Magnetic polyoxometalates:: Anisotropic exchange interactions in the CoII3 moiety of [(NaOH2)Co3(H2O)(P2W15O56)2]17-

The magnetic exchange interactions in a Co-3(II) moiety encapsulated in Na-17 [(NaOH2)CO3(H2O)(P2W15O56)(2)] (NaCO3) were studied by a combination of magnetic measurements (magnetic susceptibility and low-temperature magnetization), with a detailed Inelastic Neutron Scattering (INS) investigation. The novel structure of the salt was determined by X-ray crystallography. The ferromagnetic CO3O14 triangular cluster core consists of three octahedrally oxo-coordinated Co-II ions sharing edges. According to the single-ion anisotropy and spin-orbit coupling usually assumed for octahedral Co-II ions, the appropiate exchange Hamiltonian to describe the ground-state properties of the isosceles triangular CO3 spin cluster is anisotropic and is expressed as (H) over cap = -2Σ(a=x,y,z)(J(α)(12)(S) over cap (1α)(S) over cap (2α) + J(α)(23) (S) over cap (2α)(S) over cap (3α) + J(α)(13)(S) over cap (1α)($) over cap (3α)), where J(α) are the components of the exchange interactions between the Co-II ions. To reproduce the INS data, nonparallel anisotropic exchange tensors needed to be introduced, which were directly connected to the molecular symmetry of the complex. The following range of parameters (value ± 0.5 cm(-1)) was found to reproduce J(y)(12) Jy(3) all experimental information while taking magnetostructural relations into account: J(x)(12) = J(y)(13) = 8.6 cm(-1); J(y)(12) =J(x)(13) = 1.4 cm(-1); J(y)(12) = J(x)(13) = 10.0 cm(-1); J(x)(23) =J(y)(23) = 6.5 cm(-1) and J(z)(23) = 3.4 cm(-1).