Decomposition of a Solid Solution on the Surface of Lithium Niobate Crystals: Structure, Morphology, and Mutual Orientation of Phases

Abstract

Lithium niobate is a colorless transparent crystal belonging to the trigonal system [1]. Congruent LiNbO3 crystals are grown from the melt by the Czochralski method. The composition of the most perfect of these, most frequently used in technology, is not stoichiometric relative to the molar concentrations of lithium and niobium. A lithium deficit in the amount of ~1.4 mole % occurs in them [2]. The chemical formula of the crystals is more accurately written as Li0.945NbO3 (deviation of the composition from stoichiometric in oxygen is unknown and is taken as zero). This peculiarity, as well as the dependence of the width of the region of homogeneity for the solid solutions based on lithium niobate as a function of temperature (Fig. 1), leads to identification of a second monoclinic phase LiNb3O8 upon cooling grown crystals below the limit of the monophasic region or during their heat treatment [3, 4]. Decomposition of supersaturated solid solutions is studied in detail for metallic alloys [5, 6] and much less for crystals of complicated compounds.