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.