CORRELATIONS BETWEEN FREQUENCY OF INFRARED ACTIVE VIBRATIONAL-MODES AND COPPER-OXYGEN DISTANCE IN COPPER OXIDES - APPLICATION TO SUPERCONDUCTORS

The infra-red spectra of a large number of ternary Cu(II) oxides with at least a quasi square-planar coordination of oxygen around the copper ions have been studied. The frequency of the bands with the highest frequency, nu-max, is found to correlate extremely well with the shortest Cu-O distance. nu-max increases at an impressive rate of approximately 20 cm-1 per 0.01 angstrom when the Cu-O distance becomes less than 1.97 angstrom, which is the Cu2+ -O2- distance in square-planar CuO4 complexes as obtained from empirical ionic radii considerations. The marked sensitivity may be used as a "titration" procedure not only to assign bands but also to obtain diagnostic information about local coordination in compounds derived, for example, from the YBa2Cu3O7-d structure such as LaCaBaCu3O7-d. The only example where this correlation fails is in the two-layer non-superconducting oxides derived from La2(Ca,Sr)Cu2O6. The significance of this result is discussed. The marked dependence of frequency on the bond-distance is qualitatively examined in terms of an increased electron-phonon coupling to account for the observed tendency of the superconducting transition temperature to go through a maximum as the average basal plane Cu-O distance is decreased.