Urea Crystals Grown by Physical Vapour Transport
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C. PAORICI*, L. ZANOTTI**, M. ZHA** AND P. FRANZOSI** * University of Parma, 43100 Parma, Italy ** MASPEC Institute, Via Chiavari 18/a, 43100 Parma, Italy
ABSTRACT A new facility for growing single crystals of urea by physical vapour transport in effusive cells is reported. The best crystals, as characterised by X-ray diffraction topography, are largely dislocation-free and do not contain microprocipitates.
1. INTRODUCTION Urea is an organic material with a relatively high second-harmonic-generation (SHG) coefficient. Its transparency range (down to 200 nm), large birefringence and high optical damage threshold make this material a good candidate for non linear optics (NLO) applications in a spectral region in which few NLO materials are available [1-4]. The use of urea is, however, limited because of the difficulty of growing large high-opticalquality single crystals. Large crystals with linear size exceeding 5 cm have been prepared by solution growth [2,3,5], but the intrinsic drawbacks of this growth technique (slow growth rate in useful crystallographic directions, presence of solvent traces, microprecipitates and structural defects) still prevent any advanced NLO application. Melt growth has also been reported [5-6] but, due to incongruent melting, only small crystals, characterised by a high density of structural defects and large compositional inhomogeneity, could be grown. Since urea decomposition is strongly reduced below the melting point (132°C), vapour phase growth in the temperature range between 80-95'C would be desirable. Urea crystals have been grown in closed cells [7] and semi-open (effusive) cells[8-10] by physical vapour transport, these latter cells allow for a high linear growth rate, on the order of 0.5 -2 mm/day. In this paper, the authors report on a sophisticated crystal growth facility for vapour phase transport (PVT), based on effusive cells, in which large (>1 cm 3 ) high quality crystals can be produced.
2. VAPOUR PHASE GROWTH OF UREA Mass-spectrometry (MS) analysis of urea vapours in the 80-100°C range has evidenced [11] a molecular sublimation of urea, accompanied by a small amount of decomposition in ammonia and solid biuret. A combined MS and HPLC analysis has also evidenced [12] that when urea is sublimated in a hot point of a cell and then allowed to crystallise at a colder point, decomposition products (mainly biuret) are only observed in the sublimation charge and not in the crystallised fraction, irrespective of whether closed or effusive cells are used. But in the case of effusive cells, which are vented to vacuum through a small hole in the cell wall, the mass transport is seen to occur at a rate which is about two orders of magnitude greater than in the case of closed cells. It is thus evident that below 100 0 C, the vapour transport occurs by PVT, in which case ammonia simply behaves as an inert gas and crystallisation takes place practically congruently. 125 Mat. Res. Soc. Symp. Proc. Vol. 328. @1994 Materials Research Society
However since the ammonia pressure, even
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