LOCAL ORDERING OF THE CHROMATE GROUPS IN THE 1212-SUPERCONDUCTOR TL-0.9(CRO4)(0.2)SR2CA0.9CU2O6.2-DELTA

The oxychromate Tl-0.8(CrO4)(0.2)Sr-2(Ca0.9Tl0.1)Cu2O6.2-delta was pre pared by solid state reaction. An electron diffraction investigation s howed a tetragonal P-type subcell with a approximate to a(p) approxima te to 3.8 Angstrom and c approximate to c(1212) Angstrom. A system of satellites lying in incommensurate positions is associated with every Bragg spot. The modulation vector is qb + 1/2 c* with q approximate t o 0.23. XRD and HREM characterization confirmed the 1212-type structur e of the oxychromate and an infrared spectrum the presence of tetrahed ral CrO4 groups. The nanostructural study showed that the modulated st ructure is generated by local ordering phenomena between the CrO4 tetr ahedra and the thallium polyhedra. The optimized material: after succe ssive reducing annealings, exhibits a T-c of 110 K with a diamagnetic fraction volume of 65%. (C) 1998 Elsevier Science B.V.