Novel K/Mn phosphate hydrates, K 2 Mn 3 (H 2 O) 2 [P 2 O 7 ] 2 and KMn(H 2 O) 2 [Al 2 (PO 4 ) 3 ]: hydrothermal synthesis and crystal chemistry

Two novel K/Mn phosphate hydrates, namely, dipotassium trimanganese dipyrophosphate dihydrate, K 2 Mn 3 (H 2 O) 2 [P 2 O 7 ] 2 , (I), and potassium manganese dialuminium triphosphate dihydrate, KMn(H 2 O) 2 [Al 2 (PO 4 ) 3 ], (II), were obtained in the form of single crystals during a single hydrothermal synthesis experiment. Their crystal structures were studied by X‐ray diffraction. Both new compounds are members of the morphotropic series of phosphates with the following formulae: A 2 M 3 (H 2 O) 2 [P 2 O 7 ] 2 , where A = K, NH 4 , Rb or Na and M = Mn, Fe, Co or Ni, and AM 2+ (H 2 O) 2 [ M 3+ 2 (PO 4 ) 3 ], where A = Cs, Rb, K, NH 4 or (H 3 O); M 2+ = Mn, Fe, Co or Ni; and M 3+ = Al, Ga or Fe. A detailed crystal chemical analysis revealed correlations between the unit‐cell parameters of the members of the series, their structural features and the sizes of the cations. It has been shown that a mixed type anionic framework is formed in (II) by aluminophosphate [(AlO 2 ) 2 (PO 4 ) 2 ] ∞ layers, with a cationic topology similar to the Si/Al‐topology of the crystal structures of feldspars. A study of the magnetic susceptibility of (II) demonstrates a paramagnetic behaviour of the compound.