SANS, RAMAN and SEM studies of lead-germanate glasses doped with the manganese oxide

Abstract

The purpose of this work was to justify the induced structural modifications of the increase of MnO content in lead-germanate glasses and to explore the structural role of doping level. The local structure of the obtained glasses in the xMnO·(100-x)[7GeO2·3PbO2] composition where x = 10–50 mol% MnO was characterized by the analysis of X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) micrographs, Raman spectra and Small Angle Neutron Scattering (SANS) data. The Raman analysis shows that manganese-lead-germanate glasses are composed of [GeO4], [GeO6], [PbOn] and [MnOn] structural units and the number of [GeO6] octahedral units attains maximum values for the smaller MnO contents up to 20 mol%. Further higher MnO concentration in the host matrix implies the change in the coordination number of the germanium atoms from 6-fold to 4-fold and the breaking of short GeOGe chains from germanate network. The predicted coordination number of germanium atoms in the Raman spectra was also compared with SANS data which show strong evidence in germanate anomaly. The combining the results provided from Raman, SEM and SANS data we can conclude the maximum of tetrahedral units into octahedral germanate units occurs at compositions up to 20 mol% MnO. A further increase of doping level in host matrix up to 30 mol% MnO results in a decrease in the coordination number of germanium atoms with a contraction of skeleton of GeO2 network, an increase of non-bridging oxygen concentration and a maximum value of refractive index. For higher MnO content a depolimerisation of the disordered network lead due to the breaking of GeOGe bridges and the formation of [GeOn] structural units with 4 ≥ n < 6.