Magnetic properties of synthetic spheniscidite

We report the results of Mossbauer studies of synthetic spheniscidite, an ammonium iron hydroxide phosphate dihydrate, NH4[Fe-2(OH)(PO4)(2)]. 2H(2)O. The hyperfine structure is consistent with the existence of two high-spin ferric ions, octahedrally coordinated, in agreement with the crystallographic structure which results from a three-dimensional packing of octameric building units connected in-between by PO4 tetrahedra. Strong magnetic frustration is expected from the octameric unit, because of the presence of antiferromagnetic interactions (due to near 90 degrees Fe-O-Fe bond angles for edge-sharing iron octahedra and near 180 degrees Fe-O-Fe bond angles for corner-sharing iron octahedra) and of the cationic topology which consists of two top to bottom triangular platelets. Three-dimensional magnetic order of the octameric units occurs below 10 K due to relatively weak antiferromagnetic or ferromagnetic couplings originating from the supersuperexchange interactions through PO4 tetrahedral units. The connectivity of the ferric network is found to reinforce the frustrated magnetic behaviour, in agreement with the magnetic characteristics. Spheniscidite occurs as an antiferromagnet built upon weakly ferrimagnetic ferric octamers.