We report on comprehensive, systematic investigations into structural disorder in nominally pure lithium niobate (LN) and lithium tantalate (LT) crystals with various Li/Nb(Ta) ratios and in congruent LN crystals doped with Mg~(2+) (0.01-0.75 wt percent), Zn~(2+) (0.1-0.85 wt percent), B~(3+) (0.08-0.12 wt percent), Gd~(3+) (0.002-0.44 wt percent), Y~(3+) (0.24-0.46 wt percent), Er~(3+) (0.09-3.02 wt percent), or Ta~(5+) (1.0-5.0 wt percent). The cation type, positions, and distribution over octahedra have been shown to form structure-unit order in cation sublattices along the polar axis and to significantly govern crystal properties. Techniques for controlling crystal stoichiometry are described, and a model is advanced to determine the position of doping cations in the structure. Results on the stability of electrophysical and optical characteristics of nominally pure and doped LN crystals in the temperature range of application (300-350 K) are presented. Tools are proposed for studying the creation of a steady single-domain state in LT crystals. Raman spectra are proposed as a measure of the degree to which a crystal is a single domain. Experimental results reported here can be used in the technology of high-quality crystals for optical applications.
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