MOLECULAR-DYNAMICS SIMULATIONS OF (NAO2)(X)(SIO2)(1-X) GLASSES - RELATION BETWEEN DISTRIBUTION AND DIFFUSIVE BEHAVIOR OF NA ATOMS

Molecular-dynamics simulations of sodium silicate glasses in a range o f alkali concentration going from 1.8% to 33.33% molar of Na2O are rep orted. Our simulations show that there is a tight relationship between Na atoms and nonbridging oxygens which are mainly located in the firs t Wa coordination shell. In the whole range of composition, Na and non bridging oxygen atoms appear to segregate giving rise to a heterogeneo us distribution. For the higher alkali concentrations, formation of mi crochannels is observed. The activation energies for alkali diffusion have been computed and found in agreement with the experiment. The mec hanism for the diffusion has also been investigated and found to occur through the microchannels, if present, or across the network for low compositions, but the Na motions always appear to be assisted by nonbr idging oxygen atoms. [S0163-1829(98)09637-4].