Figure 3. Schematic and simplified copper-oxygen configurations of the CuO₂ planes in Pr₂CuO₄ (square planar coordinated cuprates) in accordance to the results deduced from neutron and X-ray scattering analysis and electronic transport data.

(a) In the as-grown state, random apical sites of copper are occupied (apical oxygen). During the first annealing step of our two-step annealing procedure, not apical but regular oxygen sites of the CuO₂ planes are being evacuated. During the second annealing step, the defective CuO₂ plane is being “healed” by an oxygen rearrangement from the apical sites to regular in-plane sites (shrinkage of c-axis). (b)–(d) Evolution of ρ(T) characteristics and lattice constants after each synthesis step. The as-grown T′-Pr₂CuO₄ thin film is insulating and the optimally reduced films (after step II) are superconducting while ρ(300 K) is reduced by more than 2 orders of magnitude. The T′-Pr₂CuO₄ thin films just after step I are even less conductive than the as-grown ones. (e)–(g) The in-plane lattice constant (a₀) remains constant throughout the annealing process while that of c-axis (c₀) shows an abrupt drop after step II. The lattice parameters a₀ and c₀ have been estimated from a Nelson-Riley function of the (h03h) and (002l) reflections, respectively.