High pressure mixed alkali disilicates in the system Na2— xKxSi2O5: hydrothermal synthesis and crystal structures of NaKSi2O5-II and Na0.67K1.33Si2O5

Abstract

Abstract Single crystal growth experiments for NaKSi2O5 and Na0.67K1.33Si2O5 have been successfully performed by hydrothermal crystallization of two glasses at 500°C and 1 kbar during 24 hours. For a glass with a Na : K ratio of 1 : 1 a previously unknown NaKSi2O5 modification named phase II was obtained. The structure of NaKSi2O5-II has been solved from a single crystal diffraction data set and refined to a residual of R(|F|)= 0.032 for 1186 independent reflections. The compound is monoclinic with space group P21/c (a = 4.852(1) Å, b = 13.594(2) Å, c = 7.463(1) Å, β = 91.20(2)° V = 492.1(2) Å3, Mr = 198.27 u, Z = 4, Dx = 2.68 g/cm3, μ(MoKα ) = 1.58 mm–1) and belongs to the group of double chain silicates. Individual double chains can be described as being built by the condensation of zweier single chains of SiO4-tetrahedra running parallel to the a-axis. Each chain contains four-membered rings. Alkali-atoms are 6- and 8-fold coordinated by the oxygen ligands. NaKSi2O5-II is isotypic with the structure of NaRbSi2O5 which has been synthesized at room-pressure. Hydrothermal treatment of a second glass with composition Na0.5K1.5Si2O5 resulted in the formation of single-crystalline material of Na0.67K1.33Si2O5, slightly Na-richer than the starting composition. The platy crystals showed twinning by pseudo-merohedry, a feature we took account of by separating the reflections from the different twin individuals during the data collection. Structure solution has been accomplished by direct methods. The subsequent refinement converged to a residual of R(|F|) = 0.037 for 1543 independent reflections. The compound is mono clinic as well with space group I2/a (a = 12.731(2) Å, b = 7.321(1) Å, c = 17.827(3) Å, β = 100.85(2)° V = 1631.9(6) Å3, Mr = 206.32 u, Z = 12, Dx = 2.53 g/cm3, μ(MoKα ) = 1.78 mm–1) and belongs to the group of single layer silicates. A single layer consists of four-, six- and eight-membered rings, which are parallel to (100), and can be described as being built by the condensation of unbranched dreier single chains running along the b-axis. The stacking of the layers results in a three-dimensional structure in which the alkali cations reside between the layers for charge compensation. The distribution of the alkali atoms among the three crystallographically different positions M(1), M(2) and M(3) shows a definite preference of the larger potassium for the M(1) and M(2) site; the M(3) site is K-free. The alkali-atoms are 5- and 6- fold coordinated by the oxygen atoms. The geometrical and topological features of the single tetrahedral sheets in Na0.67K1.33Si2O5 are very similar to those observed in the phase I of NaKSi2O5. Differences can be attributed to different ways of stacking of adjacent sheets, and different sequences of up and down pointing tetrahedra within the rings.