THE LAYERED THALLIUM CUPRATES, Tl1+xA2−yLn2Cu2O9: A TRIPLE INTERGROWTH OF THE PEROVSKITE, ROCK SALT AND FLUORITE STRUCTURE

Abstract

A new series of layered thallium cuprates — Tl 1.3 Sr 1.6 Pr 2 Cu 2 O 9, Tl 1.3 Ba 1.7 Pr 2 Cu 2 O 9 and Tl 1+x Ba 2−y NdCeCu 2 O 9 — characterized by a mixed valence of thallium Tl(I)-Tl(III) has been synthesized. These compounds crystallize in the tetragonal system (a ≃ 3.9 Å; c ≃ 30 Å). Their structure can be described as an intergrowth of three type of layers: pyramidal layers derived from the perovskite double rock salt-type layers and double fluorite-type layers. These new materials are characterized by a mixed valence of thallium, Tl(III) and T1(I), distributed over two different layers. The study of their electron diffraction patterns shows the existence of satellites like in bismuth and lead cuprates. The incommensurability of the structure, as well as the absence of superconductivity are discussed: a particular attention is drawn to the role of the lone pair cation T1(I) in such properties.