SODIUM TRISILICATE - A NEW HIGH-PRESSURE SILICATE STRUCTURE (NA2SI[SI2O7])

Crystals of sodium trisilicate (Na2Si3O7) have been grown in the prese nce of melt at 9 GPa, 1200 degrees C using the MA6/8 superpress at Edm onton, and the X-ray structure determined at room pressure (R=2.0%). N a2Si3O7 is monoclinic with a=8.922(2) Angstrom, b=4.8490(5) Angstrom, c=11.567(1) Angstrom, beta=102.64(1)degrees (C2/c), D-x=3.295 g . cm(- 3). Silicon occurs in both tetrahedral and octahedral coordination (Si -[6]:Si-[4]=1:2). The SiO4 tetrahedra form a diorthosilicate [Si2O7] g roup and are linked by the isolated SiO6 octahedra via shared corners into a framework of 6-membered (Si-[4]-Si-[4]-Si-[6]-Si-[4]-Si-[4]-Si- [6]) and 4-membered (Si-[4]-Si-[6]-Si-[4]-Si-[6]) rings: [Si-[6]-O]=1. 789 Angstrom, [Si-[4]-O]=1.625 Angstrom, Si-[4]-O-Si-[4]=132.9 degrees and the bridging oxygen is overbonded (s=2.22). Channels parallel to b-axis and [110] accommodate Na in irregular 6-fold coordination: [Na- O]=2.511 Angstrom.