Structure refinement of (NH4)3Al2(PO4)3 prepared by ionothermal synthesis in phospho-nium based ionic liquids - a redetermination

Abstract

After crystallization during ionothermal syntheses in phospho-nium-containing ionic liquids, the structure of (NH₄)₃Al₂(PO₄)₃ [tri-ammonium dialuminum tris-(phosphate)] was refined on the basis of powder X-ray diffraction data from a synchrotron source. (NH₄)₃Al₂(PO₄)₃ is a member of the structural family with formula A ₃Al₂(PO₄)₃, where A is a group 1 element, and of which the NH₄, K, and Rb forms were previously known. The NH₄ form is isostructural with the K form, and was previously solved from single-crystal X-ray data when the material (SIZ-2) crystallized from a choline-containing eutectic mixture [Cooper et al. (2004 ▸). Nature, 430, 1012-1017]. Our independent refinement incorporates NH₄ groups and shows that these NH₄ groups are hydrogen bonded to framework O atoms present in rings containing 12 T sites in a channel along the c-axis direction. We describe structural details of (NH₄)₃Al₂(PO₄)₃ and discuss differences with respect to isostructural forms.

Keywords: Cyphos 169; aluminophosphate; ethyl­tri­butyl­phospho­nium di­ethyl­phosphate; ionothermal synthesis; powder diffraction; redetermination.