Modeling of raman spectra of H : LiNbO3

Raman spectra of proton-exchanged LiNbO3 waveguides are analyzed in a simple small-cluster approximation. For this purpose, the Raman-active transverse optical modes of virgin LiNbO3 are modeled using a cluster of two Li, two Nb and six O atoms with imposed periodic boundary conditions. Then, the calculated modes of a cluster, in which one or both Li atoms are replaced by protons, are considered and compared with the polarized spectra of a highly protonated sample. The results show that the extra peaks at about 65 cm(-1) and in the range 500-700 cm(-1) observed in the Raman spectra of HxLi1-xNbO3 are directly due to formation of Nb-O-H groups into the Nb-O network. The additional Raman peaks, appearing in the spectral range 100-300 cm(-1) after proton exchange for x > 0.56, are due to breakdown of the symmetry of the bulk structure and appearance of new crystalline phases. (C) 2000 Published by Elsevier Science Ltd.