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

Abstract

It is well known that ${\text{PbO-SiO}}_{2}$ exhibits a wide glass formation composition range, up to $90\text{ }\text{mol}\text{ }\mathrm{%}$ PbO. Earlier studies suggest that the existence of ${\text{PbO}}_{3}$ or ${\text{PbO}}_{4}$ 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 ${\text{SiO}}_{4}$ tetrahedra and ${\text{PbO}}_{x}$ 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 ${\text{PbO}}_{x}$ polyhedra in the $34\text{ }\text{mol}\text{ }\mathrm{%}$ PbO glass and that of ${\text{SiO}}_{4}$ tetrahedra in the $65\text{ }\text{mol}\text{ }\mathrm{%}$ PbO glass yield a prepeak at $Ql1.5\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$, a sign of a characteristic length of the inhomogeneity in the diffraction patterns. Furthermore, ${\text{PbO-SiO}}_{2}$ 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 ${\text{SiO}}_{2}$ glass and in its mixtures with another network former such as ${\text{GeO}}_{2}$. We classify ${\text{PbO-SiO}}_{2}$ glass as a ``binary network-former glass'' with large amounts of free volume.