Ag20-B2O3 ionic glasses: Network coherence as seen from molecular dynamics calculations

A study of molecular dynamics in ionic glasses Ag2O-B2O3 is presented. The analysis of the random structures obtained indicates that they are built up from triangular and tetrahedral distorted units BO3 and BO4. These units are connected by bridging oxygen atoms. The coordination change of the boron from three (typical of the pure boron oxide) to four is due to the addition of the metallic oxide, which acts as a network modifier. The formation rate of the tetrahedral BO4 groups follows the x/(1-x) law, x being the molar fraction of Ag2O, up to approximately 0.25 molar fraction, giving rise to a corresponding increase of the network coherence degree that defines the network connectivity. For a higher content of Ag2O an additional mechanism of non-bridging oxygen formation starts. These two mechanisms work in opposition to each other, decreasing the network coherence. The silver ions seem to be almost homogeneously distributed throughout the network to ensure charge neutrality.