No abstract

The fundamental vibrational transition nu 01 of the higher vibrational transitions nu 02, nu 03 and nu 04 and the OH and the OD oscillator in the bulk of congruent LiNbO3 and in proton/deuteron exchanged LiNbO3:PE/DE surface layers are investigated with respect to their spectroscopic properties and their temperature dependence. All observed vibrational transitions are completely polarized perpendicular with respect to the ferroelectric z direction. The spectral positions of the vibrational transitions follow closely a Morse type potential with De=4.75 eV and a=22.23 nm-1 ( omega e,OH=3684 cm-1 and Omega exe,OH=88.5 cm-1 for LiNbO3:PE). No hint of possible hydrogen bonding was found. The integrated absorption intensity ratios of the higher vibrational transitions exhibit the existence of an electrical anharmonicity, i.e. a non-linearity of the dipole moment versus O-H separation. The electrical anharmonicity reveals a noticeable temperature dependence whereas the vibrational transition energies and therefore the mechanical anharmonicity shows no temperature dependence in the range from room temperature to liquid helium temperature.

No abstract

The presence of hydrogen (deuterium) defects in oxide ABO3 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 ABO3 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 ABO3 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/sub ////Iperpendicular to approximately 0.55 for OH and an intensity ratio I/sub ////Iperpendicular to approximately 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.

No abstract