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

Rare earth (RE) phosphate glasses, (R 2 O 3 ) x (P 2 O 5)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 PO 4 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 On on Q 3 groups. RE coordination of 8 for x ∼ 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.