Growth kinetics and adsorption trapping of impurity during crystallization of NH₄Cl in the NH₄Cl-CuCl₂-H₂O-CONH₃ system

Abstract

The growth of NH₄Cl crystals and their trapping of copper impurity in the NH₄Cl-CuCl₂-H₂O-CONH₃ quaternary system have been experimentally studied. The epitaxial adsorption of copper complexes on (100) faces leads to a sharp decrease in the growth rate in good correspondence with the Bliznakov-Chernov equation. The copper impurity enters the crystal composition in amounts up to 6.5 mol %. The impurity distribution coefficient nonlinearly changes with the copper concentration in the solution: it is much larger than unity at low concentrations and sharply decreases with an increase in supersaturation. Such behavior is indicative of the adsorption mechanism of copper trapping by NH₄Cl crystals. Single-crystal X-ray study shows that the impurity is incorporated in NH₄Cl crystals in the form of oriented intergrowths of different complex coppercontaining compounds. The concentration and variety of impurity phases increase with an increase in the copper content in the solution and decrease with an increase in supersaturation. Heterogeneous 2D isomorphous trapping of copper impurity by NH₄Cl crystals induces high (up to 60 MPa) internal stresses in them, as a result of which anomalous birefringence and splitting of crystals occur.