Résumé:

Computer simulations have become an important exploring tool of the properties of a vast class of systems belonging to various areas as physics, chemistry, biology or earth sciences. In the particular case of silicate glasses, intensive efforts have been made also during the last two decades to probe their microscopic properties by means of computer simulations. Most of these materials that man has used and produced for thousands of years, have technological, geophysical or biomedical importance, and hence they are widely studied from experimental and theoretical points of view. In this contribution, we will present a detailed structural characterization of the local environment of atoms in some (Li/Na) single-alkali silicate glasses with different cation content (20% and 33%), and obtained by combined classical and Car–Parrinello molecular dynamics simulations. The attention given to these systems is justified by the fact that they can be used as prototypes for more complicated ternary silicate glasses and/or to obtain a precise microscopic insight into one of the unusual features of oxide glasses, namely the so-called mixed alkali effect. The influence of the nature and proportion of the network modifier cation on the relevant structural parameters (bond length and bond angle distributions, distribution of alkali cations) will be discussed in terms of their correlations to the first-principles NMR spectra and parameters computed for our glassy models using the GIPAW method [1,2]. We will equally show a detailed analysis of the nature of the vibrational modes computed for our models and how they are affected by the alkali content and nature. [1] C. J. Pickard, F. Mauri, Phys. Rev. B 63 (2001) 245101. [2] S. Ispas, T. Charpentier, F. Mauri, D.R. Neuville, Solid State Sci. 12 (2010) 183-192.