Lead silicate glasses: Binary network-former glasses with large amounts of free volume

It is well known that PbO-SiO₂ exhibits a wide glass formation composition range, up to 90 mol% PbO. Earlier studies suggest that the existence of PbO₃ or PbO₄ structural units, which act like network formers, may be the reason for the wide glass formation range but the structure beyond short-range order is still unclear. Here we found that the network formation in the glass is governed by the interplay of SiO₄ tetrahedra and PbO_x polyhedra (x=3-5, x=4 is major) as a network former while the distribution of other (non-network) units is inhomogeneous. The inhomogeneous distribution of PbO_x polyhedra in the 34 mol% PbO glass and that of SiO₄ tetrahedra in the 65 mol% PbO glass yield a prepeak at Q<1.5 Å⊃-1, a sign of a characteristic length of the inhomogeneity in the diffraction patterns. Furthermore, PbO-SiO₂ glasses contain extraordinarily large amounts of free volume (voids), which cannot be found in conventional binary silicate glasses (network former-network modifier) but only in network-former glasses such as SiO₂ glass and in its mixtures with another network former such as GeO₂. We classify PbO-SiO₂ glass as a "binary network-former glass" with large amounts of free volume.