Abstract
HgI~2~ crystallizes under ambient conditions from various solvents and by sublimation into three concomitant polymorphs whose colors are red, orange, and yellow. The orange and yellow phases are metastable and transform into the red phase when touched. A phase transition from red to yellow occurs at 400โ
K. The reverse transition from yellow to red shows a huge hysteresis. We established that the structures of the metastable yellow^M^ phase (determined by singleโcrystal Xโray diffraction) and the highโtemperature yellow^HT^ phase (determined by powder synchrotron Xโray diffraction and secondโharmonic generation) are different, albeit closely related. Both show analogous packings of I๏ฃฟHg๏ฃฟI molecules, which are straight in the first and bent with an angle of ca. 160ยฐ in the second. The red and orange phases are tetrahedral semiconductor structures that sublime even at room temperature. The growth of the yellow^M^ phase from 2โchloroethanol and the kinetics of the reconstructive phase transition red to yellow^HT^ and back were studied by optical microscopy, Raman spectroscopy in solution, luminescence, and powder synchrotron Xโray diffraction as a function of time at various temperatures. Both yellow phases grow by accretion of HgI~2~ molecules, present in the solution or liberated from the red crystals, on the surface of the crystal. In contrast, the reverse transformation from yellow to red occurs in the bulk of the crystal, presumably by migration of Hg in the packing of I and subsequent rearrangement of I. The displacement parameters of Hg in both structures are considerably larger than those of I and apparently not dominated by disorder effects.