The Structure and Mechanism of Melt Crystallization of CZX-1, a Templated ZnCl2 Network Material

Abstract

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 CuZn5Cl121-. 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 Å 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 oC the templates disorder, forcing a phase transition of α-CZX-1 to β-CZX-1 at 170 oC. A peritectic point is observed at 175 oC where CZX-1 disproportionates into 20 % β-ZnCl2 and 80 % template- and copper(I)-rich liquid. β-ZnCl2 dissolves forming an isotropic liquid at 205 oC. Rapid quenching of the isotropic melt to below 164 oC produces α-CZX-1, while slow cooling produces β-ZnCl2 and the template- and copper(I)-rich liquid below 205 oC. The disproportionation of CZX-1 into β-ZnC2 and liquid is reversed if the material is rapidly cycled from below 164 oC 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 oC 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.