Investigation of structural peculiarities of impure lithium niobate crystals by Raman spectroscopy

Abstract

Polarized Raman spectra of lithium niobate single crystals doped with Mg 2+, Gd 3+, Y 3+ and Ta 5+ cations were studied. Threshold concentrations for Mg 2+ and Gd 3+ cations were shown to exist below which these dopants cause an ordering effect of the Li + and Nb 5+ cations and an octahedral alignment of the hollows along the crystal polar axis. This ordering can be seen in spectra as a narrowing of the 254 and 274 cm −1 modes corresponding to fundamental vibrations of the Li + and Nb 5+ ions located inside the oxygen octahedra and as a splitting into two components of the two-phonon line in the region of 100–120 cm −1 caused by the light scattering on two bound acoustic phonons of A 1 symmetry. Above the maximum concentrations Mg 2+ and Gd 3+ ions disorder the crystal structure, which leads to the loss of the fine structure for the two-phonon line whereas the 254 and 274 cm −1 modes considerably broaden and almost merge into one band. The Y 3+ and Ta 5+ ions worsen the ordering of the cations and the octahedral hollows along the polar axis and cause, in addition, deformation of the crystal oxygen carcass.