Reverse Monte Carlo modelling of Eu and Tb metaphosphate glasses

It is possible to make phosphate glasses (R2O3)(x)(P2O5)(1-x) With contents of rare-earth elements, R, close to the metaphosphate composition, R(PO3)(3). These glasses are particularly stable to water, and have interesting optical, acoustic and magnetic properties. We have modelled Eu and Tt, metaphosphate glasses using the reverse Monte Carlo (RMC) technique with X-ray diffraction data. The model building process must include additional constraints, and we have derived these from knowledge of metaphosphate structures, including metaphosphate crystals. The model building procedure was first to fit structural constraints alone, and then to fit the diffraction data while maintaining the structural constraints. The models constructed consist of chains of phosphate tetrahedra with Eu (or Tb) 6-fold coordinated to non-bridging oxygens (NBOs), and are in good agreement with the diffraction data. Thus the models provide a demonstration of validity of this structural description of rare-earth metaphosphate glasses. In addition, two different values were used for minimum rare-earth separation, 3.9 and 5.4 Angstrom, based on two different metaphosphate crystals, and more satisfactory models were obtained when using the shorter distance.