Molecular dynamics, diffraction and EXAFS of rare earth phosphate glasses compared with predictions based on bond valence

Rare earth (RE) phosphate glasses, (R2O3)(x)(P2O5)](1-x), commonly form with x < 0.3. As previously discussed by Hoppe, in this composition range Q(2) groups provide 6 O-nb (non-bridging oxygens) per RE, but higher RE coordinations can occur if RE are bonded to O-nb on Q(3) groups, or if RE-O-nb-RE configurations occur (where Q" refers to a PO4 tetrahedra with connectivity of n). The values of N-RO and R-RO from the majority of the previous diffraction and EXAFS studies of RE phosphate glasses have been surveyed. Overall, the experimental results for 0.18 <= x <= 0.28 indicate RE coordination is from 6.5 to 7.0 for large RE, and slightly lower for small RE. For x >= 0.23 this implies the occurrence of RE-O-nb-RE configurations, as observed in recent diffraction studies. The experimental results for x <= 0.15 indicate RE coordination is from 7.5 to 8, which can be attributed to RE bonding to O-nb on Q(3) groups. RE coordination of 8 for x similar to 0.15 requires 2 Q(3) groups (per RE) to be connected to Q(2) groups. Comparison with estimated connectivities between Q(2) and Q(3) groups indicate that chemically ordered Q(2)-Q(3) linkages are required. A recent MD model of a Tb metaphosphate glass has N-RO = 6, but includes Tb bonded to O-nb on Q(3) groups and Tb-O-nb-Tb configurations, because it has a broad Q" distribution.