F. Goutenoire

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.

New mercury-based superconductors with the “1201” structure Hg0.4Pr0.6Sr2−xPrxCuO4+δ and Hg0.3Pb0.7Sr2−xLaxCuO4+δ

Abstract Two families of mercury-based superconductors, Hg 0.4 Pr 0.6 Sr 2− x Pr x and Hg 0.3 Pb 0.7 Sr 2− x La x CuO 4+δ , with a T c onset of 41 K, have been isolated. The praseodymium phase exhibits a narrow existence range x ⩽ 0.1; in the lead phase, lanthanum can be substituted for strontium up to x = 0.65. The X-ray diffraction and high-resolution electron microscopy analyses show that they both belong to the “1201” series; the structures can be described from the intergrowth of single perovskite layers with double oxygen-deficient rock-salt layers built up from the mixed layers [Hg 0.4 Pr 0.6 O δ ] ∞ and [Hg 0.3 Pb 0.7 O δ ] ∞ . Evidence is found for ordering between mercury and praseodymium and a structural model is proposed.

Substitution of mercury for thallium in the 2223-cuprate : The 130K-superconductor Tl1.6Hg0.4Ba2Ca2Cu3O10−δ

Abstract A new superconductor, Tl1.6Hg0.4Ba2Ca2Cu3O10+δ, has been synthesized, with a Tc of 130K and a diamagnetic volume fraction of 38%. The possibility to substitute mercury for thallium in the 2223 cuprate is demonstrated. The refined cell parameters, a = 3.843 (1) A and c = 35.816 A (7) are also to those of the undoped oxide. The electron diffraction and high resolution electron microscopy investigations evidence that the stacking of the layers along c is more perfect than in the pure 2223 thallium cuprate. Some regular intergrowths of the 2223 and the 2212 members are observed. Critical temperature and diamagnetic volume fraction are slightly improved by annealing under reducing atmosphere. The role of mercury is discussed.

A mercury based cuprate with the “2212” structure: Hg2−x(Cu, Pr)xBa2PrCu2O8−δ.

Abstract A mercury based cuprate, Hg2−x(Cu,Pr)xBa2PrCu2O8−δ, (x≈0.5) with double mercury layers has been synthesized for the first time. This oxide exhibits the 2212-type structure with a tetragonal symmetry: a =3.9236(1) A , c =28.993(1) A . HREM, XRD and EDS studies show that the double mercury layers are partially occupied by copper and praseodymium according to the formula [Hg1.5Pr0.3Cu0.2O2−δ]∞, whereas the fluorite cages between the pyramidal copper layers are occupied by praseodymium. The material is not superconducting above 4K, in agreement with the presence of praseodymium in the fluorite cages.

The calcium thallate Ca3Tl4O9, an intergrowth of the CaTl2O4 and Ca2Tl2O5 structures, member n = 1.5 of the series CanTl2On+3

Abstract A new calcium thallate Ca 3 Tl 4 O 9 has been isolated. It crystallizes in a monoclinic cell, space group C 2/ m , with a = 11.118(3) A, b = 3.341(1) A, c = 12.287(3) A, and β = 102.88(2)°. The ab initio determination of this strucure shows that it consists of a regular intergrowth of the first two members of the calcium thallate family Ca n Tl 2 O 3+ n : CaTl 2 O 4 ( n = 1) and Ca 2 Tl 2 O 5 ( n = 2). Intergrowth defects have been characterized by electron microscopy. Among all oxides and sulfides that belong to the lillianite family, Ca 3 Tl 4 O 9 is the only one that exhibits the 2,3 L structure, i.e., where layers are built up from ribbons of two and three edge-sharing octahedra alternately.

Mercury and thallium based superconducting oxycarbonates

Abstract Five layered oxycarbonates involving mercury or thallium have been synthesized and characterized on a structural point of view and on that of the physical properties. Critical temperatures up to 70 K and superconducting volume fractions higher than 50 % can be achieved.