Ilmenite-like stacking defect in nonstoichiometric lithium niobate crystals investigated by Raman scattering spectra

The Raman spectra of three pure lithium niobate (LiNbO3) crystals with different content of Li and several doped LiNbO3 (Mg, In and Ti) crystals have been investigated. A weak peak at about 738 cm(-1) reported as a very strong Raman peak of ilmenite phase LiNbO3 was observed for nonstoichiometric LiNbO3 crystals. The dependence of this peak on Li content and doping concentration in crystal suggested that there is a new kind of defect structure which has ilmenite-like stacking and direct relationship with antisite Nb-Li(4+) ion. This result experimentally proved the existence of ilmenite-like stacking defect in nonstoichiometric LiNbO3 for the first time. This defect is in good agreement with the Li vacancy model and has a different structure from that proposed by Smyth [Ferroelectrics 50, 93 (1983); Proc. IEEE, p. 115 (1986)]. (C) 2000 American Institute of Physics. [S0021-8979(00)00907-5].