Hanspeter Andres, Juan M. Clemente-Juan, Michael Aebersold, Hans U. Güdel, Eugenio Coronado, Herma Büttner, Gordon Kearly, Julio Melero and Ramon Burriel
Magnetic Excitations in Polyoxometalate Clusters Observed by Inelastic Neutron Scattering: Evidence for Anisotropic Ferromagnetic Exchange Interactions in the Tetrameric Cobalt(II) Cluster [Co₄(H₂O)₂(PW₉O₃₄)₂]^10-. Comparison with the Magnetic and Specific Heat Properties
J. Am. Chem. Soc. 121, 10028-10034 (1999)                 

Abstract: The ground-state properties of the tetranuclear Co²⁺ cluster [Co₄(H₂O)₂(PW₉O₃₄)₂]^10– were investigated by combining specific heat, magnetic susceptibility, and magnetization measurements with a detailed inelastic neutron scattering (INS) study on a fully deuterated sample of K₁₀[Co₄(H₂O)₂(PW₉O₃₄)₂]·22H₂O. As a result of the single ion anisotropy of the octahedral Co²⁺, the appropriate exchange Hamiltonian of the Co₄ spin cluster is anisotropic. INS turns out to be essential for the determination of energy splittings in the ground state resulting from the coupling. Besides the energy pattern, INS provides information about the wave functions of the split ground-state components of the spin cluster. Based on the Hamiltonian _AN = _i=x,y,z -2J_i(S₁iS₃i + S₁iS₄i + S₂iS₃i + S₂iS₄i) - 2J'_i(S₁iS₂i) the following set of parameters is obtained: J_z = 1.51 meV, J_x = 0.70 meV, J_z' = 0.46 meV, J_x' = 0.44 meV, r = 1.6, where r = J_x/J_y = J_x'/J_y'. These parameters also reproduce the magnetic specific heat, susceptibility, and magnetization properties of the compound.