Structure refinement of (NH 4 ) 3 Al 2 (PO 4 ) 3 prepared by ionothermal synthesis in phosphonium based ionic liquids – a redetermination

After crystallization during ionothermal syntheses in phosphonium-containing ionic liquids, the structure of (NH 4 ) 3 Al 2 (PO 4 ) 3 [triammonium dialuminum tris(phosphate)] was refined on the basis of powder X-ray diffraction data from a synchrotron source. (NH 4 ) 3 Al 2 (PO 4 ) 3 is a member of the structural family with formula A 3 Al 2 (PO 4 ) 3 , where A is a group 1 element, and of which the NH 4 , K, and Rb forms were previously known. The NH 4 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 NH 4 groups and shows that these NH 4 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 (NH 4 ) 3 Al 2 (PO 4 ) 3 and discuss differences with respect to isostructural forms.