Juan M. Clemente-Juan, Eugenio Coronado, Alejandro Gaita-Ariño, Carlos Giménez-Saiz, Grégory Chaboussant, Hans-Ulrich Güdel, Ramón Burriel and Hannu Mutka
Magnetism in Polyoxometalates: Anisotropic Exchange Interactions in the Co^II₃ Moiety of [Co₃W(D₂O)₂(ZnW₉O₃₄)₂]^12- - A Magnetic and Inelastic Neutron Scattering Study
Chem. Eur. J. 8, 5701-5708 (2002)                 

Abstract: The ground-state properties of a Co^II₃ 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 Na₁₂[Co₃W(D₂O)₂(ZnW₉O₃₄)₂]·40 D₂O (Co₃). The ferromagnetic Co₃O₁₄ cluster core consists of three octahedrally oxo-coordinated Co^II ions. According to the single-ion anisotropy and spin-orbit coupling of the octahedral Co^II ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co₃ spin cluster is anisotropic and is expressed as H = -2 S_a=x,y,z (J_a¹²_1a2a + J_a²³_2a3a), where J_a 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_x¹²=J_y²³, J_y¹²=J_x²³, J_z¹²=J_z²³. This result was correlated with the molecular symmetry of the complex. The following set of parameters was obtained: J_x¹²=J_y²³=1.37, J_y¹²=J_x²³=0.218, and J_z¹²=J_z²³=1.24 meV. This set also reproduces in a satisfactory manner the specific heat, susceptibility, and magnetization properties of Co₃.