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Problems of beta-BeB2O4 crystal growth for fabrication of large optical elements

Boundaries of the electric macrotwins (antipolar parts of crystal) have always been found in X- or Y-axis crystals. Boundaries are nearly planar and located in subseed zone of crystal along the (00.1) plane. In large crystals two more of such boundaries are usually found at a distance of 15 - 18 mm in both sides from the central boundary. In Z-axis crystals, boundaries of macrotwins have never been found. There are only microtwins up to a few mm in size and insuperable tendency to cellular breakdown across the entire growth interface of crystals after 15 - 30 mm of growth. When Na2O and NaF are used as a solvent, antipolar microtwins can be found near macrotwin boundaries or cellular growth zone and in the whole volume of crystals, respectively. In the latter case, crystals are stressed and show a partial fracture along the (00.1) plane. Misoriented blocks and low-angular boundaries can be found at the growth stages later than seeding and growing up to the required sections. Most of the inclusions in BBO crystals are of flux type with a partially void space and a soli state core. If reactions of synthesis had not been finished before melting, enormous amounts of CO2 and probably some amounts of OH were detected, along with near eutectical alignment of Ba and Na in the core of inclusions. Possible causes of formation of the above-mentioned features in BBO structure are discussed.