Coordination of Pb2+ in oxide glasses determined by ESR and properties of binary lead glasses

ESR spectra of paramagnetic Pb3+ with 6s1 electronic configuration induced by γ-irradiation at 77K were measured for a wide variety of oxide glasses. It has been found that the Pb 65-character of the magnetic electron, evaluated from the analysis of the hyperfine structure due to a 207Pb nucleus (nuclear spin 1/2, abundance 21.7%) highly depends on the stereo-chemistry of precursor Pb2+, i.e., the s-character of the Pb3+ produced from Pb2+ having a nearly spherical-symmetric ligand field (coordination I) is fairly greater than that produced from Pb2+ with a trigonal- or tetragonal-pyramidal ligand field (coordination II). The determination of stereochemistry of Pb2+ in oxide glasses has been attempted by measuring ESR spectra of as-induced Pb3+ and by using the relation between the s-character of Pb3+ and the coordination of Pb2+ (i.e., coordination I or II). The results obtained are summarized as follows; (1) In phosphate glasses and low alkali borate glasses, most of Pb2+'s exist in coordination I. (2) Lead ions take predominantly coordination II in silicate and high alkali borate glasses. The relation has been also considered between the properties of binary lead glasses and the stereochemistry of Pb2+. A marked difference of limiting PbO concentrations for glass-formation in phosphate system from those in silicate and borate systems has been interpreted by considering the dissimilarity of the Pb2+-coordination structure. It has been found for binary silicate, borate and phosphate glasses that the partial molar refractivity of PbO increases with decreasing the s-character of Pb3+. This relatin leads to a conclusion that the polarizability of Pb2+ taking coordination II is considerably greater than that having coordination I. (author)