The increased intensity of the first sharp diffraction peak of a NaPO3 melt

The most striking change in the x-ray diffraction pattern of NaPO3 glass which takes place during melting is the generation of a strong first sharp diffraction peak (FSDP); Starting from a model of KPO3 glass and using reverse Monte Carlo simulations, the structure of molten NaPO3 is approximated by replacing the K+ ions with Na+ ions and displacing them in a random manner within the extended cages originally occupied by the larger K+ ions. In this way the increased vibrations and the additional diffusive motion of the Na+ ions are taken into account. This motion requires considerably more room in the melt than is occupied by the cations in the glass. The model reproduces the structure factor of the melt, including the FSDP, and explains the disappearance of the peak at 0.35 nm in the total pair distribution function.