Crystal growth of the acentric organic non-linear optical material methyl-p-hydroxybenzoate (MHB): morphological variations in crystals grown by physical vapour transport.

Single crystals of the acentric compound Methyl-p-hydroxybenzoate were grown by self-nucleation and seeded growth from the vapour phase by the Physical Vapour Transport (PVT) process. In the temperature range 80 – 95 °C, (nucleation supersaturation 0.97 to 0.88) all crystals were of the polymorphic form as produced by room temperature solution growth. Self-nucleated crystals varied in macro-morphology from columnar to octahedral to skewed octahedral and finally to skewed columnar but retained the same crystal forms indicated by theoretical calculations. Micro-morphological studies of growth faces indicated that these variations result from changes in growth mechanisms which influence both the defect structure and perfection of the growing crystal. X-ray topographic studies confirmed that growth under the most ideal conditions when the dominant faces of the crystals were growing by a dislocation induced BCF (Burton, Cabrera and Frank) mechanism, yielded the structurally most perfect crystals. Preliminary s...