Synthesis, single crystal structure, optical and magnetic properties of a new rubidium nickel iron phosphate RbNiFe(PO4)2

New RbNiFe(PO4)2 phosphate, both in single and powder forms, was synthesized by flux method and solid state reaction. It is characterized by crystal-X-ray diffraction, HRTEM, IR and Raman spectroscopy and UV-diffuse reflectance. Its crystal structure adopts the monoclinic P21/c space group with the cell parameters: a= 5.134 (4) & ANGS;, b= 14.547(2) & ANGS;, c= 9.230(2) & ANGS;; & beta;=103.76(3) degrees and Z= 4. The RbNiFe(PO4)2 framework has a threedimensional structure, consisting of chains of edge-sharing NiO6, isolated FeO5 and PO4 tetrahedra sharing vertices and edge to form large tunnels along the [100] direction, where Rb + cations are placed. The IR and Raman data are in good agreement with the number of PO4 groups determined in crystal structure. Diffuse reflectance spectroscopy leads to a gap energy of 3.15 eV. It also proved the presence of isolated Fe3+ ions from low energy due to the inter-electronic transition from oxygen to Ni (II) and the non-existence of d-d transition related to iron. The colorimetric properties of the compound result mainly from d-d visible transitions related to the Ni2+ cation in octahedral coordination. The magnetic study revealed that below the temperature of TN =21K an antiferromagnetic character appears with a Curie-Weiss constant of ? =-97 K.