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

Abstract

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)ų, Z= 2, R= 23 and R_w= 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 Pmn2_l 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 R_wp= 7.37, R_B= 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.