Modifier effects on the properties and structures of aluminophosphate glasses

Properties and structures of potassium, barium and magnesium alumino-metaphosphate glasses (O/P≈3) have been investigated. Adding Al(PO3)3 to an alkali or alkaline earth metaphosphate glass improves the chemical durability, increases the glass transition temperature and reduces thermal expansion and residual OH-content. Infrared spectra are dominated by metaphosphate vibrational modes, the frequencies of which are dependent on the field strengths of the modifying cations. 27Al MAS NMR spectroscopy indicates that aluminum ions are incorporated into the aluminophosphate network primarily in octahedral sites, although tetrahedral and pentahedral sites are also present. The average aluminum co-ordination number decreases in the series K+>Ba2+>Mg2+ and this is related to a reduction in the basicity of glasses with higher field strength modifiers.