR spectroscopy is applied in the temperature range 9—300K to study the vibrational modes (stretching, bending, combination) of impurity-OH defects in alkali halides and fluoroperovskites. Proper thermal treatments and isotopic substitutions (i.e. 18OH, 16OD, 18OD, 16OT for 16OH) allowed the assignment of the vibrational mode lines to defects in which OH is embedded, and supplied models for them. Anharmonicity effects, monitored by weak overtone lines, are discussed in the framework of the Morse model. Electric anharmonicity is also reported. The phonon coupling of the OH-vibrational modes in different defects is studied by analyzing the temperature dependence of the line-position and width: in most cases the single phonon coupling model accounts for the experimental data and supplies the frequencies of the coupled phonons.