Novel aluminophosphate Na[AlPO] with the original microporous crystal structure established in the study of a pseudomerohedric microtwin.

The synthesis and characterization of a new aluminophosphate, Na[AlPO], obtained as single crystals in the same experiment together with Cl-sodalite, Na[AlSiO]Cl, is reported. Na[AlPO], with a strongly pseudo-orthorhombic lattice, is described by the monoclinic crystal structure established in the study of a pseudomerohedric microtwin. The design of Na[AlPO] can be interpreted as an alternative to sodalite, with a monoclinic (pseudo-orthorhombic) 2×4×1 super-structure and unit-cell parameters multiples of those of sodalite: a ≃ 2a, b ≃ 4b and c ≃ c. The triperiodic framework is built by AlO, AlO and PO polyhedra having vertex-bridging contacts. While all the oxygen vertices of the Al-centred octahedra and tetrahedra are shared with phosphate groups, some of the PO tetrahedra remain `pendant', e.g. containing vertices not shared with other polyhedra of the aluminophosphate construction. Na atoms occupy framework channels and cavities surrounded by eight-, six- and four-membered windows with maximal effective pore widths of 4.86 × 3.24 and 4.31 × 3.18 Å. The generalized framework density is equal to 19.8, which means that the compound may be classified as a microporous zeolite. The Na[AlPO] crystal structure is discussed as being formed from octahedral rods arranged in two perpendicular directions, similar to the rods elongated in one direction in the NASICON-type compounds, which have been intensively investigated as promising materials for batteries. Analogous properties can be expected for phases with a modified composition of the NaAlPO topology, where the Al atoms at the centres of octahedra are replaced by Fe, V or Cr.