Morphology of β-BaB₂O₄ (BBO) in relation to its crystal structure and growth conditions

A periodic bond-chain (PBC) analysis has been made of the structure of β-BaB₂O₄ (BBO) and related to crystal growth conditions, in order to explain the sometimes tangerine-shaped, sometimes needle shaped crystals that are observed when BBO is grown experimentally. The periodic bond chains generate eight-connected nets in BBO. The order of morphological importance (MI) of these F faces is determined by comparing the nearest-neighbour interaction ionic bond energies. The results are compared with experimental data. Six faces, sometimes observed parallel to the c axis of BBO, were identified as the hexagonal prism {110}. Two other forms are either the trigonal pyramids {012} and {01} or the trigonal pyramids {102} and {01}. {0001} is an important cleavage plane. The tangerine habit is a result of the strong temperature gradient, which is often used when growing BBO. The isotherms of the furnace impose their habit on the growing crystal.