A molecular dynamics model of the atomic structure of Tb metaphosphate glass (Tb2O3)0.25(P2O5)0.75

There have been many recent experimental studies of rare earth (RE) phosphate glasses, (RE 2 O 3 ) x (P 2 O 5 ) 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 (O b ) and from P to non-bridging oxygen (O nb ). The phosphate network has tetrahedra denoted Q n (where n is the number of O b ) with an average of n = 2.1 and mostly Q 2 groups, but with some Q 1 and Q 3 groups. Most Tb are coordinated to six O nb , and the average coordination is N TbO nb = 5.7, which compares favourably with experimental results that indicate N RE−O ∼6 in metaphosphate glasses with small RE ions. The great majority of O nb are bonded to only one Tb, but there are a few shared O nb occurring in Tb–O nb –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.