Effect of Cu-Co and coupled Er-Ca/Cu-Co substitution upon superconductivity in ErBa2Cu3O7-delta

The structural and superconducting properties of Cu-Co and coupled Er-Ca/Cu-Co substitution in ErBa2Cu3O7-delta have been studied using X-ray diffraction, a.c. susceptibility, resistivity and oxygen, content measurements. The oxides (Er1-yCay)Ba-2(Cu1-xCox)(3)O-z have been synthesized for 0 less than or equal to x less than or equal to 0.15 and 0 less than or equal to y less than or equal to 0.45. The effect of increasing the Co-concentration in ErBa2(Cu1-xCox)(3)O-z for 0 less than or equal to x less than or equal to 0.15 is to lower the carrier concentration and decrease T-c, which is attributed to the hole filling by Co. This suppression in T-c has been compensated for by appropriate hole doping with Ca, in (Er1-yCay)Ba-2(Cu1-xCox)(3)O-2, which shows that for a fixed Co-content T-c increases dramatically as the Ca-content increases, leading, for instance, to the oxide (Er0.55Ca0.45)Ba-2(Cu0.85Co0.15)(3)O-6.81 (to a T-c of 71 K), whereas the non-substituted phase ErBa2(Cu0.85Co0.15)(3)O-6.70 does not superconduct. As a result, the 'compensated oxide' (Er1-yCay)Ba-2(Cu1-xCox)(3)O-z exhibits an almost x-independent T-c ranging from 86 to 84 K for 0.12 less than or equal to y = 3x less than or equal to 0.24. (C) 1996 Elsevier Science Limited.