Study of α-HgI2 crystals grown in space by differential scanning calorimetry, scanning cathodoluminescence microscopy and optical microscopy

Abstract

Several α-HgI2 crystals grown in space during the SL3 mission of Spacelab have been analysed: (1) by spark source mass spectrography, (2) by α to β phase transition temperature measurements using differential scanning calorimetry, (3) by scanning cathodoluminescence microscopy, (4) by optical microscopy, and have been compared with other crystals grown on the ground under the same conditions. All the crystals show: (1) an important concentration of inorganic impurities (exceeding 100 at.ppm with respect to Hg), (2) an even larger concentration of carbon bound as hydrocarbons (several hundreds of at.ppm), (3) a deviation from stoichiometry (probably Hg-rich), (4) a characteristic cellular structure having cell diameters ranging from 2 to 6 μm, and (5) growth instabilities, revealed by the presence of many macrosteps and terraces, leading to the formation of grooves and of lens-shaped clusters rich in impurities having diameters ranging from 1 to 6 μm. By comparing the transition temperatures and the average cell diameter of the grown crystals with those of the source polycrystals, we conclude that the composition of the crystals grown in space are closer to the stiochiometric composition than the composition of the crystals grown on the ground.