Phase separation and cation distributions in Tl-2201 from resonant synchrotron X-ray diffraction

Abstract

Orthorhombic and tetragonal Tl-2201 (‘Tl 2Ba 2CuO 6+δ’) samples have been studied using resonant synchrotron X-ray powder diffraction data at energies just below the Cu K and Tl L III absorption edges. The resonant contributions enable us to demonstrate that Cu substitutes at the Tl site; the tetragonal structure is stabilised by a higher (5.5%) degree of substitution than the orthorhombic material whereit is <1%. A slight splitting of some diffraction peaks of orthorhombic Tl-2201 is consistent with separation into two phases due to the segregation of interstitial oxygens, giving one phase with δ = 0 and another with δ = 0.3 that displays an incommensurate superstructure. These two phases are respectively too under- and over-doped to superconduct.