Crystal structure of a new high pressure polymorph of Na₂Si₂O₅

Information on the composition and structure of liquid silicates is of substantial importance in an effort to understand the bulk chemical and physical properties of magmas. Typically the best known and most fundamental aspect of these structures is the regular coordination of silicon by four oxygen atoms (Si^IV) in a tetrahedral fashion. This is the geologically important coordination in silicon glasses and melts at ambient pressures and temperatures. However, at pressures that are present throughout much of the earth's interior, the coordination of silicon varies, leading to the higher coordination number of six, with Si^VI, in an octahedral environment. We have prepared two new high pressure polymorphs of Na₂Si₂O₅, and have characterized them by x-ray diffraction, ²⁹Si MAS NMR, and Raman spectroscopy. The crystal structure of the ζ-phase of Na₂Si₂O₅, synthesized at 8 GPa and 1200°C, has been determined by single-crystal x-ray diffraction. The unit cell is rhombohedral, with the hexagonal cell parameters of a = 9.899(1) angstrom and c = 13.009(1) angstrom. The principal feature of the structure of the ζ-phase is the six (Si^IV) tetrahedra surrounding the Si^VI atoms in an octahedral manner. Comparison will be made to other structures, like that of garnet-MSiO₃, wadeite-K₂Si₄O₉, or silicon phosphates like SiP₂O₇.