Hanspeter Andres, Michael Aebersold, Hans U. Güdel, Juan M. Clemente, Eugenio Coronado, Herma Büttner, Gordon Kearly and Markus Zolliker
Anisotropic exchange coupling in the Keggin derivative K₈[Co₂(D₂O)(W₁₁O₃₉)] · n D₂O
Chem. Phys. Lett. 289, 224-230 (1998)                 

Abstract: 20 g of the fully deuterated title compound have been prepared in polycrystalline form and investigated by inelastic neutron scattering using both thermal and cold neutrons. Magnetic dimer excitations were observed and the energy-splitting pattern resulting from the exchange coupling within the Co²⁺ dimer was determined. The coupling is highly anisotropic with the parameter values J=-2.24 meV and h=0.33 based on the effective coupling Hamiltonian H = -2J[S_1zS_2z + h(S_1xS_2x + S_1yS_2y)]. The anisotropy results mainly from the single-ion anisotropy of the Co²⁺ ion in the distorted octahedral coordination.