Absence of metallic character at certain Cu sites in YBa2Cu3O6.7

We report Cu63 nuclear-quadrupolar-resonance spectra and nuclear-spin relaxation data for the oxygen-deficient phase YBa2Cu3O6.7. Compared with YBa2Cu3O7.0, the spin-lattice relaxation rates are reduced by more than three orders of magnitude for a class of sites identified as nonmetallic Cu(1) "chain" sites with twofold oxygen coordination. Generation of these 3d10 Cu+ sites by ordering of oxygen vacancies onto "empty" chains increases the carrier concentration and may help maintain the observed 60-K plateau in the variation of Tc with oxygen concentration.