The growth and perfection of organic non-linear optical crystals: N-methyl urea (NMU) from methanol solution. I. Kinetics and growth anisotropy

Abstract

As a preliminary to the preparation of large single crystals of N-methyl urea (NMU) for evaluation and use as a non-linear optical material, we have examined the growth characteristics of this material from solution in methanol using an interferometric technique. NMU shows high growth anisotropy which cannot be linked to the interaction of the solvent with the growing surfaces. We find no experimental evidence to suggest that the methanol molecules have either a blocking or decelerating effect on step movement on all crystal faces. The principal limitation to growth is volume diffusion. This influence and its variation from face to face is shown to increase with the kinetic coefficient of the crystal face and probably represents the dominant cause of the growth anisotropy.