Effects of nonstoichiometry and doping on the Curie temperature and defectstructure of lithium niobate

LiNbO3 crystals doped with Gd, Zn, Cu, and Er to various levels were grown by the Czochralski technique. The structural parameters, Curie temperature, and density of the LiNbO3(Gd) crystals were determined as a function of co mposition. With increasing Gd content, the a parameter of the hexagonal cel l increases, c remains constant within the experimental error, density incr eases, and Te gradually decreases. The defect structure and properties of L iNbO3 doped with Gd, Zn, Cu, Er, IMg, and Ta are contrasted with those of n ominally undoped LiNbO3 crystals differing in Li/Nb ratio. The results sugg est that the variation of the Curie temperature: with composition is determ ined primarily by the position that the dopant cations occupy in the struct ure of lithium niobate.