THE GROWTH OF LARGE SINGLE-CRYSTALS OF THE ORGANIC NONLINEAR OPTICAL MATERIAL 2-(ALPHA-METHYLBENZYLAMINO)-5-NITROPYRIDINE

The crystal growth behaviour of the important organic nonlinear optical material 2-(alpha-methylbenzylamino)-5-nitropyridine (MBANP) (space group P2(1)) has been investigated and techniques have been developed for the preparation of large (up to 7 cm x 4 cm x 3 cm) high-quality single crystals suitable for detailed optical, dielectric, ultrasonic and piezoelectric studies and for device fabrication. Observation of the crystal growth behaviour and measurement of growth kinetics revealed a strong and anomalous dependence of growth rate on supersaturation. At relative supersaturations less than 0.08, slow growing irregular faces were formed. Once the crystal was completely bounded by such faces, little or no further growth occurred. At supersaturations below 0.24 no growth was observed in either [010] direction. Above this, rapid, but highly imperfect, growth occurred in one [010] direction while the opposite end of the crystal exhibited zero growth rate at all achievable supersaturations. At relative supersaturations greater than 0.4 considerable instability in the growth along [010] was observed. The factors potentially responsible for this behaviour are discussed. A novel technique is described for the preparation of seed crystals elongated parallel to [010]. The use of such seeds allows the growth of large crystals of MBANP at supersaturations less than 0.24, thereby avoiding the highly imperfect and unstable growth at higher supersaturations and yielding crystals of very high quality. The nature, density and distribution of growth induced defects in large crystals of MBANP have been investigated using X-rav diffraction topography. In addition to revealing growth dislocations, growth sector boundaries, inclusions and striations, these studies provide a detailed insight into the growth history of the crystals.