CRYSTALLOGRAPHY OF SURFACE NUCLEATION AND EPITAXIAL-GROWTH OF LITHIUMTRINIOBATE ON CONGRUENT LITHIUM-NIOBATE

The decomposition of congruent lithium niobate (LN) crystals proceeds by surface nucleation and growth of LiNb3O8 precipitates which exhibit an epitaxial orientation relationship. The same orientation relations hip is observed on x-, y-, and z-cut LN substrates. The epitaxy arises from similarities between the two crystal structures and provides for an essentially continuous oxygen-ion framework from parent to product . On y-cut LN, the precipitates have a well-defined habit plane, and t he interfacial misfit between the two structures is accommodated by a rectangular grid of misfit dislocations. The density, geometry, and im aging behavior of the misfit dislocations suggest that their Burgers v ectors serve to accommodate the disregistry in the oxygen-ion framewor k. Based on these observations, it is concluded that the mechanism of formation of LiNb3O8 consists of preferential nucleation on the surfac e and subsequent growth, possibly by counterdiffusion of cations in a closed system where the oxygen-ion framework remains essentially fixed in space.