COMBINATION BANDS OF LIBRATION PLUS VIBRATION OF OH/OD CENTERS IN ABO(3) CRYSTALS

The presence of hydrogen (deuterium) defects in oxide ABO(3) crystals is revealed by a characteristic OH (OD) stretching vibration near 3500 cm(-1) (2600 cm(-1)). Low-frequency OH (OD) librational modes have not been reported so far for these crystals. However, we have found in a number of ABO(3) crystals (LiNbO3, LiTaO3, SrTiO3 and KTaO3) weak high-energy sidebands to the stretching vibration with positions shifted by about 950 cm(-1) for OH and about 700 cm(-1) for OD from the vibrational band position. The bandshape of these new OH/OD absorption bands is similar to the bandshape of the OH/OD absorption bands of the stretching vibration in the corresponding ABO(3) crystals. The polarization of these sidebands in general is different from the stretching vibration and reflects more the properties of a three-dimensional oscillator. While the OH/OD stretching vibrations in LiNbO3 and LiTaO3 are completely polarized perpendicular to the ferroelectric z-axis, the libration + vibration combination bands exhibit in both crystal systems an intensity ratio I-parallel to/I-perpendicular to similar to 0.55 for OH and an intensity ratio I-parallel to/I-perpendicular to similar to 0.75 for OD with respect to the ferroelectric z-axis. These different intensity ratios for OH and OD support the identification of the new absorption bands as libration + vibration combination bands of the OH (OD) oscillator. The OH/OD libration + vibration combination bands have also been detected as sidebands to higher vibrational transitions in proton- (deuteron-) exchanged LiNbO3. In a first approximation the energetic positions of the observed OH and OD combination bands can be well described with a simple oscillator model for the libration. The intensity of these sidebands is directly proportional to the intensity of the stretching vibration (or the concentration) of the H (or D) centres.