Orientation of the main axis of the electric field gradient at the iron nucleus in some iron doped copper oxide compounds

We have performed 57Fe Moessbauer measurements on aligned iron-doped crystallites of La1.85Sr0.15CuO4-y and Pb2Sr2R0.50Cu3O8-y where R=Eu or Ho. From the intensity ratio of the two lines of the quadrupole doublets we deduce that the main axis of the electric field gradient does not coincide with the c axis of the lattice. This surprising result indicates that at the iron probe, substituted in the Cu site of the CuO2 plane, the local electric charge symmetry is different from the lattice symmetry. We discuss this result in relation with the electronic configuration nearby this Cu site. The Pb based samples without Ca are magnetic at room temperature. Therefore in these samples Ca plays the same role as Sr in (La, Sr)2CuO4-y. (orig.)