NEW THALLIUM OXYCARBONATES BUILT-UP FROM ROCK-SALT LAYERS - A(4)TL(2)CO(3)O(6) (A=CA, SR, BA)

The investigation of the systems AO-Tl2O3-ACO(3) has allowed three new oxycarbonates A(4)Tl(2)CO(3)O(6) to be synthesized for A = Ca, Sr, Ba . A study of the system SrO-Tl2O3, under the same conditions, i,e., in sealed tubes, has not been able to detect the well-known oxide Sr4Tl2 O7. An XRD and ED study of these oxycarbonates leads to a tetragonal I -centered cell with a approximate to 5 Angstrom and c approximate to 1 8 Angstrom, previously obtained for Sr4Tl2O7. The detailed ED study of the oxycarbonate Ca4Tl2CO3O6 has shown the following features: (i) th ere exist very weak extra reflections leading to an orthorhombic super cell a' = 2a, b' = a, c' = c, and (ii) streaks are observed along c c orrelated to a disordered stacking of some (001) layers. The powder XR D and HREM investigation of this phase shows that its structure consis ts of distorted rock salt-type layers interleaved with layers of carbo nate groups. The close relationships between this structure and that o f the 1201 layered cuprates are discussed. (C) 1995 Academic Press, In c.