Anisotropic exchange coupling in the Keggin derivative K8[Co2(D2O)(W11O39)] · n D2O

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 η=0.33 based on the effective coupling Hamiltonian Ĥ=−2J[S1zS2z+η(S1xS2x+S1yS2y)]. The anisotropy results mainly from the single-ion anisotropy of the Co2+ ion in the distorted octahedral coordination.