Chemical phase separation in carbon doped YBa2Cu3O6+x: a Cu-63,Cu-65 NQR study

We report on Cu-63,Cu-65 NQR spectra and nuclear spin-lattice relaxation rate (1/T-1) measurements at 4.2 K in YBa2Cu3O6+x powder samples with carbon concentrations ranging from 500 to 3000 ppm. A new NQR line appears at 30.92 MHz (Cu-63 line), the intensity of which is strongly correlated with the amount of carbon in the sample. The measurement of the spin-lattice relaxation rate for Cu corresponding to the different lines measured shows different relaxation behaviors. Furthermore a drastic change is observed for T-1 between the two samples. We discuss the implications of these findings in the framework of chemical phase separation and compare to previous carbon retention studies in YBa2Cu3O6+x by other techniques. (C) 1997 Elsevier Science B.V.