A STUDY OF STRUCTURAL DEFECTS IN VAPOR-GROWN ALPHA-HGI2 SINGLE-CRYSTALS BY GAMMA-RAY DIFFRACTION

3D distribution maps of the gamma-ray rocking curves of 002 and 200 reflections were systematically assessed in as-grown mercuric iodide crystals and compared with the growth data. Narrow rocking curves (full width at half maximum, FWHM approximate to 0.025 degrees) were obtained in some regions of the crystals. Depending on the thermal growth conditions, three kinds of distortion of the rocking curves were observed in other parts of the crystals: (a) broadening of peaks, (b) appearance of additional peaks, (c) shift in the position of the peak maxima. They indicate an increased mosaicity grade (subgrain boundaries), the occurrence of larger grains (domains) and bending of the lattice planes, respectively. Most defects appear at the bottom or at the top of the crystals (with respect to the crystal support). Bending of the (001) crystallographic planes occurs in these regions as demonstrated by rotating a crystal with respect to the incident beam. Improvement in the temperature control during growth resulted in very uniform crystals with low mosaicity grade.