Effect of glass composition on the interactions between structural elements in Cu-containing silicate–phosphate glasses

The structure of silicate–phosphate glasses from the SiO2–P2O5–K2O–CaO–MgO–CuO system acting as slow release fertilizers, containing different amounts of copper ions replacing magnesium or calcium ions, was examined by DSC, XRD, 31P, and 29Si MAS–NMR techniques. The chemical activity of the glasses in the 2 mass% citric acid solution was measured by the ICP-AES method. The studies showed that regardless of the kind of replaced components, the Cu ions prefer to associate with the phosphorous Q1 units, forming the domains composed of orthophosphate with P–O–Cu bonds. The formation of domains with chemically stable P–O–Cu bonds resulted in poorer glass solubility under conditions simulating the natural soil environment.