Some concepts concerning twinning and cellular growth of bulk barium metaborate (BBO) crystals

摘要

Bulk single β-BaB_2O_4 (BBO) crystals have been grown by the TSSG method in Czochralski variant, using Na_2O and NaF as the solvents. It was found that formation of twins (electric type) or cellular substructures of different scales is their specific growth feature. We believe that their formation is governed by changes in the composition and structure of the double-electric layer (DEL) at the interface of crystal growth. In Z-axis crystals only microtwins structures were revealed whose number and localization are determined by the composition of used solvent. The cellular growth of these crystals at a particular stage is a result of the impossibility of frontal formation of an antipolar structure (macro twin) under the conditions of increasing charge density in the DEL. In the Y(X)-axis crystals the conditions for formation of one or three (five, etc.) macrotwin boundaries and, hence, noncellular growth are always realized. Model concepts, characterizing seeding and growth of Y(X)- and Z-axis BBO crystals are discussed.