Structural study of binary phosphate glasses with MgO, ZnO, and CaO by x-ray diffraction

The X-ray diffraction profiles of magnesium, and calcium phosphate glasses have been measured and the pair function distributions (PFDs) were calculated. Using the pair function method, distance and coordination numbers for the pairs of PO, OO, and MO (M = Mg, Zn or Ca) were determined. A fundamental unit of the structure in these phosphate glasses is confirmed to be a PO4 tetrahedron. It is found that in the calcium phosphate glass the PO4 tetrahedra from long chains and almost all Ca2+ are surrounded by 6 oxygens, which agrees with the structural model by other workers. In contrast to this normal-type calcium phosphate glass, it is also realized that the structures of the abnormal-type magnesium and zinc phosphate glasses consist of long chains with many branches, and most of Mg2+ and Zn2+ are surrounded by 4 oxygens. Thus, the distinct structural difference for abnormal-type phosphate glasses was recognized by the present X-ray diffraction results. However, an abrupt change of an oxygen coordination number around a metallic ion at the metaphosphate concentration was not observed, which is previously considered to be a main reason for the abnormal behavior in the abnormal-type phosphate glasses.