Role of Lead in Lead Phosphomolybdate Glasses and a Model of Structural Units

$PbO---MoO_3$---$P_2O_5$ 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 $Mo^{5+}$ 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 $[Mo0_{6/2}]$ 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, it is assumed to be present as four coordinated tetrahedral $[PbO_{4/2}]^{2-}$ units again with share corber. This is accompanied by the simulataneous conversion of $[P00_{3/2}]$ into $[P0_{4/2}]^+$ 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 $Pb^{2+}$ ions. The composition dependent structural changes are represented using a structural phase diagram