Anomalous temperature dependence of the OH− absorption band in stoichiometric LiNbO3 crystals

Infrared absorption spectra of the stretching vibration of OH− and OD− ions in stoichiometric LiNbO3 crystal have been measured in the temperature range 10-310 K. The band parameters, halfwidth and position, have been determined with high accuracy by assuming quasi-Voigt line shapes. Anomalous behaviour of the OH− band position has been observed and interpreted by phonons coupled to the stretching vibration with coupling constants of alternate signs.