Combination bands of libration+vibration of OH/OD centres in ABO₃crystals

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.

Proton Tunneling: A Decay Channel of the O-H Stretch Mode inKTaO3

“…Thus, for the O-H stretch mode, the most probable accepting channel is the bend mode as its frequency is localized above the phonon spectrum [14]. We find the O-H bend mode at 1058 cm À1 [15], well above the highest bulk phonon frequency at $850 cm À1 [16], and for the O-D bend mode a frequency of approximately 755 cm À1 has been estimated, a result that is consistent with the position of the combination mode and a small anharmonic shift [17]. Since the excited local vibrational modes couple much stronger to other localized modes [18], we argue that the most likely decay channel of the O-H and O-D stretch mode involves three bend modes plus a phonon mode at 312 cm À1 .…”

Phonon spectra and electro-optic properties of crystalline phases in proton-exchanged LiTaO3 wave guides

“…lattice vibrational spectrum of LiTaO 3 , to a direct OHlibrational mode. Note that a direct OH-librational mode was previously observed at 960 cm Ϫ1 in proton-exchanged LiNbO 3 wave guides examined by Raman scattering 21 and IR reflection spectroscopy. 13 The OH-librational wave numbers measured in LiTaO 3 are in agreement with the results of a simple harmonic oscillator model.…”

“…On the other hand, the OHlibrational band can contain, at the same frequency, components either parallel or perpendicular to the c axis depending on whether it is an unstrained or axially strained LiTaO 3 lattice. These facts point to an energetically isotropic, threedimensional nature 21 of libration in these simple cases. Unlike these cases and as opposed to their counterparts in Z cut, PE wave guides in X-cut LiTaO 3 have nonzero off-diagonal strain components 17 and thus represent quite a specific case of nonaxial strain, which affects crystal symmetry and distorts chemical bonding.…”

“…͑3͒ and ͑4͒, each optical mode polarized along the Z axis in LiTaO 3 produces an individual contribution to the electro-optic coefficient r 33 , which is proportional to the product of the Raman scattering efficiency and infrared oscillator strength. Since the contribution of Ta-O bonds to the linear and nonlinear susceptibility is substantially larger than the corresponding contribution of Li-O bonds, 21 we can restrict our analysis to the fundamental A modes of the intrinsic vibrations of TaO 6 octahedra. It is important to note that the data reported previously 14,16,23,25 on the phonon spectrum assignment and point group symmetry of LiTaO 3 are mutually contradictory.…”

Interface and thickness dependent domain switching and stability in Mg doped lithium niobate

“…A new peak at 960 cm À1 appears and is assigned to OH -librational bands that are strongly dependent on the proton exchange phase. 54,55 Having assigned spectral characteristics, it is possible to monitor the influence of PE across the sample as well as strains indicated by peak broadening and position shift. 30 In Raman spectra (Figure 6(a)), extracted at different positions on the sample (red: 20 lm thick Mg:LN; blue: exposed PE area; green: 14.1 lm thick Mg:LN above 5.9 lm PE, as also approximately indicated by colored crosses in the intensity map, Figure 6(b)), this peak is unexpectedly visible as a shoulder on the Mg:LN stripe and is even more pronounced above the PE channel (spectral area of interest shaded in light blue).…”