SHARP, TEMPERATURE-DEPENDENT OH/OD IR-ABSORPTION BANDS IN NEARLY STOICHIOMETRIC (VTE) LINBO3

In comparison with congruent LiNbO3 the OH/OD IR-absorption bands in the bulk of nearly stoichiometric LiNbO3 (prepared by the VTE technique) are about an order of magnitude narrower at room temperature and show a further narrowing to low temperatures. The fundamental stretching absorption band of OH, polarized completely perpendicular to the ferroelectric z-axis (RT at 3465.9 cm(-1) FWHM = 3.25 cm(-1)), and of OD (RT at 2561.9 cm(-1), FWHM = 2.05 cm-l), as well as the next higher vibrational transition for OH (RT:(v) over bar(02) = 6745.1 cm(-1), FWHM = 10.0 cm(-1)) and for OD (RT: (v) over bar(02) = 5025.5 cm(-1), FWHM = 6.5 cm(-1)) and their position, halfwidth and integrated absorption are measured as a function of temperature in the range from RT to LHeT. Electrical and mechanical anharmonic contributions are discussed on the basis of a Morse-type potential for a diatomic oscillator. In contrast to the absorption of the OH/OD centers in the bulk of nearly stoichiometric (VTE) LiNbO3, the absorption of OH/OD in proton/deuteron exchanged surface layers (at 3508/2589 cm(-1), polarized completely perpendicular z) in the VTE-treated crystals is as broad (FWHM approximate to 30/21 cm(-1)) and nearly temperature independent as in proton/deuteron exchanged congruent LiNbO3. Tempering of the proton exchanged nearly stoichiometric (VTE) crystals at 350 degrees C in dry N-2-atmosphere shows the diffusion of protons from the PE-layer into the bulk of the VTE-LiNbO3 crystal. The integrated optical density of the total OH absorption stays approximately constant for the first 8-10 h of the tempering treatment, indicating about equal oscillator strength for both (bulk and surface layer) absorption bands.