Short-range and medium-range order in solid and molten metaphosphate glasses

X-ray diffraction measurements of molten binary metaphosphate glasses are described. The changes in the short-range order in the temperature range greater than the glass transition temperature, T(g), are discussed and the differences between the glass and liquid structures are emphasized.. The phenomenon of a thermally inducible peak at the smallest Q nm(-1) which exists for sodium metaphosphate, but is not detected for diffraction from magnesium or calcium metaphosphate, is regarded as a first sharp diffraction peak (FSDP) and interpreted in terms of the cluster-void picture. The amplitude of the FSDP increases with temperature due to the increase of scattering contrast by the thermal expansion of the NaO,, polyhedra. We assert that the FSDP is a sufficient, but not necessary signature of medium-range order. (C) 1998 Elsevier Science B.V. All rights reserved.