9431
2007, 111, 9431-9433 Published on Web 07/21/2007
Sudden Reversal of the Defect-like Behavior of Small Rings in Vitreous Silica S. Sen Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616 ReceiVed: June 1, 2007; In Final Form: July 9, 2007
The structural response of vitreous silica to changes in temperature and pressure is of fundamental importance in earth and materials sciences and is largely controlled by the ring-size distribution that controls the topology of the structure. The 3- and 4-membered rings constitute only a small fraction of the structure but are known to be sensitive to and increase in concentration with increasing pressure and temperature. We present new experimental results that show a sudden reversal in the temperature dependence of the concentrations of these rings near the recently discovered density minimum of vitreous silica. These results invalidate our conventional wisdom regarding the entropically stabilized, defect-like behavior of the small rings and indicate that these rings are possibly key players in controlling the density of silica.
Introduction The structure of vitreous silica at ambient pressure is known to be a three-dimensional network of corner-shared SiO4 tetrahedra. The topology of this network is controlled by the size distribution of rings or closed loops of connected tetrahedra where the size of a ring corresponds to the number of constituent tetrahedra.1 Despite its compositional simplicity, vitreous silica shows many anomalous variations in physical properties that are manifestations of the complexities in its structural topology and, therefore, of the ring-size distribution. The structural disorder in vitreous silica allows the coexistence of rings with sizes ranging from 3 to up to ∼10, with 5, 6, and 7 -membered rings being the most abundant.2 Although the structure has been studied in exceptional detail with a variety of techniques in the last several decades, the statistics of the ring-size distribution and its response to temperature and pressure are only poorly understood.3 Direct spectroscopic identification has so far been possible only for the two smallest rings, that is, the 4- and 3-membered rings, which appear as sharp features at ∼495 and 606 cm-1 (designated as D1 and D2 bands), respectively, in the Raman spectrum.4 The populations of these rings in the glass structure are known to be very small (
