CHARGE-TRANSFER AND COMPRESSION EFFECTS OF ISOMORPHOUS SUBSTITUTIONS IN YBA2CU3O7

The influence of partial isomorphous substitutions at the Cu and Ba sites of YBa2Cu3O7-delta on the phonon spectrum and the structure has been investigated systematically by Raman scattering and neutron diffraction. For partial substitution of Sr for Ba the vibrational frequency of the apex oxygen, O(4), increases as expected from the observed compression of the structure in the region between the CuO2 planes and the CuO chains. Substitutions of trivalent ions such as Co, Fe, and Al for Cu change the coordination of the apex oxygen in a way consistent with an elimination of holes in the CuO2 planes. The O(4) vibrational frequency decreases rapidly with increasing trivalent substitution. The changes cannot be explained in a simple way by the bond-length changes observed in the neutron-diffraction experiments. A remarkable correlation is found between T(c) and the O(4) Raman mode for trivalent substitutions on the Cu(1) site and for oxygen deficiency. We suggest that this correlation as well as the rapid decrease of T(c) and the structural changes we observe are caused by a hole charge transfer from the CuO2 planes to the CuO chains.