PbI2 single crystals have considerable potential for use in optical informa-tion recording systems, nonlinear optical devices, and x-ray and gamma detectors. The purpose of this work was to assess the effect of growth conditions on the habit of PbI2 grown from the vapor phase in the presence of excess iodine. The growth conditions of PbI2 crystals were optimized using thermodynamic analysis of the equilibrium vapor-phase composition in the Pb-I2 system and theoretical and experimental studies of mass transport: source temperature, 750-800 K; deposition temperature, 630-660 K; iodine overpressure, 4-10 kPa; growth time 2-4 h. The structural perfection of the crystals was examined by optical and SEM techniques and x-ray diffraction. By varying growth conditions, platelike, ribbon, needle, twinned, and dendritic crystals were prepared, as well as combinations and intergrowths of these habits. In most cases, under optimal growth conditions, we obtained platelike or ribbon crystals. The growth habit was responsive to the thermal conditions, which determine the stoichiometry of the growing crystal. Typically, the platelets and ribbons had the same orientations. It is believed that, in vapor growth, platelike morphology is due to the lateral growth of needle crystals, acting as growth leaders. Platelike crystals may result not only from the vapor-liquid-solid mechanism but also from layer-by-layer growth by the vapor-solid mechanism. The observed twinning may be due to the significant supersaturations during growth, thermal stress, and second-phase inclusions. The amount of twinning was found to rise with increasing PbI2 vapor pressure.

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1. Equilibrium vapor composition in the Pb - I₂ system and mass transport in the PbI₂ - I₂ system