Structure of (GeO2)1−x(PbO)x glasses by X-ray scattering

Glasses from the (GeO2)1−x(PbO)x system were synthesised and their structure was studied by X-ray scattering analysis. Simultaneously we have checked the relative Ge/Pb composition of the samples by the measurement of the X-ray fluorescence spectra. In our analysis the radial electron density distribution curve as calculated from the X-ray scattering diagram showed several pronounced peaks. It was easy to ascribe the first peak at 1.75 Å to Ge–O interactions, however the pair function analysis of the area was not able to discriminate between a model based on corner-bound tetrahedra Ge–O=1.75 Å and a model consisting of a mixture of corner-bound tetrahedra (Ge–O=1.72) with corner-bound GeO6 octahedra (Ge–O=1.88). The relation between octahedra and tetrahedra in the mixture model was (1:7). The second peak located at 2.43 Å for x=0.20 and at 2.40 Å for x=0.36 was simply ascribed to Pb–O interactions. The packing of PbO4 tetrahedra is relatively close and is characterised by a distorted tetrahedral co-ordination of O atoms by Pb atoms similar to that one found in the orthorhombic form of PbO (massicot) stable above 760 K. The pair function analysis of the radial electron distribution curves shows that the medium-range order structure may be interpreted so that parts of the GeO2-like structure formed by segments of GeO4 helices (α-quartz like) are interpenetrated by a PbO-like phase formed by parts of sheets/layers built of Pb atoms square-pyramidally bonded to four O atoms.