The Science & Technology
of Glass
Cambridge - Monday 4th to
Wednesday 6th September 2017



Natalia M. Vedishcheva
<ionatali386@gmail.com>

article posted 21 June 2017

Dr. Natalia M. Vedishcheva is Leading Scientist at the Institute of Silicate Chemistry of the Russian Academy of Sciences, where she has been working since graduating from Leningrad State University (Chemistry) and has gained her Ph.D. degree in Physical Chemistry. Her research interests include calorimetric studies of the thermodynamic properties of oxide systems (glasses and crystals), and thermodynamic modelling of the structure-property relationships for a variety of oxide glasses. At present, her attention is mainly focused on thermodynamic modelling of the intermediate-range order in borate and borosilicate glasses.

In 2001, Dr. Vedishcheva shared the Otto Schott Research Award for her contributions to the field of thermodynamic modelling of the physical properties of oxide glasses, and in 2004 she was elected to the Fellowship of the Society of Glass Technology. She has subsequently served the Society as a member of its Board of Fellows, and is an active member of Technical Committee TC03 (Glass Structure) of the International Commission on Glass. She represents Russia on the International Advisory Boards of the series of conferences on Borate Glasses, Crystals & Melts and on the Structure of Non-Crystalline Materials.


Non-bridging oxygen atoms: Can they be present in low-alkali borate glasses?
1Natalia M. Vedishcheva* & 2Adrian C. Wright

As is known from structural studies, the presence of non-bridging oxygen atoms in borate glasses becomes noticeable when the content of modifying oxides approaches ~30 mol. %. This experimental observation can be explained if it is remembered that glasses are products of chemical interactions between the substances forming the batch, rather than the result of direct melting. This approach allows glasses to be considered as solutions formed from chemical groupings whose stoichiometry and structure are similar to those of crystalline compounds that form in a given system. Using the formalism of this model, the equilibrium concentrations of a variety of groupings present in glasses, which determine their chemical structure, are calculated as a function of the glass composition. The results of these calculations, performed with the observation of the principle of the minimum Gibbs energy of a system, reveal that metaborate groupings, M2O·B2O3, that introduce non-bridging oxygen atoms into glasses [1] dominate the chemical structure only over the high-alkali region where the content of M2O exceeds 35-40 mol. %. In low-alkali glasses, with the M2O content below 25 mol. %, pentaborate (M2O·5B2O3) and tetraborate (M2O·4B2O3) groupings form, whose structures do not incorporate non-bridging oxygen atoms. This trend is explained by comparing the degree of ionicity of the metal-oxygen bonds (εM-O) in low-alkali and high-alkali chemical groupings, the values of εM-O being calculated from the enthalpies of formation of the given glasses, together with various atomic parameters.

Reference:

1. N.M. Vedishcheva and A.C. Wright, In: Glass. Selected Properties and Crystallization, Ed. J.W.P. Schmelzer (De Gruyter, Berlin/Boston, 2014), Chapter 5, p. 269-299.

Institutions:

1Institute of Silicate Chemistry of the Russian Academy of Sciences, Nab. Makarova 2, St. Petersburg, 199034, Russia.
2J.J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading, RG6 6AF, U.K.