ON THE ELECTRIC-FIELD GRADIENT AT COPPER NUCLEI IN OXIDES

A useful interpretation is presented of the material dependence of Cu electric field gradient (EFG) in a great variety of insulating and superconducting copper oxides. The present study is concerned only with copper sites in nearly tetragonal symmetry and in stoichiometric compositions. The experimental data of Cu EFGs have been analyzed in terms of ionic picture. The analysis has revealed for the first time a systematic correlation between the observed Cu EFG and the ionic contribution to the EFG. By using the correlation, we have extracted empirical values of the Sternheimer antishielding factor gamma(infinity) and the hyperfine constant [r-3] for Cu2+ and Cu1+ ions. Those values are somewhat different from the traditional ones of the results of unrestricted Hartree-Fock (UHF) calculations for free ions.