Synthesis, synchrotron diffraction study and twinning in Na2Ca4Mg2Si4O15 – a heteropolyhedral framework compound

Abstract

Abstract The formation of polycrystalline Na2Ca4Mg2Si4O15 from solid state reactions has been studied between 800 and 1050°C. Single crystals of the compound have been grown in a closed platinum capsule by slow cooling in the temperature range between 1300 and 1000°C. Basic crystallographic data are as follows: monoclinic symmetry, space group P12/c1, a=7.1717(3) Å, b=5.3512(2) Å, c=16.4789(7) Å, β=90.911(4)°, V=632.33(4) Å3, Z=2. A conspicuous feature of the crystals is an intensive lamellar non-merohedral twinning clearly observable already under a petrographic microscope. The diffraction pattern can be explained as a superposition of two reciprocal lattices with a two-fold axis parallel to [001] being the twin element. Using synchrotron radiation it was possible to solve the crystal structure of Na2Ca4Mg2Si4O15 from a twinned data set. Least-squares refinements resulted in a residual of R(|F|)=0.031 for 2899 observed reflections with I>2σ(I) and 127 parameters. The crystal structure contains both [Si2O7]-dimers and insular [SiO4]-moieties. Tetrahedra and [MgO6]-octahedra form a three-dimensional framework whose topological characteristics have been studied. The remaining calcium and sodium cations are distributed among four crystallographically independent positions located in voids of the network. On a microscopic scale the twinning observed in the diffraction experiments could be explained by the existence of a 21-screw axis parallel to [001] in (¼, 0, z) mapping both domains onto each other. A comparison with related compounds having an A+ 2B2+ 6Si4O15 stoichiometry is presented. More than 25 years after its first observation in refractories our investigation clarifies the crystal structure of a silicate that is of relevance for both Materials science and high pressure research.