CRYSTAL-GROWTH AND CHARACTERIZATION OF UREA BY PHYSICAL VAPOR TRANSPORT IN SEMIOPEN CELLS

Crystals of urea have been grown by physical vapour transport (PVT) in semi-open (effusive) cells. The reason why effusive cells are used is in relation with the particular vaporisation scheme of urea, which is briefly discussed. X-ray topography (XPT) and diffraction experiments point to a very high structural quality, as demonstrated by the absen ce of extended defects and narrow Bragg peak. However, dark and bright bands parallel to the c-axis, whose nature is not yet understood, hav e been revealed. Optical characterisation has also been performed by a dsorption spectra in the near-IR-visible-near-UV. The results put in e vidence intrinsic absorption in the near IR region and possible defect (impurity)-related transparency losses whose distribution is not homo geneous through the sample.