Role of lead in lead phosphomolybdate glasses and a model of structural units

Abstract

PbO---MoO₃---P₂O₅ glasses have been studied over a wide range of compositions. Their physical, thermal, and spectroscopic properties such as density, molar volume, microhardness, heat capacity, glass transition temperature and IR spectra have been investigated. Variation of Mo⁵⁺ concentration as a function of PbO concentration has also been studied using ESR spectroscopy. A structural model is presented to rationalize the experimental observations. According to this model, binary phosphomolybdate glasses are built up of [MoO_6/2] octahedral and [POO_3/2 tetrahedral units which share corner Further, PbO plays a dual strutural role, both as a network former and as a network modifier. This aspect is discussed in the light of various experimental results. When PbO enters into the network modifier, itis assuuned to be present as four coordinated tetrahedral [PbO_4/2]^2- units again with share corber. This is accompanied by the simulataneous conversion of [POO_3/2] into [PO_4/2]⁺ units which thereby achieves charge neutrality in the system. When PbO acts as a network modifier, glasses are characterized by the presence of a number of polyhedral structural units carrying a net charge, while lead remains in the structure as Pb²⁺ ions. The composition dependent structural changes are represented using a structural phase diagram.