Effect of impurity on the habit of nickel molybdate (NiMoO4�xH2O) crystals in silica gel

Abstract

Listen

Translate article

The effects of impurity on crystal growth have been the subject of many investigators (for example [1-3]) . The authors [4] have reported the growth of nickel molybdate crystals in silica gel by controlled diffusion of reagents. The present paper is an investigation of the effect of impurities on these crystals under the same growth conditions. The double diffusion technique was employed using a U-tube, as described by Henisch [5]. Neutral gels were prepared by mixing sodium silicate of density 1.03 g cm -3 with dilute nitric acid. About 30 ml of the mixture was poured into the U-tube and was permitted to set for 24h. Equal volumes of solution of 1 M NiC12" 6H20 and 0.025M (NH4)2MoO4 were then added on either side of the U-tube. Polycrystalline aggregates with lath-shaped branches were formed below the gel interface within 20 to 25 days after the start of the experiment. On being broken, the individual branches appeared as lozenge-shaped 6 m m x 4 m m x 0 . 3 m m in size. After 25 to 35 days flake-like platelets of average dimensions 3 mm x 2 mm x 0.2 mm were obtained at the gel interface. The flake-like platelets consist of thin layers piled-up, almost all of which have the same crystallographic axes. The basal (0 0 1)plane was found to be the cleavage plane. Small thin platelets of different habits were obtained after 40 to 50 days. The method described above was also used for the preparation of doped crystals. A series of experiments were performed to study the effect of impurities on the mode of crystallization of nickel molybdate crystals in silica gel at neutral pH. By adding certain dopants like Ca 2+, Co z+, Mn 2+, Zn 2+, Cr 3+ and Mg 2+ in the form of chlorides and nitrates in the supernatant solution, in solution form, a dramatic effect in the crystallization was observed. Impurities like Ca 2+, Co 2+, Mn 2+ and Cr 3+ were found to inhibit spurious nucleation and facilitate the growth of good-quality crystals of perceptible size and shape. Significant changes in the growth kinetics and morphology were observed and the crystals harvested had few growth