Spectroscopic investigations of sharp Oh/Od Ir-absorption bands in nearly stoichiometric (VTE) LiNbO₃

Abstract

A narrow stretching absorption band of OH/OD, polarized completely perpendicular to the ferroelectric z-axis, (at RT for OH at 3465.9 cm⁻¹, FWHM = 3.25 cm⁻¹ and for OD at 2561.9 cm⁻¹, FWHM = 2.05 cm⁻¹) is observed in the bulk of nearly stoichiometric LiNbO₃ (prepared by the VTE technique [1]). These bands are about an order of magnitude more narrow than the corresponding bands in the bulk of congruent LiNbO₃ at RT. The position, halfwidth and integrated absorption of the fundamental as well as the next higher vibrational transition for OH and OD are measured as a function of temperature (RT - LHeT). Electrical and mechanical anharmonicities are discussed on the basis of a Morse-type potential for a diatomic oscillator.

Additional OH and OD absorption bands [2] for OH at 4417 cm⁻¹ and for OD at 3271 cm⁻¹ in the bulk of nearly stoichiometric LiNbO₃, are shown to belong to a libration-vibration combination band. The OH libration was also observed directly at 960 cm⁻¹ in Raman scattering and the OD libration at 698 cm⁻¹ in IR-reflection measurements.

The stretching absorption of OH in proton exchanged surface layers was observed in the VTE treated stoichiometric crystal to be as broad as (FWHM ≈30 cm⁻¹, polarized completely perpendicular z) in congruent LiNbO₃:PE. In both materials (congruent and stoichiometric) this structural broadered OH absorption band in the PE-layer is nearly independent of temperature.