Sharp, temperature dependent OH/OD IR-absorption bands in nearly stoichiometric (VTE) LiNbO 3

Abstract

In comparison with congruent LiNbO 3 the OH/OD IR-absorption bands in the bulk of nearly stoichiometric LiNbO 3 (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 = 3.25cm −1), and of OD (RT at 2561.9cm −1, FWHM = 2.05cm −1), as well as the next higher vibrational transition for OH ( RT: v ̄ 02 = 6745.1 cm −1 , FWHM = 10.0cm −1) and for OD ( RT: v ̄ 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) LiNbO 3, 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 ≈ 30 21 cm −1 ) and nearly temperature independent as in proton/deuteron exchanged congruent LiNbO 3. Tempering of the proton exchanged nearly stoichiometric (VTE) crystals at 350 °C in dry N 2-atmosphere shows the diffusion of protons from the PE-layer into the bulk of the VTE-LiNbO 3 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.