Hydrothermal synthesis and structural characterization of κ-Na 2Si 2O 5 and Na 1.84K 0.16Si 2O 5

Abstract

Single crystals of a new sodium disilicate modification labeled κ-Na 2Si 2O 5 have been prepared from the hydrothermal crystallization of a glass at 700 °C and 3 kbar. The structure has been solved and refined to a residual of R(| F|)=0.035 for 750 independent observed reflections. The compound is orthorhombic with space group Pn2 1 a ( a=8.128(1) Å, b=4.8322(8) Å, c=11.977(3) Å, V=470.4(3) Å 3, Z=4, D x =2.57 g/cm 3, μ( Mo K α)=0.87 mm −1) and belongs to the group of single layer silicates. Individual sheets can be described as being built by the condensation of zweier single chains of SiO 4-tetrahedra parallel to the b-axis or, alternatively, by condensation of vierer single chains parallel to the a-axis. The layers contain six-membered rings in UDUDUD conformation. The stacking of the layers parallel to the c-axis results in a three-dimensional structure in which the alkali cations reside on two crystallographically independent sites (M(1) and M(2)) between the layers for charge compensation. The geometrical and topological features of the single tetrahedral sheets in κ-Na 2Si 2O 5 are almost identical to those observed in β- and C-Na 2Si 2O 5. Differences between the structures can be attributed to different ways of stacking of adjacent sheets and are discussed in detail. Small amounts of potassium can be substituted for sodium without changing the structure type. At slightly different synthesis conditions (600 °C, 1 kbar) we obtained an isostructural mixed alkali disilicate with composition Na 1.84K 0.16Si 2O 5 ( a=8.172(2) Å, b=4.849(1) Å, c=12.078(3) Å, V=478.6(3) Å 3). The distribution of the alkali atoms among the two M(1) and M(2) positions shows a definite preference of the larger potassium for the M(1) site; the M(2) site is K-free.