Structure and magnetic properties of Li3Fe2(AsO4)(3-x)(PO4)(x) [x=0, 1, 1.5, 2]: two sublattice weak ferromagnets

The Li3Fe2(AsO4)(3 - x)(PO4)(x) (x = 0, 1, 1.5, 2) solid solution was prepa red by the ceramic method. Li3Fe2(AsO4)(3) crystallizes in the monoclinic P 112(1)/n space group with the cell parameters, a = 8.608(1), b = 12.215(1), c = 8.929(1) Angstrom and gamma = 90.76(1)degrees. The structure consists of a three-dimensional framework of vertex-sharing FeO6 octahedra and AsO4 tetrahedra with the lithium ions distributed in three four-coordinated crys tal sites. There are two independent iron positions in the structure of thi s compound, Fe(1) and Fe(2). The cell parameters of the isostructural Li3Fe 2(AsO4)(3 - x)(PO4)(x) (x = 0, 1, 1.5, 2) phases follow Vegard's law. The I R spectra show the evolution of the intensity for the bands corresponding t o the AsO4 and PO4 tetrahedra in the solid solution. An isotropic signal wi th a g value of 2.0 is observed in the ESR spectra of all compounds, in goo d agreement with the presence of a high spin Fe(III) ion with slightly dist orted octahedral symmetry. The magnetization measurements and the magnetic DC and AC susceptibility data show an antiferromagnetic behavior for these compounds, with the presence of a ferromagnetic component below the orderin g temperature. T-N decreases from 35.5 to 31 K with decreasing the amount o f arsenate. The magnetic results are explained by the existence of a phenom enon of ferrimagnetism in these compounds, due to the existence of two diff erent magnetic sublattices, corresponding to the Fe(1) and Fe(2) sites. The presence of canting between the magnetic moments of these two sublattices has been also detected. This fact agrees with the relative orientation of t he FeO6 magnetic octahedra in the Li3Fe2(AsO4)(3 - x)(PO4)(x) (x = 0, 1, 1. 5, 2) compounds.