ARTICLES Influence of Small Additions of Al 2 O 3 on the Properties of the Na 2 O‚3SiO 2 Glass C. Leonelli,* G. Lusvardi, M. Montorsi, M. C. Menziani, L. Menabue, P. Mustarelli, ² and L. Linati ‡ Department of Chemistry, UniVersity of Modena and Reggio Emilia, Modena, Italy, Department of Chemical Physics and INFM, UniVersity of PaVia, PaVia, Italy, and Centro Grandi Strumenti, UniVersity of PaVia, Italy ReceiVed: May 23, 2000; In Final Form: October 18, 2000 Changes in the structural properties of sodium alumino-silicate glasses of general formula Na 2 O‚xAl 2 O 3 ‚(3- x)SiO 2 were investigated as a function of Al 2 O 3 concentration. The experimental evidences provided by density, elastic modulus, glass chemical resistance measures, 29 Si and 27 Al MAS NMR investigations were complemented by molecular dynamics simulations. While neither of the experimental techniques or computational investigation utilized in this study were able to furnish unequivocable responses for the rationalization of the measured properties of sodium alumino-silicate glasses, the synergistic application of experimental and computational techniques showed that the anomalies observed in bulk properties like density and elastic modulus find their origin in medium-range structural features. Introduction The silicate-based glasses have long been of interest in the past because of their broad applications; hence, structural models for their amorphous network have attracted numerous authors. It is well-known that there are few oxides which added in small percentages to soda-silicate or industrial soda-lime silicate glasses remarkably influence the glass properties by perturbing the short rang order (SRO) and the intermediate range order (IRO) of the original network. The SRO describes the environ- ment of the atoms around the network former ions; in particular, the oxygens atoms can be classified as bridging oxygens (BO), non bridging oxygens (NBO) and three bridging oxygens (TBO). The IRO defines how the tetrahedra are linked to each other. Alumina is probably the most studied in terms of modification induced in glass network, being classified among those oxides whose role depends on its concentration and the overall glass composition. Sodium alumino-silicate glasses can be considered as modi- fied silica glasses, where the modifications are made to reduce the melt viscosity and to ease formation of the glasses. 1 Alumina additions to sodium silicate glasses improve durability, and some of these glasses are characterized by interesting anomalies in physical properties such as refractive index, viscosity, density, and electrical conductivity, as a function of composition. For example, sodium alumino-silicate glasses of general formula Na 2 O‚xAl 2 O 3 ‚(3-x)SiO 2 show a maximum in the viscosity and a minimum in the activation energy for electrical conductivity at R ) Al/Na ) 1.0, and an anomalous property change, i.e., elastic modulus and microhardness, at about R ) 0.2-0.4. 2 Structural models derived by different experimental data have been proposed to explain these property changes. They are based upon changes in aluminum coordination, 3-7 introduction of triclusters, 8,9 and degree of network polymerization. 10-13 The updated random network model derived from that pro- posed by Zachariasen, known as “modified random network” theory accounts for the described anomalies. 14 An interpretation at atomic level was given by molecular dynamics (MD) computer simulations, which were successful in explaining the minimum in the electrical conductivity activation energy for a ratio of Al/Na ) 1. 15 In the computational procedure adopted in this paper, the Al ions were constrained to a 4-fold coordination; therefore, the initial hypothesis of a coordination change from 4-fold to 6-fold for the Al(III) ions at R g 1 was not tested. Moreover, the maximum in the Na diffusion activation energy and minimum in the Na site potential experimentally observed at R ) 0.2 remained to be explained. An improvement on this topic was provided by Cormack et al. 16-18 that simulated the sodium alumino-silicate glass struc- tures without structural constraints on the Al ions. They did not observe changes in the aluminum coordination and repro- duced correctly the extrema at R ) 0.2 and R ) 1. It was suggested that the Al(III) ions act as network formers all over the compositional range considered, the electrostatic neutrality being guaranteed by the formation of three bridging oxygen (TBO) species. The change in oxygen coordination from 2-fold to 3-fold was observed to be paralleled by a general change in the ring size distribution that stabilizes the alkali potential sites leading to a general decrease of the Na(I) ions diffusion mechanism. Therefore, they hypothesized that the ob- served anomalies in the experimental properties of these glasses might be a result in the shift of the percolated alkali diffusion channels from NBO-rich regions to AlO 4 rich regions. 16,19 Detailed information on short-range structure of alumino- silicate glasses may be obtained by 27 Al and 29 Si solid-state ² Department of Chemical Physics. ‡ Centro Grandi Strumenti. * Corresponding author. Dip. di Chimica, Facolta’ di Ingegneria, Universita’ degli Studi di Modena e Reggio Emilia, Via Campi 183, 41100 Modena. E-mail: leonelli@unimo.it. Tel.: +39 059 2056247. Fax: +39 059 373543. 919 J. Phys. Chem. B 2001, 105, 919-927 10.1021/jp0018908 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/12/2001