We present a Nuclear Quadrupole Resonance (NQR) study of Cu(2) (planes)
and Cu(1) (chains) sites in two YBa2Cu3Oy≈7 powder samples with carbon concentration equal to
500 ppm and 3000 ppm, and in a free-carbon sample. By comparison with standard
spectra in YBa2Cu3Oy, we show that a new NQR line appears at 30.90 MHz, the intensity of which is
strongly correlated with the amount of carbon in the sample. Moreover, according to the
spin-lattice relaxation rate 1/T1 measurements at different frequencies of the spectrum, we
attribute this anomalous line to the formation of clusters in which the superconductivity is
destroyed by the presence of carbonate groups, which substitute preferentially for copper
chain site. Those results are in good agreement with those of
Wang et al. [1], obtained by high
resolution electron microscopy.