Structural and magnetic dimers in the spin-gapped system CuTe2O5
  We investigated the magnetic properties of the system CuTe2O5 by susceptibility and electron spin resonance measurements. The anisotropy of the effective g-factors and the ESR linewidth indicates that the anticipated structural dimer does not correspond to the singlet-forming magnetic dimer. Moreover, the spin susceptibility of CuTe2O5 can only be described by taking into account interdimer interactions of the same order of magnitude than the intradimer coupling. Analyzing the exchange couplings in the system we identify the strongest magnetic coupling between two Cu ions to be mediated by super-super exchange interaction via a bridging Te ligand, while the superexchange coupling between the Cu ions of the structural dimer only results in the second strongest coupling. 
J. Deisenhofer, R. M. Eremina, A. Pimenov, T. Gavrilova, H. Berger, M.   Johnsson, P. Lemmens, H.-A. Krug von Nidda, A. Loid...