Effects of magnesium for calcium substitution in P2O5–CaO–TiO2 glasses

• Magnesium-substitution improves glass formation and processing in phosphate glasses.
• P2O5–CaO–MgO–TiO2 glasses show mixed cation effect.
• P2O5–CaO–MgO–TiO2 metaphosphate and invert glasses show comparable solubility.
• Full Mg for Ca substitution increases solubility of invert glasses significantly.

Many phosphate glasses dissolve in aqueous solutions, and they are increasingly studied for use as degradable biomaterials. As magnesium has been suggested to improve glass formation in phosphate glasses, its effect on glass structure, thermal properties and dissolution were studied. Two phosphate glass series (one in the metaphosphate range with 50 mol.% P2O5 and one invert series with 30 mol.% P2O5) in the system P2O5-CaO-TiO2, where calcium was partially (0 to 100% on a molar base) replaced by magnesium, were produced by a melt-quench route.Substitution of magnesium for calcium reduced the glass density (owing to the lower atomic weight of magnesium compared to calcium), made the phosphate network more compact, and caused Raman bands to shift to higher frequencies. Magnesium substitution significantly improved the processing of the glasses in both series by widening the processing window. The solubility of both glass series was low as shown by negligible pH changes and low ionic concentrations, with less than 7.5% of ions being released over 1 week. There were no significant differences in the solubility of the two series, which is surprising as metaphosphate glasses are generally considered to be more soluble than invert glasses. Magnesium substitution had a marked influence on invert glass dissolution, increasing the solubility about three-fold compared to the calcium-containing glasses, while no such effect was observed for metaphosphate glasses. Results suggest that magnesium substitution affects metaphosphate glasses and invert glasses differently, but improves glass formation and processing in both.