Abstract
Single crystals of the acentric compound methyl-p-hydroxybenzoate were grown by self-nucleation and seeded growth from the vapor phase by the physical vapor transport (PVT) process. In the temperature range of 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 macromorphology from columnar to octahedral to skewed octahedral and finally to skewed columnar but retained the same crystal forms indicated by theoretical calculations. Micromorphological studies of growth faces indicated that these variations result from changes in growth mechanisms that 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 Burton, Cabrera, and Frank mechanism, yielded the structurally most perfect crystals. Preliminary studies of seeded growth were performed as a prelude to using PVT for the growth of larger crystals. The seeded growth followed a different pattern of supersaturation dependence. All crystals showed the same asymmetric growth along the polar axis that has come to be regarded as characteristic of these highly polar acentric materials when grown from solution.
Highlights
The requirement for highly efficient nonlinear optical (NLO) materials for use in the fabrication of optical switching and amplification devices in the areas of optical processing and communication has led to the investigation of the potential of organic materials for this purpose.[1−3] The essential structural requirement for the materials to be useful is that the molecules should crystallize into an acentric crystallographic form: that the material should be both structurally and chemically stable in the region of temperature over which the optical devices would be expected to operate
Much effort has been expended on the formulation of the required molecular and structural basis for high nonlinear optical performance.[4−6] This has resulted in the realization that the basic requirements are fulfilled by highly polar organic molecules when assembled into the acentric structure, the highest efficiencies of operation being achieved when the constituent molecules are mutually aligned in very specific orientations.[4]
The morphology of crystals of MHB grown from the vapor phase follow the general pattern of unidirectional growth that has become expected of this class of structurally acentric solids
Summary
The requirement for highly efficient nonlinear optical (NLO) materials for use in the fabrication of optical switching and amplification devices in the areas of optical processing and communication has led to the investigation of the potential of organic materials for this purpose.[1−3] The essential structural requirement for the materials to be useful is that the molecules should crystallize into an acentric crystallographic form: that the material should be both structurally and chemically stable in the region of temperature over which the optical devices would be expected to operate. This relationship for MHB was confirmed by assessing the polarity of the pyroelectric current developed on heating a morphologically well-defined crystal and from the absolute configuration studies described above.[40] This indicated that the fast-growing faces were negative and that the slow and even possibly zero growth faces were positive.
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