Topotactic reductive synthesis of A-site cation-ordered perovskite YBaCo₂O_x (x = 4.5-5.5) epitaxial thin films

A-site cation-ordered perovskite YBaCo₂O_x epitaxial films were synthesized by combining pulsed-laser deposition and topotactic reduction using CaH₂. The oxygen contents (x) of the films could be controlled in a range of 4.5-5.5 by adjusting the reaction temperature. The c-axis length of the YBaCo₂O_x films decreased with decreasing x when x ≥ 5.3 but drastically increased when x ∼ 4.5. In contrast, the in-plane lattice constants remained locked-in by the substrate after the reaction. The metal insulator transition observed in bulk YBaCo2O5.5 was substantially suppressed in the present film, likely because of the epitaxial strain effect. The resistivity of the films was significantly enhanced by changing the x value from ∼ 5.5 to ∼ 4.5, reflecting the distortion of the CoOx layers.