FTIR Spectroscopy to Monitor OH-Impurity Interactions in Insulating Crystals

Hydroxyl ion is a common impurity in insulating crystals: by interacting with other impurities, it gives rise to new complexes. The OH-stretching frequency is a very sensitive probe of the hydroxyl environment. High resolution (0.04 cm^_1) FTIR spectroscopy in the temperature range 9–300 K was applied to study the OH-cation impurity interaction in alkali halides, fluoroperovskites, and sillenites, suitable for optoelectronic applications. Proper thermal treatments and isotopic substitutions allowed to assign the stretching mode absorption lines to the defects in which OH^– is embedded and to supply possible models for them. Anharmonicity effect of the OH^– -stretching modes, monitored by weak overtone lines, were well described in the framework of the Morse model. Electric anharmonicity was also detected. The phonon coupling of the OH^–-stretching mode in different defects was studied by analysing the temperature dependence of the line-position and -width: in most cases the single-phonon coupling model accounted for the experimental data and supplied the coupled phonon frequencies.