Experimental studies investigating the structure of soda-lime glasses after silver-sodium ion exchange

Abstract

Structural rearrangements as a result of silver-sodium ion exchange have been investigated systematically on commercial soda-lime glasses by EXAFS experiments, X-ray diffraction, Fourier transform infrared spectroscopy, and density measurements. It was found that the atomic structure of the base glass is closely related to binary sodium silicates and that the addition of calcium causes a CaO bond length of ≌ 0.25 nm. Silver has been incorporated in an extended range of concentration (0.7 to 11 mol% Ag 2O) well below the glass transformation temperature. Nevertheless, the typical silver environment consisting of two oxygens at an average distance of 0.215 nm was found. The second sphere around silver is dominated by an AgAg correlation at an approximately 0.298 nm spacing. Additionally, AgO and AgSi correlations have to be considered. The determined AgAg bond length indicates a d 10-d 10 interaction between silver ions. The so formed silver pairs or clusters should be precursory of the precipitation of crystalline silver particles at elevated temperatures. The interpretation of the difference function of diffraction data yields a pair correlation at 0.342 nm that can be ascribed to AgSi bond lengths. Comparing the EXAFS data at the Na and Ag K-edge structural relaxation is observed around the silver ions during low-temperature ion exchange although the glass volume remains constant in a wide range of composition. This relaxation entails essential consequences with respect to diffusion and conductivity behavior. Thus, it yields the structural basis of the mixed mobile effect in ion-exchanged glasses. The suggested memory effect means the inability of sodium to re-convert ‘silver-like’ sites below T g which leads to local electric fields.