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 Q2 groups provide 6 Onb (non-bridging oxygens) per RE, but higher RE coordinations can occur if RE are bonded to Onb on Q3 groups, or if RE–Onb–RE configurations occur (where Qn refers to a PO4 tetrahedra with connectivity of n). The values of NRO and RRO 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–Onb–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 Onb on Q3 groups. RE coordination of 8 for x ∼ 0.15 requires 2 Q3 groups (per RE) to be connected to Q2 groups. Comparison with estimated connectivities between Q2 and Q3 groups indicate that chemically ordered Q2–Q3 linkages are required. A recent MD model of a Tb metaphosphate glass has NRO = 6, but includes Tb bonded to Onb on Q3 groups and Tb–Onb–Tb configurations, because it has a broad Qn distribution.