Anisotropic exchange coupling in the Keggin derivative K 8[Co 2(D 2O)(W 11O 39)] · n D 2O

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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 Co 2+ 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 H ̂ =−2J[S 1zS 2z+η(S 1xS 2x+S 1yS 2y)] . The anisotropy results mainly from the single-ion anisotropy of the Co 2+ ion in the distorted octahedral coordination.