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 powd er diffraction and bulk magnetometry, and the corresponding deuterio c ompounds, by neutron powder diffraction. They are isomorphous (space g roup Pmn2(1)). The divalent metal ions form approximately square-plana r layers of corner-sharing octahedra, the layers being bound by hydrog en bonds in the ammonium compounds and by electrostatic interactions i n the potassium ones. In all cases, the M(II)O(6) octahedra are severe ly distorted, the point symmetry being C-2v Bulk susceptibility measur ements 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 struct ures. All three materials have simple two-sublattice structures, with the antiferromagnetic moments along +/-[010], perpendicular to the pla ne of the metal-containing layers. Analysis using the irreducible repr esentations of the space group shows that the ferromagnetic moments ar e parallel to [001].