Vacuum annealing treatment effects on structure and superconductivity in RBa₂Cu₃O_7-δ and YBa_2-xK_xCu₃O_7-δ

We report the effects of heat treatment in a very low pressure at 800°C on the structure and superconducting properties of RBa₂Cu₃O_7-δ R=Y, Nd and Eu, and YBa_2-xK_xCu₃O_7-δ. These systems undergo an orthorhombic (and superconducting) to amorphous (and non superconducting) transition, caused by large oxygen losses (δ = 1.3-1.8) and disordering of oxygen vacancies. Then by a controlled technique we restored the oxygen content to a value of δ = 0 and the orthorhombic crystal structure and superconductivity were fully recovered. The substitution of barium with potassium is reported for the first time. The recovered NdBa₂Cu₃O_7-δ (δ = 0.7) is superconducting with T_C = 89 K.