Diffusion zone evolution around growing crystals in combeite glass

The precipitation of nonstoichiometric crystals (of different composition from the parent glass) motivated us to study the stoichiometric Na2O center dot 2CaO center dot 3SiO2 ( N2CS3) glass-forming system comprehensively. The concentration profiles of Na and Ca in the diffusion zone around Na4+2xCa4-x[Si6O18], 0<1$0{\mathrm{\;}} < {\mathrm{\;}}x{\mathrm{\;}} < {\mathrm{\;}}1$, crystals in an N2C3S glass were characterized by energy-dispersive spectroscopy and were then theoretically described. We show that the diffusion zone is formed because of the rejection of part of calcium by the growing crystal. The radial variation of the Ca concentrations and the crystal nucleation rate through the diffusion zone were analyzed simultaneously. We found that the nucleation rate in the diffusion zone near the crystal-glass interface drastically decreases due to the increase in the work of critical cluster formation resulting from the composition change. In contrast, the diffusion activation barrier remains practically constant. Moreover, the Ca diffusion coefficients, DCa, in Na2O center dot 2CaO center dot 3SiO2 and CaO center dot SiO2 glasses show the same Arrhenius dependence, demonstrating that (surprisingly) DCa does not depend on the glass composition in the CaO center dot SiO2-Na2O center dot SiO2 metasilicate joint. This work provides the quantitative spatial distribution of chemical components in a partially crystallized glass, shedding light on solid solution crystallization in a glass-forming liquid and its consequences. image

Automated summary

This study comprehensively investigates a stoichiometric glass-forming system, analyzing the concentration profiles of sodium and calcium during crystal growth. The rejection of calcium leads to a decrease in crystal nucleation rate near the crystal-glass interface, while the diffusion activation barrier remains constant. Additionally, the calcium diffusion coefficients are independent of glass composition.