Order-disorder transition in (Cu_0.5Cr_0.5)Sr₂CuO_x under high-pressure and high-temperature conditions

The (Cu_0.5Cr_0.5)Sr₂CuO_x phase is the first member of the (Cu_0.5Cr_0.5)Sr₂Ca_n-1Cu _nO_2n+3 homologous series, having a layered structure with a single CuO₂ plane. Numerous samples of this system were prepared under high-pressure/high-temperature conditions while varying the x value. The phases crystallize with tetragonal symmetry when the samples are quenched to room temperature, while they show orthorhombic symmetry when slow cooled (with the exception of the x = 5.0 sample). Transmission electron microscopic data collected on several samples have allowed us to suggest that the origin of the orthorhombic distortion in the slowly cooled phases lies in the ordered arrangement of copper, chromium, and oxygen atoms in the (Cu_0.5Cr_0.5) plane. We have also concluded on the absence of superconductivity in the (Cu, Cr)-1201 phases.