Crystallization and Thermal Behavior of 2MX·CuX2·2H2O (M+ = K, NH4, Rb, Cs; X- = Cl, Br)

Abstract

The crystallization in the MX – CuX2 – H2O system (M+ = K, NH4, Rb, Cs; X- = Cl, Br) was studied and 2MX.CuX2.2H2O was found to be the predominant type of double salts. All of them are isostructural (tetragonal crystal system, sp. gr. P42/mnm) and the structure consists of discrete [Me(H2O)2X4] octahedra and separate M+ ions. The thermal behavior of 2MX.CuX2.2H2O was studied in the range 20-780°C. Ammonium salts showed analogous behavior but higher thermal stability (up to 100-120°C) than the other salts (up to 50-60°C). The first step of 2MX.CuX2.2H2O decomposition was dehydration to anhydrous double salts that occurred up to 100-210°C. The second step was the decomposition of the anhydrous double salts followed by CuX2 decomposition. Formal kinetic parameters (E*deh and A) were calculated and kinetic equations of the dehydration processes were derived from the TG curves using the differential method of non-isothermal decomposition. The results pointed to a diffusion-controlled dehydration process in all cases.