Low-temperature Raman study of zinc-doped lithium niobate crystal powders

Low-temperature (10 K) Raman scattering was used to examine powder samples ground from congruently grown Zn-doped lithium niobate crystal. The Raman lineshapes clearly showed three major variations in phonon frequency and half width, at the 279, 335, 580 and 633 cm^-1 Raman peaks. The change in phonon lineshapes is barely noticeable below 5 mol% Zn doping. Above this value of doping, the phonon frequency sharply decreased and the half width increased. Both reached extreme values at approximately 7.5 mol% doping. Phonon frequency dramatically increased and half width decreased at doping concentrations above 7.5 mol%. Based on the Li-vacancy model, we suggest that Zn atoms entered Li sites and fewer Nb atoms replace Li atoms at doping concentrations below 5 mol%. Above 5 mol% Zn doping, replacement of the Li atom by the Nb atom is unlikely. Zn atoms begin to enter the Nb sites at a concentration of approximately 7.5 mol%. The concentration of Li vacancies decreases rapidly at doping over 7.5 mol%.