INCORPORATION OF HYDROGEN IN CUBIC AND UNIAXIAL OXIDIC CRYSTALS DEDUCED FROM POLARIZED RAMAN-SCATTERING

Polarized Raman scattering investigations of the ir OH stretch mode vi bration have been performed in a variety of prototype oxidic crystals, KTaO3, SrTiO3, K1-xLixTaO3, TiO2, LiTaO3, with the aim of providing i nformation on the sites of the incorporation of hydrogen. The traditio nally used polarized ir absorption is limited by ensemble averages and its lacking sensitivity to crystallographic orientations of defects. Contrarily Raman scattering is sensitive to the symmetry of the dynami cal mode. The problem of extracting information on a single defect fro m an ensemble is treated in the behavior-type theory, which has been d eveloped recently for defects in cubic crystals and was extended to te tragonal and trigonal crystals in the course of these investigations. The principles of the method and the extensive tables are summarized. We describe an experimental method for determining the precise optical alignment of the scattering geometry. In the cubic phase of the perov skites SrTiO3 and KTaO3 with isotropic ir absorption, the Raman data a llow us to reject one out of three models, where the protons vibrate b etween the two next-nearest oxygen ions along the cubic axes, by exclu ding the related C2[100] symmetry. In tetragonal SrTiO3 the absorption band splits into three components. In the second model the protons vi brate between the O2- and the Sr2+ ions on the face diagonal of the cu be, in the last along the edges of the oxygen octahedron between the t wo nearest-neighbor oxygen ions. The second model can be rejected, bec ause the claimed vertical mirror plane sigma(010) is not observed in t he Raman spectra. The third model claims no mirror symmetry because of the tilting of the octahedra. In K1-xLixTaO3, x=0.023, a polar tetrag onal phase occurs due to the off-center freezing of the Li(K) ions alo ng the polar axis. Again three subbands of the OH mode occur. But the bands, which reflected the breakdown of the sigma(010) symmetry in SrT iO3, do not occur in K1-xLixTaO3. The observed vertical mirror plane s igma(010) reflects the nontilted arrangement of the octahedra in K1-xL ixTaO3. In TiO2 (rutile) the Raman data reveal a C1h(001) symmetry for the prominent OH stretch mode. This firmly establishes the model, whe re the protons vibrate in the center of the open channels between the next-nearest O2- ions in the basal plane. In trigonal LiTaO3 the cryst al symmetry is so low that hardly any models can be discriminated on t he basis of their symmetry, because most of them deal with C1 symmetry . This is detected in the experiment and thus any orientation not alon g the trigonal axis or the vertical mirror plane is compatible with th is result. Still one model proposed for LiNbO3 can be excluded.