Ab initio crystal structure determination of Na_2Si_3O_7 from conventional powder diffraction data

摘要

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 X H_2O by careful thermal decomposition at 440 deg C. Sodium trisilicate adopts monoclinic symmetry, space group P2_1/c with unit cell parameters a = 7.1924(5) A, b = 10.6039(8) A, c = 9.8049(7) A, beta= 120.2478(4) deg, V = 646.0(9) A~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/l000 A~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 X H_2O. Differences can be attributed to tetrahedral rotations. In agreement with the structure analysis the ~(29)Si 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.