Influence of side groups on 90 degrees superexchange: A modification of the Goodenough-Kanamori-Anderson rules

A mechanism is suggested which can modify in certain cases the Goodenough-Kanamori-Anderson rules determining the character of superexchange in magnetic insulators; namely, side groups coupled to ligands, which are often present but are usually ignored, may contribute significantly, and may in certain cases even lead to a change in sign of the superexchange interaction. Thus this factor can make the 90 degrees superexchange of half-filled shells antiferromagnetic, in contrast to the usual case. Qualitative arguments and numerical estimates show that this mechanism may be important in the inorganic spin-Peierls compound CuGeO3.