Cationic substitution and role of oxygen in the n-type superconducting T′ system ${\mathrm{Nd}}_{2\mathrm{-}\mathrm{y}}$${\mathrm{Ce}}_{\mathrm{y}}$${\mathrm{CuO}}_{\mathrm{z}}$

The effect of oxygen content and cationic substitution on the structural and superconducting properties of the T′ phase were studied by investigating the ${\mathrm{Nd}}_{2\mathrm{-}\mathrm{y}}$ ${\mathrm{Ce}}_{\mathrm{y}}$ ${\mathrm{CuO}}_{\mathrm{z}}$ and ${\mathrm{R}}_{1.85\mathrm{-}\mathrm{x}}$ ${\mathrm{R}}_{\mathrm{x}}^{\prime }$ ${\mathrm{M}}_{0.15}$ ${\mathrm{CuO}}_{\mathrm{z}}$ (R=Nd,Eu; R′=La,Y; M=Ce,Th; z≅4) series. We find that solid solutions of the T′ phases can be prepared, and the solubility range of La and Y depends on the sizes of the host versus the dopant ions. The transition from tetragonal T′ phase to the orthorhombic T phase is quite abrupt, without evidence for the T* phase. In this system, as for the La-Sr-Cu-O system, we find that superconductivity seems to correlate with the in-plane Cu-Cu distance, and there is a critical distance (3.95 Å) for which ${\mathrm{T}}_{\mathrm{c}}$ is maximum. The as-prepared Ce- or Th-doped materials have an oxygen content per unit formula greater than four, and when heated under nitrogen they lose oxygen in two steps at temperatures of 400 and 800 °C, with the amount of oxygen lost through each step being strongly dependent on sample processing. The total amount of mobile oxygen lost decreases with increasing Ce or Th content. From thermogravimetric-analysis measurements we show the crucial importance of the reducing step at temperatures greater than 850 °C for inducing superconductivity as well as the importance of the cooling rate on the superconducting properties of the T′ phases. The importance of oxygen disorder to the normal-state properties is demonstrated.