Zdeněk Černošek, Petr Hejda
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic,
Glasses are commonly described as a formal mixture of constituting oxides, which is particularly advantageous from a technological point of view, and also allows the use of ternary diagrams to describe the composition series. Glasses are not the mixture of oxides but they are composed as a mixture of stable chemical compounds created during the high-temperature synthesis, whose crystallization was prevented by a suitable method. Thus, the properties of the glass cannot be simply derived from the ratio of oxides but from the glass forming chemical compounds.
The simples and frequently used way for characterization of phosphate glasses is O/P ratio Unfortunately, this ratio is ambiguous with respect to chemical composition, even when all oxygen is bound only in phosphates and is completely inapplicable when besides phosphates another species containing oxygen are formed (complex cations,…). So, more accurate and better would be to use a thermodynamic model. However, this model is based on molar Gibbs energy of crystalline compounds and at least an entropic part could be problem when used for non-crystalline matter. The chemical model based on real compounds verified by analysis of experimental results, mainly MAS NMR and vibrational spectroscopy for electronegative part of glasses and, when it is possible, EPR for cationic part all together with balance of elements and charges can shown more realistic view of chemical composition of glasses. For non-transient elements calculation of the chemical model is simpler – only the anionic part plays main role because all of oxygen is bound into phosphate units. Complications occur in the case of d-elements that form both complex cations and oxygen anions. By the way, in this moment popular O/P ratio completely loses meaning.
The aim of this work is to demonstrate chemical model created for two series of glasses with transition metals and discuss its use for explanation of selected physico-chemical properties.