New 1212-type superconductors with a Tc upt to 85 K in the system Hg-Pr-Sr-Ca-Cu-O

Abstract A new family of 1212-type superconductors, Hg 0.4 Pr 0.6 Sr 2 (Sr 1− x Pr x (0.2⩽ x ⩽0.7) and Hg 0.4 Sr 0.6 Sr 2 (Ca 1− x Pr x )Cu 2 O 6+δ (0.2⩽ x ⩽0.7) has been synthesized. Electron-microscopy studies show that all these oxides belong to the “1212” family; nevertheless, they from the classical “1212” mercury-substited captures because of the ordering between Pr and Hg in the [Hg 1− x Pr x Oδ]∞ layers. Two kinds of superstructures called O I (a I ≈a p , b I ≈a p , c I ≈c 1212 and O II ( a II ≈ a p , b II ≈ a p , c II ≈2 c 1212 ) are indeded observed . In both types, the Pr and Hg rows alternate along the a -axis within each [Hg 1− x Pr x O δ ]∞ layer; two successive layers are in phase in the O I structure, whereas they are shifted over a p in the O II structure, leading to a doubling of the c 1212 parameter. Evidence is also found for more complex cationic distributions responsible for the presence of diffuse streaking along c ∗ . All these oxides are superconducting for x ≤ 0.50; a T c up to 85 K, a sharp transition and a superconeducting volume fraction of 100% are observed. The influence of the oxygen pressure during synthesis upon the supeconducting properties of these materials is also studied.