Order-disorder transition in (Cu0.5Cr0.5)Sr2CuOx under high-pressure and high-temperature conditions

The (Cu0.5Cr0.5)Sr2CuOx phase is the first member of the (Cu0.5Cr0.5)Sr2Can -1CunO2n+3 homologous series, having a layered structure with a single CuO2 plane. Numerous samples of this system were prepared under high-pressure/h igh-temperature conditions while varying the x value. The phases crystalliz e with tetragonal symmetry when the samples are quenched to room temperatur e, while they show orthorhombic symmetry when slow cooled (with the excepti on of the x = 5.0 sample). Transmission electron microscopic data collected on several samples have allowed us to suggest that the origin of the ortho rhombic distortion in the slowly cooled phases lies in the ordered arrangem ent of copper, chromium, and oxygen atoms in the (Cu0.5Cr0.5) plane. We hav e also concluded on the absence of superconductivity in the (Cu, Cr)-1201 p hases. (C) 2001 Academic Press.