Measurements of the electric field gradient at cadmium in the high Tc superconductor YBa2Cu3O6+x

The electric field gradient (EFG) at¹¹¹Cd probe atoms in YBa₂Cu₃O_6+x has been studied by the perturbed angular correlation (PAC) spectroscopy. After annealing at temperatures below 1000 K the probe atoms are located at the Cu(1) site; above 1100 K they preferentially occupy the Y site, whereas in the intermediate temperature range they also reside on the Ba site. A reduction of the oxygen content from x=1.0 to 0.2 resulted in a decrease of the asymmetry parameter measured at the Ba site, but in an increase of it at the Y site, indicating there a local distortion of the YBa₂Cu₃O_6+x lattice by the Cd impurity. Two different surroundings for the Cd atoms at the Cu(1) site were observed.