Compositional dependence of cationic displacements in lithium niobate and lithium tantalate crystals

Cationic displacements in both lithium niobate (LN) and lithium tantalate (LT) crystals are quantitatively studied by using the measured crystallographic data and the bond valence model. On the basis of the structural analysis by the bond valence model, it is found that intrinsic defects in both LN and LT crystals lead to local contractions around NbLi4+/TaLi4+-□Li sites and the decrease of Nb5+/Ta5+-Li+ distances. Therefore, with the increasing Li concentration in both crystals Li+ and Nb5+/Ta5+ cations begin to leave each other. Cationic locations in the LN crystallographic frame at room temperature and high temperatures (up to those near the ferroelectric Curie temperature) are also studied, respectively, which strongly support our present conclusions.