Novel aluminophosphate Na6[Al3P5O20] with the original microporous crystal structure established in the study of a pseudomerohedric microtwinстатья
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Аннотация:The synthesis and characterization of a new aluminophosphate, Na6[Al3P5O20],obtained as single crystals in the same experiment together with Cl-sodalite,Na8[Al6Si6O24]Cl2, is reported. Na6[Al3P5O20], with a strongly pseudo-orthorhombiclattice is described by the monoclinic crystal structure established inthe study of a pseudomerohedric microtwin. The design of Na6[Al3P5O20] can beinterpreted as an alternative to sodalite, with a monoclinic (pseudoorthorhombic)241 super-structure and unit-cell parameters multiples ofthose of sodalite: a ’ 2as, b ’ 4bs, and c ’ cs. The triperiodic framework is builtby AlO6, AlO4, and PO4 polyhedra having vertex-bridging contacts. While all theoxygen vertices of the Al-centred octahedra and tetrahedra are shared withphosphate groups, some of the PO4 tetrahedra remain ‘pendant’, e.g. containingvertices not shared with other polyhedra of the aluminophosphate construction.Na atoms occupy framework channels and cavities surrounded by eight-, six andfour-membered windows with maximal effective pore widths of 4.86 3.24and 4.31 3.18 A ˚. The generalized framework density is equal to 19.8, whichmeans that the compound may be classified as a microporous zeolite. TheNa6[Al3P5O20] crystal structure is discussed as being formed from octahedralrods arranged in two perpendicular directions, similar to the rods elongated inone direction in the NASICON-type compounds, which have been intensivelyinvestigated as promising materials for batteries. Analogous properties can beexpected for phases with a modified composition of the Na6Al3P5O20 topology,where the Al atoms at the centers of octahedra are replaced by Fe, V or Cr.