Synthesis, crystal structure and spectroscopic properties of the NH4NiPO4·nH2O (n= 1,6) compounds; magnetic behaviour of the monohydrated phase

NH4NiPO4·6H2O and NH4NiPO4·H2O have been obtained by adding different concentrations of H3PO4 to dilute solutions of NiCl2·6H2O with special attention to the control of the pH in the solvent medium, which was regulated by addition of NH4OH. The NH4NiPO4·6H2O compound crystallizes in the orthorhombic Pmn21space group with cell parameters a= 6.9032(8), b= 6.0907(5) and c= 11.1402(8)Å, V= 468.39(7)Å3, Z= 2, R= 23 and Rw= 2.3%. The structure is three-dimensional and consists of Ni[O(w)]6 octahedra [O(w)= oxygen from a water molecule] linked to PO4 and NH4 tetrahedra by hydrogen bonds. All polyhedra are quite regular in this compound. The NH4NiPO4·H2O phase crystallizes in the Pmn2l space group with cell parameters a= 5.5698(2), b= 8.7668(2) and c= 4.7460(2)Å. The structure of this compound has been refined with the Rietveld method using the coordinates of the KMnPO4·H2O phase as a starting model. The final residual factors were Rwp= 7.37, RB= 2.68%. The structure is formed from sheets of distorted NiO6 corner-sharing octahedra bridged through the oxygen atoms of the phosphate tetrahedra. These layers are pillared along the b direction and are interconnected by hydrogen bonds with the NH4+ cations, which are inserted between the sheets. The spectroscopic properties of both compounds are in good agreement with the symmetry observed in each phase. The values of the nephelauxetic ratio, β, are 0.89 and 0.94 for the hexahydrated and monohydrated compounds respectively. Magnetic susceptibility and specific heat results obtained for NH4NiPO4·H2O show an essentially two-dimensional antiferromagnetic exchange coupling, which becomes of a more three-dimensional behaviour with decreasing temperature.