New Thallium Oxycarbonates Built Up from Rock Salt Layers: A4TL 2CO 3O 6 ( A = Ca, Sr, Ba)

Abstract

The investigation of the systems A O-TL 2O 3-ACO 3 has allowed three new oxycarbonates A 4TL 2CO 3O 6 to be synthesized for A = Ca, Sr, Ba. A study of the system SrO-TL 2O 3, under the same conditions, i.e., in sealed tubes, has not been able to detect the well-known oxide Sr 4Tl 2O 7. An XRD and ED study of these oxycarbonates leads to a tetragonal I-centered cell with a ≈ 5Å and c ≈ 18Å, previously obtained for Sr 4Tl 2O 7. The detailed ED study of the oxycarbonate Ca 4Tl 2CO 3O 6 has shown the following features: (i) there exist very weak extra reflections leading to an orthorhombic supercell a' = 2a, b' = a, c ' = c, and (ii) streaks are observed along c* correlated to a disordered stacking of some (001) layers. The powder XRD and HREM investigation of this phase shows that its structure consists of distorted rock salt-type layers interleaved with layers of carbonate groups. The close relationships between this structure and that of the 1201 layered cuprates are discussed.