Synthesis, Spectroscopic and Magnetic Properties of the Co2(OH)(PO4)1-x-(AsO4)x [0 ≤ x ≤ 1] Solid Solution

The Co-2(OH)(PO4)(1-x)(AsO4)(x) [0 <= x <= 1] solid solution was prepared by hydrothermal synthesis and, polycrystalline samples characterized by X-ray powder diffraction and spectroscopic measurements. The cell parameters of the isostructural phosphate-arsenate phases follow Vegard's law in the whole range of composition. The IR spectra are characteristic of three distinct features corresponding principally to the vibrations of both the hydroxide and tetrahedral (XO4)(3-) (X = P, As) groups together with the evolution of the intensity of stretching [vas (X-O)] vibration modes corresponding to the PO4 and AsO4 tetrahedra in the solid solution. Diffuse reflectance data show bands belonging to the octahedral and trigonal bipyramidal geometries of the Co2+ ions with a high degree of covalence in the Co-O bonds of the arsenate phases. Magnetization measurements of Co-2(OH)(PO4)(1-x)(AsO4)(x) [0 <= x <= 1] show the existence of antiferromagnetic interactions with the presence of a ferromagnetic component below the ordering temperature. TN decreases from 71 to 19 K as the arsenate amount is increased. ZFC-FC curves show irreversibility just below T-N for Co-2(OH)(PO4)(1-x)(AsO4)(x) [x = 0.1-0.75]. The magnetic behavior of Co-2(OH)(PO4)(0.1)(AsO4)(0.9) is completely different from the rest of the hydroxyphosphate-arsenate members and similar to that observed in the undoped arsenate compound where an incommensurate magnetic phase is observed.