Structure and dynamics of ionic borate glasses

The structural and dynamic properties of ionic borate glasses with composition xM(2)O-(1-x)B2O3 (M=Li, Cs) are revisited in view of recent results of molecular dynamics simulations. Simulations were carried out in collections of similar to 512 atoms in the canonical ensemble for compositions 0(.)20 <= x <= 0(.)50 and temperatures 300, 600 and 1250 K. The short range order structure of these glasses was found to depend on temperature and on the alkali oxide modifier content, as well as on the nature of the alkali ion. ne structural trends resulted from simulations are compared with those derived from different experimental techniques. At the nanoscale size, it was possible to distinguish long lived and well distinguished network sites hosting alkali metal ions. The distribution of cations in the glassy network was found to be inhomogeneous and consistent with the predictions of the modified random network model. Cation dynamics were evaluated in both short and long time scales. In particular, the vibrational properties of cations were found to depend strongly on the nature of their hosting sites, whereas their diffusivity is facilitated by the presence of nonbridging oxygen (NBO) atoms. ne detailed analysis of the ion transport mechanism revealed discrete hopping of ions mostly between similar neighbouring sites. The calculated cluster size distribution curves for cations and NBO atoms were found to obey the predictions of percolation theory. The formation of percolating clusters above a certain composition threshold supports strongly the existence of a percolating diffusion mechanism for ionic borate glasses.