CRYSTAL AND MAGNETIC-STRUCTURES OF LAYER TRANSITION-METAL PHOSPHATE HYDRATES

NH(4)M(II)PO(4) . H2O (M(II) = Mn, Fe, Co, Ni) and KM(II)PO(4) . H2O (M(II) = Mn, Co, Ni) have been prepared and characterized by X-ray powder diffraction and bulk magnetometry, and the corresponding deuterio compounds, by neutron powder diffraction. They are isomorphous (space group Pmn2(1)). The divalent metal ions form approximately square-planar layers of corner-sharing octahedra, the layers being bound by hydrogen bonds in the ammonium compounds and by electrostatic interactions in the potassium ones. In all cases, the M(II)O(6) octahedra are severely distorted, the point symmetry being C-2v Bulk susceptibility measurements on NH4FePO4 . H2O, NH4-MnPO4 . H2O, and KMnPO4 . H2O show they are weakly ferromagnetic, with Neel temperatures of T-N = 24.0(2), 17.5(3), and 18.0(3) K, respectively. powder neutron diffraction was used to determine the underlying antiferromagnetic component of the structures. All three materials have simple two-sublattice structures, with the antiferromagnetic moments along +/-[010], perpendicular to the plane of the metal-containing layers. Analysis using the irreducible representations of the space group shows that the ferromagnetic moments are parallel to [001].