Synthesis, Crystal Structure, and Electrochemical and Magnetic Study of New Iron (III) Hydroxyl-Phosphates, Isostructural with Lipscombite

Abstract

Two novel iron (III) hydroxyl phosphates, of general formula Fe2-y□y(PO4)(OH)3-3y(H2O)3y-2 (y = 2/3 or 0.82; □ represents vacancy), have been synthesized by the solvothermal method. The Rietveld refinement of the crystal structure from the X-ray powder diffraction was performed in a tetragonal cell with space group I41/amd. The structure is isotypic with the mineral caminite Mg1.33[SO4(OH)0.66(H2O)0.33] and is closely related to the mixed-valence lipscombite Fe2-yPO4(OH) (0≤ y ≤ 2/3). The interconnection of the chains of face-sharing iron octahedra forms the “rod-packing” structure. In Fe1.18(PO4)(OH)0.57(H2O)0.43 (y = 0.82), about 60% of the chain sites are occupied, whereas about 2/3 of the chain sites are occupied in Fe1.33(PO4)(OH) (y = 2/3). The partial occupancy of the Fe3+ sites allows the incorporation of other cations into the structure. When ZnCl2 and NiCl2 were added into the hydrothermal mix, iron was partially substituted by these metal ions, giving Fe4/3-zMz□2/3(PO4)(OH)1-z(H2O)z (M = Ni, Zn; z = 0.28 and 0.26 for Ni and Zn, respectively), and increasing the cation occupation of the chains to about 2/3. The protons of the hydroxyl groups in these compounds can be replaced by lithium ions with structure retention. Lithium can also be incorporated electrochemically into the lattice, and the disordered compounds are good candidates for the cathode for secondary lithium batteries. The compounds exhibit magnetic phase transitions in the temperature range 60 to 90 K; the transition temperature increases with the number of magnetic ions in the chains.