A first Mn member in the struvite morphotropic series, CsMn(H 2 O) 6 (PO 4 ): hydrothermal synthesis, crystal structure and interconnections within the family of related phosphates.

Caesium manganese hexahydrate phosphate, CsMn(H2 O)6 (PO4 ), was synthesized under hydrothermal conditions. Its crystal structure was determined from single-crystal X-ray diffraction data. The novel phase crystallizes in the hexagonal space group P63 mc and represents the first manganese member in the struvite morphotropic series, AM(H2 O)6 (TO4 ). Its crystal structure is built from Mn(H2 O)6 octahedra and PO4 tetrahedra linked into a framework via hydrogen bonding. The large Cs atoms are encapsulated in the framework cuboctahedral cavities. It is shown that the size of the A+ ionic radius within the morphotropic series AM(H2 O)6 (XO4 ) results is certain types of crystal structures and affects the values of the unit-cell parameters. Structural relationships with Na(H2 O)Mg(H2 O)6 (PO4 ) and the mineral hazenite, KNa(H2 O)2 Mg2 (H2 O)12 (PO4 )2 , are discussed.