Modeling of raman spectra of H : LiNbO3

Raman spectra of proton-exchanged LiNbO3 waveguides are analyzed in a simpl e small-cluster approximation. For this purpose, the Raman-active transvers e 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 ca lculated 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-xN bO3 are directly due to formation of Nb-O-H groups into the Nb-O network. T he additional Raman peaks, appearing in the spectral range 100-300 cm(-1) a fter 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 Publi shed by Elsevier Science Ltd.