Radionuclide microscopic diagnostics of the translational mobility of atoms and of the crystal defectiveness, with the crystallization of CaSO4·2H2O from supersaturated aqueous solution as example

Abstract

The specific features of the behavior and interaction of nonequilibrium defects arising in fluctuation growth of gypsum crystals from aqueous solution and the effect of these defects on the self-diffusion coefficients of the crystallizant atoms were revealed by radionuclide microscopic diagnostics using a pair of tracers (45Ca and 35S). In the course of the experiment, the kinetics of the radionuclide transfer from the solution to the solid phase was compared with the mathematical model of the diffusion transfer of radionuclides from a limited volume of the solution into particles of a polydispersed solid and with the pH variation in the course of formation and existence of the solid phase. In the course of gypsum crystal growth, the expanding surface captures the CaOH+ and HSO4− ions which form nonequilibrium vacancies in the anionic and cationic sublattices. Excess defects interact with each other to form on the crystal surface and in its bulk molecular vacancies, dipolons, or are annealed in the course of aging. Dipolons are responsible for high self-diffusion coefficients of the crystallizant atoms. Perfection of the solid phase leads to a decrease in these quantities by two orders of magnitude.