Experimental and modelling studies of glass structure

Abstract

Elastic properties of alkali phosphomolybdate and phosphotungstate glasses are discussed in terms of their structure. Elastic moduli, Poisson’s ratio and Debye temperature are sensitive to ring structures in the network, which change depending on the type and concentration of alkali ions present in the glass. Highly modified glasses tend to exhibit bulkmodulus-volume relationships characteristic of ionic systems. The elastic properties of lead phosphomolybdate and phosphotungstate glasses confirm the dual structural role of lead, which occupies both networkforming and networkmodifying positions. The extent of PbO incorporated into the network is quantitatively determined by the concentration of the P2O5P_2O_5P2O5 system. Elastic properties of phosphomolybdate glasses containing (Na/K) and (Li/K) alkali couples exhibit a pronounced mixedalkali effect. The nonlinear dependence of elastic moduli, Poisson’s ratio and Debye temperature has been explained using a structural model in which atomic ring reformations take place around alkali ions. In the mixedalkali region, these ring reformations occur such that the minimum possible number of small rings are opened for modification, leading to a positive deviation from linearity in the compositiondependence of elastic properties. The structure of PbO-PbF glasses, simulated by molecular dynamics, substantiates several predictions of the proposed structural model based on Xray diffraction and EXAFS studies. However, some discrepancies-mainly concerning the presence of [PbO2F4][PbO_2F_4][PbO2F4] units-were observed. These differences may arise from the unphysically high quenching rates in the simulation runs, which do not allow truly equilibrated structures to form. The structure of AgO-nBO glasses, simulated using molecular dynamics, shows Ag-Ag and Ag-O correlations comparable to Xray diffraction and EXAFS results reported in the literature. The low oxygen coordination number of Ag, observed by EXAFS, was not reproduced in the simulation, presumably due to the nondirectional nature of pair potentials used. Radial distribution functions were obtained from PDFs using the pairfunction method. The structure of PbO-SiO has been studied in the glassy and melt states using moleculardynamics simulations. Pb-Pb correlations corresponding to Pb-O-Pb chains persist in the melt state as well. Oxygencoordination numbers for Pb were higher than those suggested by Xray diffraction studies. Possible reasons for this discrepancy have also been discussed.