Ab initio crystal structure determination of Na 2Si 3O 7 from conventional powder diffraction data

Abstract

The crystal structure of Na 2Si 3O 7 has been determined by direct methods using integrated intensities of conventional X-ray powder diffraction data and subsequently refined with the Rietveld technique. The title compound was prepared from Na 2Si 3O 7×H 2O by careful thermal decomposition at 440°C. Sodium trisilicate adopts monoclinic symmetry, space group P2 1/ c with unit cell parameters a=7.1924(5) Å, b=10.6039(8) Å, c=9.8049(7) Å, β=120.2478(4)°, V=646.0(9) Å 3 and Z=4. It belongs to the group of interrupted framework silicates of four- and three-connected [SiO 4]-tetrahedra with a ratio of Q 3:Q 4=2:1. Within the framework the sodium atoms are coordinated by 4 to 6 oxygen ligands. The porous character of the new phase is reflected in a framework density FD=18.6 T-atoms/1000 Å 3, a value which is comparable to those observed in zeolitic materials. The topology of the tetrahedral network is identical to the one observed in the hydrous sodium silicate Na 2Si 3O 7×H 2O. Differences can be attributed to tetrahedral rotations. In agreement with the structure analysis the 29Si MAS NMR spectrum of Na 2Si 3O 7 shows two peaks at −91.9 ppm and −102.5 ppm with an intensity ratio of 2:1 which are assigned to three- and four-connected [SiO 4]-tetrahedra, respectively.