MICROSCOPIC ORIGIN OF DIMERIZATION IN THE CUO2 CHAINS IN SR14CU24O41

The interplay between structure and magnetic properties of Sr14Cu24O41 +delta with CuO2 chain and Cu2O3 ladder building blocks is studied as a function of oxygen nonstoichiometry. The characteristic decrease in the magnetic susceptibility below 80 K for delta approximate to 0 disa ppears both with increasing and decreasing delta, and, correspondingly , the periodicity of the superstructure, which arises from the lattice mismatch along the chain direction between the two structure blocks, shows a significant change. The microscopic origin of the singlet grou nd state is suggested to be the localization of electrons at low tempe rature in dimers on structurally modulated CuO2 chains.