TlBa2NdCu2O7 AND TlBa2YCu2O7 TWO NON-SUPERCONDUCTIVE OXIDES WITH THE “1212” STRUCTURE

Abstract

Two new thallium cuprates isostructural with the “1212” superconductors were synthesized. These materials do not exhibit any superconductivity in spite of the presence of Tl(III), contrary to the oxides such as Tl 2 Ba 2 CaCu 2 O 8 or Tl 2 Ba 2 Ca 2 Cu 3 O 10. Their X-ray powder diffraction study confirms their close relationships with the superconductor TlBa 2 CaCu 2 O 7, i.e., their structure is built up from distorted double rock salt-type layers [(TIO) (BaO)] intergrown with oxygen deficient double perovskite layers [ BaY ( CuO 2.5)2] or [ BaNd ( CuO 2.5)2]. Although the determination of the positions of the oxygen atoms cannot be considered as accurate, one significant difference with respect to TlBa 2 CaCu 2 O 7 must be pointed out: the oxygen atoms forming the basal planes of the CuO 5 pyramids are brought closer together along c, leading to an almost cubic environment for Y and Nd, whereas the CaO 8 prisms are elongated along c in the thallium superconductors.