Rare-earth site distributions in R(PO3)3 (R = La, Nd, Er, Yb) metaphosphate glasses by reverse Monte Carlo simulations

The R - R pair distribution functions of rare-earth metaphosphate glasses R(PO3)3 with R = La, Nd, Er, Yb are determined from total x- ray and neutron scattering data by the reverse Monte Carlo method. Characteristic features of the resulting pair distribution functions gRR(r) are small peaks at similar to 0.44 nm followed by large broad peaks at similar to 0.63 and similar to 0.85 nm and a smooth feature at 1.1 nm. With the decrease of the radii of the R3+ ions by similar to 0.023 nm if R is changed from La to Yb, the peaks of the R - R distances shift to smaller lengths. Accordingly, the features in the SRR(Q) factors shift to greater Q-values except for the first diffraction peaks which appear at a constant position of similar to 12 nm(-1). The decrease in the area of the peak at 0.44 nm with decreasing R radii is interpreted by considering the decreasing fractions of O atoms in terminal P - O bonds, which possess two R first-neighbours. The resulting gRR(r) functions are modelled with structures based on the monoclinic Yb(PO3)(3) crystal. The characteristics of the series of gRR(r) and S-RR(Q) functions give support to the g(TbTb)( r) and S-TbTb(Q) data reported for a magnetic difference neutron diffraction experiment on Tb metaphosphate glass.