The structure and mechanism of melt crystallization of CZX-1, a templated zinc chloride network material

A glassforming templated derivative of ZnCl2, called CZX-1, is melt synthesized by combining stoichiometric amounts of ZnCl2, CuCl and (CH3)3NHCl in a 5:1:1 ratio. CZX-1 consists of templated sodalite cage structures with the empirical formula of CuZn 5Cl121-. A cage-template charge interaction occurs where templates ((CH3)3NH1+) are positioned at one of four tetrahedrally distributed sites within a sodalite cage. Molten CZX-1 shows significant extended-range order including structural features up to 50 A long. This network organization in molten CZX-1 does not allow for nanoparticle formation during melt crystallization.;CZX-1 exhibits several temperature-induced phase transitions. Upon heating above 164°C the templates disorder, forcing a phase transition of alpha-CZX-1 to beta-CZX-1 at 170°C. A peritectic point is observed at 175°C where CZX-1 disproportionates into 20% beta-ZnCl2 and 80% template- and copper(I)-rich liquid. beta-ZnCl2 dissolves forming an isotropic liquid at 205°C. Rapid quenching of the isotropic melt to below 164°C produces alpha-CZX-1, while slow cooling produces beta-ZnCl2 and the template- and copper(I)-rich liquid below 205°C. The disproportionation of CZX-1 into beta-ZnC2 and liquid is reversed if the material is rapidly cycled from below 164°C to the isotropic melt.;Time-resolved X-ray diffraction experiments are exploited to independently measure the rates of nucleation and crystal growth. These experiments clearly indicate that condensed-phase crystallization is not explained by Classical Nucleation Theory. Rather, melt crystallization kinetics fit a new model treating CZX-1 nucleation and growth separately as a serial process. Three types of nucleation are observed, (1) intrinsic: the inherent nucleation of CZX-1, (2) extrinsic: nucleation is influenced by outside parameters like defects or internal domains, and (3) growth-initiated: continuous nucleation that follows the rate of growth. The rate of growth increases with increasing temperature to a critical temperature of 164°C where template disordering forces the rate of growth to drop dramatically---demonstrating that the respective structure of the melt and crystalline phases exhibit a strong controlling influence on the crystallization mechanism.