FTIR SPECTROSCOPY TO MONITOR OH-IMPURITY INTERACTIONS IN INSULATING CRYSTALS

Hydroxyl ion is a common impurity in insulating crystals: by interacti ng with other impurities, it gives rise to new complexes. The OH-stret ching frequency is a very sensitive probe of the hydroxyl environment. High resolution (0.04 cm(-1)) FTIR spectroscopy in the temperature ra nge 9-300 K was applied to study the OH-cation impurity interaction in alkali halides, fluoroperovskites, and sillenites, suitable for optoe lectronic applications. Proper thermal treatments and isotopic substit utions allowed to assign the stretching mode absorption lines to the d efects 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 mode l. Electric anharmonicity was also detected. The phonon coupling of th e OH--stretching mode in different defects was studied by analysing th e temperature dependence of the line-position and -width: in most case s the single-phonon coupling model accounted for the experimental data and supplied the coupled phonon frequencies.