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

Abstract

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 yieldwell resolved P–O and Ge–O bond distances. The Ge–O coordination number(NGeO) increasedfrom 4.0 ± 0.2 to4.5 ± 0.2 with the decrease inx from 0.98 to 0.81.The increase in NGeO is consistent with a structural model that assumes that all oxygen form Ge–O–Geand P–O–Ge linkages between Ge polyhedra and P tetrahedra and that newGeO5 orGeO6 polyhedra areformed with isolated PO4 units when P2O5 is added to GeO2. The bond valencies in the P–O bonds of thePO4 tetrahedra are greater than unity and are balanced in P–O–Ge bridges with underbonded Ge–O links inthe GeO5 or GeO6 polyhedra. Mixed site connections are expected for theGeO5 (orGeO6) andPO4 units in glasseswith relatively low (<20 mol%) P2O5 content due to the overwhelming fraction ofGeO4 tetrahedra. The structural changes are compared with those reported for alkali germanateglasses. Several features indicate different characteristics for the compositional dependence ofNGeO forthe GeO2–P2O5 and alkali germanate glasses. However, the distributions of thefirst-neighbour Ge–O distances are found to be nearly identical for theGeO2–P2O5 and K2O–GeO2 glasses ofequimolar K2O and P2O5 content.