Magnetism in polyoxometalates:: Anisotropic exchange interactions in the Co3II moiety of [Co3W(D2O)2(ZnW9O34)2]12- -: A magnetic and inelastic neutron scattering study

The ground-state properties of a Co-3(II) moiety encapsulated in a polyoxometalate anion were investigated by combining measurements of specific heat, magnetic susceptibility, and low-temperature magnetization with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na-12[Co3W(D2O)(2)(ZnW9O34)(2)].40 D2O (Co-3). The ferromagnetic Co3O14 cluster core consists of three octahedrally oxo-coordinated Co-II ions. According to the singleton anisotropy and spin-orbit coupling of the octahedral Co-II ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co-3 spin cluster is anisotropic and is expressed as H = -2Sigma(alpha=x,y,z)(J(alpha)(12)(S) over cap (1alpha)(S) over cap (2alpha) +J(alpha)(23)<(S) over cap>(2alpha)(S) over cap (3alpha)), where J(alpha) are the components of the exchange interactions between the Co-II ions. To reproduce the INS data, different orientations of the two anisotropic J tensors must be considered, and the following conditions had to be introduced: J(x)(12) = J(y)(23), J(y)(12) = J(x)(23), J(z)(12) = J(z)(23). This result was correlated with the molecular symmetry of the complex. The following set of parameters was obtained: J(x)(12) = J(y)(23) = 1.37, J(y)(12) =J(x)(23) = 0.218, and J(z)(12) = J(z)(23) = 1.24 meV. This set also reproduces in a satisfactory manner the specific heat, susceptibility, and magnetization properties of Co-3.