Structure of GeO2-P2O5 glasses studied by x-ray and neutron diffraction

The structures of three xGeO2-(1- x)P2O5 glasses, where x = 0.98,0.88, and 0.81, have been studied by neutron and x-ray diffraction experiments that yield well resolved P-O and Ge-O bond distances. The Ge-O coordination number (NGeO) increased from 4.0 ± 0.2 to 4.5 ± 0.2 with the decrease in x from 0.98 to 0.81. The increase in NGeO is consistent with a structural model that assumes that all oxygen form Ge-O-Ge and P-O-Ge linkages between Ge polyhedra and P tetrahedra and that new GeO5 or GeO6 polyhedra are formed with isolated PO4 units when P2O5 is added to GeO2. The bond valencies in the P-O bonds of the PO4 tetrahedra are greater than unity and are balanced in P-O-Ge bridges with underbonded Ge-O links in the GeO5 or GeO6 polyhedra. Mixed site connections are expected for the GeO5 (or GeO6) and PO4 units in glasses with relatively low (<20 mol%) P2O5 content due to the overwhelming fraction of GeO4 tetrahedra. The structural changes are compared with those reported for alkali germanate glasses. Several features indicate different characteristics for the compositional dependence of NGeO for the GeO2-P2O5 and alkali germanate glasses. However, the distributions of the first-neighbour Ge-O distances are found to be nearly identical for the GeO2-P2O5 and K2O-GeO2 glasses of equimolar K2O and P2O5 content