Structures and dissolution behaviors of MgO-CaO-P₂O₅-Nb₂O₅ glasses

MgO-CaO-P₂O₅-Nb₂O₅ glasses with the substitution of MgO for CaO consisting predominantly of meta-, pyro-, and orthophosphate groups were prepared using a melt-quenching method, and their structures and dissolution behaviors were investigated. Magnesium in the metaphosphate compositions was shown to act as a modifier oxide and coordinate phosphate groups. Magnesium in the pyro- and/or orthophosphate compositions was shown to act as a network former and to form P-O-Mg bonds, which crosslink short phosphate groups. The chemical durability of the metaphosphate glasses increased with increasing the substitution of MgO for CaO because magnesium exhibits a larger field strength than calcium. However, the durability of the pyro- and/or orthophosphate glasses decreased with increasing the MgO content: P-O-Mg bonds weakened the glass network structure and induced hydrolysis.