A molecular dynamics model of Tb metaphosphate glass Tb0.25(PO3)0.75

There have been many recent experimental studies of rare earth (RE) phosphate glasses, (RE2O3)x(P2O5)1−x, but only two previous reverse Monte Carlo (RMC) modelling studies. The current study reports the first molecular dynamics model of an RE phosphate glass, for Tb metaphosphate glass, with x = 0.25. The model is in good agreement with experimental results for nearest-neighbour distances and coordination numbers, and in reasonable agreement for x-ray and neutron diffraction structure factors. There is a tetrahedral phosphate network, with marked splitting of distances from P to bridging oxygen (Ob) and from P to non-bridging oxygen (Onb). The phosphate network has tetrahedra denoted Qn (where n is the number of Ob) with an average of n = 2.1 and mostly Q2 groups, but with some Q1 and Q3 groups. Most Tb are coordinated to six Onb, and the average coordination is NTbOnb = 5.7, which compares favourably with experimental results that indicate NRE−O~6 in metaphosphate glasses with small RE ions. The great majority of Onb are bonded to only one Tb, but there are a few shared Onb occurring in Tb–Onb–Tb configurations. These cause a small peak in Tb–Tb correlations around 4 Å, prior to the main peak around 6 Å. The corresponding Tb–Tb partial structure factor shows promising agreement with a recent experimental measurement using magnetic neutron scattering.