Contribution of structural defects to nucleation process of metastable phase

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Disordered stack ings are commonly 'observed in the metastable phase. In the cases of opal-CT and moganite, the disordered domains have following common features; (1) the stacking sequence are completely disorder without periodic sequences such as highly ordered polytypes; (2) the planar faults are parallel to each other in the disordered domain; (3) the domain of disordered stackings exhibits a thin and platy form, which elongates parallel to the planar defects; (4) the domains of well ordered stackings overgrew on the domain of disordered stackings with very sharp boundary between the two domains (5) The basal plane of the platyshaped domain of disordered stackings is parallel to a singular surface of the crystal, and the singular plane. The disordered, structure is caused by two-dimensional nucleation growth mechanism under highly supersaturated conditions. The results of observations suggest that important factors for formation of metastable phase are supersaturation degree and admissibility of crystal structure for defects.