PHASE-SEPARATION AND CATION DISTRIBUTIONS IN TL-2201 FROM RESONANT SYNCHROTRON X-RAY-DIFFRACTION

Orthorhombic and tetragonal Tl-2201 ('Tl2Ba2CuO6+delta') 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 re sonant contributions enable us to demonstrate that Cu substitutes at t he Tl site; the tetragonal structure is stabilised by a higher (5.5%) degree of substitution than the orthorhombic material where it is <1%. A slight splitting of some diffraction peaks of orthorhombic Tl-2201 is consistent with separation into two phases due to the segregation o f interstitial oxygens, giving one phase with delta = 0 and another wi th delta = 0.3 that displays an incommensurate superstructure. These t wo phases are respectively too under- and over-doped to superconduct.