A Novel Topological Compression Mechanism in a Covalent Liquid

Monte Carlo simulations of tetrahedrally bonded SiO$_2$ liquid show that pressure induces large changes in the topology of its four-coordinated framework structure but leaves the average properties of local coordination environments virtually unchanged. Ring statistics are used to describe the liquid's topology; the observed changes paradoxically indicate that the liquid compresses primarily by increasing the size of its rings. A theory for the effects of ring formation on density, which also explains the density of crystalline tectosilicates, accounts for the compression of the liquid.