Medium-range order in alkali metaphosphate glasses and melts investigated by reverse Monte Carlo simulations and diffraction analysis

Reverse Monte Carlo simulations have been performed on the alkali metaphosphate glasses ${\mathrm{Na}}_{0.5}{\mathrm{Li}}_{0.5}{\mathrm{PO}}_3$ and ${\mathrm{LiPO}}_3$ concerning structural experimental data obtained by neutron and x-ray diffraction at 300 K for both systems and versus temperature up to the melting point for the mixed composition. It appears that the contrast effect due to the negative scattering length of Li is not the only reason for the difference in the intensity of the prepeak observed in both systems. The main structural difference lies in the intermediate-range order, while the short-range order is quite similar in both systems. Moreover, it is shown that the intensity increase of the prepeak in the ${\mathrm{Na}}_{0.5}{\mathrm{Li}}_{0.5}{\mathrm{PO}}_3$ structure factor is due to the partial structure factors of the ${\mathrm{PO}}_4$ tetrahedron, sustaining the hypothesis of an ordering between several ${\mathrm{PO}}_4$ tetrahedra and voids with temperature.