Electron microscopic study of the morphology of lead sulphide and silver sulphide crystals obtained by the silica gel crystal growth technique
An electron microscopic study has been performed of the morphological changes in PbS and Ag2S crystals during their growth by the silica gel technique in different inorganic acidic gel media using U-tubes. The formation and growth of the PbS and Ag2S crystals were followed by a binocular lens and an optical microscope. The morphological changes of the PbS crystals obtained in an HCl gel medium were the same as those observed for the Ag2S crystals in HClO4 and HNO3 gels. The sequence began with the formation of single crystals showing dendritic growth, and was followed by growth along the  direction. The changes were remarkably different for PbS crystals obtained in HClO4 and HNO3 gel media. Whereas the first steps also involved the formation of dendritic crystals, subsequently the crystals grew along . Therefore, morphological changes are due to different nucleus formation and growth rates, which depend on the nature and concentration of the acidic medium. The most perfect cubic structure is obtained for the slowest rate.
These changes were confirmed by electron microscopy, and the compounds obtained were identified by X-ray diffraction. Using these techniques it was observed that PbS and Ag2S crystals grew independently in gels which diffused thioacetamide and a mixture of Pb and Ag ions, instead of yielding PbS-containing AgI ions as an impurity.