Ca and Na environments in Na2O-CaO-Al2O3-SiO2 glasses:: influence of cation mixing and cation-network interactions

X-ray absorption spectroscopy at Ca and Na K-edges and Molecular Dynamics (MD) simulations are used to investigate the Ca and Na environment in soda lime alummosilicate glasses. Both X-ray Absorption Near Edge Structure (XANES) spectra and MD calculations indicate a coordination number of about 7 for Ca and 6 for Na, in a distorted polyhedron. Ca and Na XANES spectra are poorly affected either by the Si/Al substitution or the Ca/Na substitution while MD simulations present some variations in the cationic environment. We show an affinity for non-bridging oxygens (NBOs) to be connected with Ca rather than Na. Evidence of Ca-Na mixing is shown by the MD models and there is a tendency toward forming more regions enriched in (Ca, NBOs) for CaO-rich alummosilicate glasses. Variations in the cationic mobility upon the Na/Ca substitution are correlated with the observed structural modifications. (C) 2004 Elsevier B.V. All rights reserved.