Effect of nominal copper concentration in yttrium barium copper oxide (YBa_2Cu_{3\pmx}O_{7-\delta} ) on the superconducting transition

Variation of [Cu] in the nominal compn. YBa_2Cu_{3\pmx}O_{7-\delta} markedly influences the superconducting transition. The transition occurs at the highest temp. with the lowest width, when $x = 0.25

Structure and superconducting properties of YBa2−xLnxCu3O7±δYBa_{2-x}Ln_xCu_3O_{7\pm\delta} (Ln = La, Nd or Gd) and (Ln,Ln′)3−xBa3+xCu6O14+δ(Ln,Ln')_{3−x}Ba_{3+x}Cu_6O_{14+\delta}

In $YBa_{2-x}Ln_xCu_30_{7\pm\delta}$ (Ln = La, Nd or Gd), the structure becomes tetragonal with increase in x and the $T_c$ of the orthorhombic phase decreases with x. Members of $(Ln,Ln')_{3_x}Ba_{3+x}CU_60_{14+\delta}$ (Ln = Nd or Gd)are generally tetragonal and non superconducting. $Ln_3Ba_3Cu_60_{14+\delta}$ and $LnBa_2Cu_30_7\pm\delta$ (Ln =Nd or Gd) prepared by low-temperature decomposition of nitrates are also tetragonal and nonsuperconducting. Random occupation of Ba in $LnBa_2Cu_30_{7\pm\delta}$ or $La_3Ba_3Cu_60_{14+\delta}$ appears to favour the tetragonal structure. Furthermore oxygen is less labile when La partly substituting Ba in these oxides