G. Van Tendeloo

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.

A 94 K Hg-based superconductor with a “1212” structure Hg0.5Bi0.5Sr2Ca1−xRxCu2O6+δ (R = Nd, Y, Pr)

Abstract A new mercury-bismuth-based cuprate, Hg 0.5 Bi 0.5 Sr 2 Ca 1− x Nd x Cu 2 O 6+δ has been syntahesized for x ranging fro m 0.35 to 1. The high-resolution electron microscopy and the X-ray diffraction analyses give evidence for a 1212-type structure, without any ordering between Hg and Bi. For 0.35⩽ x ⩽0.5, the samples are superconducting with a T c conset of 94 K and a diamagnetic volume fraction of 32%. Annealings at low temperatures under different atmospheres do not modify the superconducting behavior. Substitutions of Pr and Y for Nd allow isotypic oxides to be synthesized with similar superconducting properties.

A new “1201” mercury cuprate The 27 K-superconductor Hg0.5Bi0.5Sr2−xLaxCuO5−gd

Abstract A new superconductor, Hg 0.5 Bi 0.5 Sr 2− x La x CuO 5− δ with a T c onset of 27 K has been isolated for 0≤ x ≤0.75. This phase crystallizes in the tetragonal system with a ≈ a p ≈3.8 A and c ≈8.9 A. Its X-ray diffraction and high resolution electron microscopy analysis show that it belongs, like the 94 K-superconductor HgBa 2 CuO 4+ δ , to the “1201” series. Its structure can be described as the intergrowth of single perovskite layers with double oxygen deficient rock salt layers built up from mixed layers [Hg 0.5 Bi 0.5 O 1− δ ]∞. This unique “Sr-Hg-1201” cuprate opens the route to the exploration of “mixed” superconductors resulting from the substitution of mercury for different metallic cations such as Tl, Bi, Pb in the “normal” layers that ensure the junction between the superconducting copper layers.

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.

A mercury-based superconducting cuprate, intergrowth of the 2201 and 1201 structures Tl2HgBa4Cu2O10+y

Abstract A new superconductor Tl2HgBa4Cu2O10+y has been prepared. It crystallizes in the space group I4/mmm with a = 3.8584 (1) A ; c = 42.2031 (9) A . Electron diffraction and high-resolution microscopy investigations have allowed a structural model to be established: the structure of this phase consists of the intergrowth of 2201-layers, [Tl2Ba2CuO6]∞, with 1201 layers, [HgBa2CuO4+y]∞. Susceptibility measurements show that the as-synthesized cuprate under oxygen pressure does not superconduct, but that, by annealing the sample at very low temperature in an argon—hydrogen flow, superconductivity with a Tc onset up to 50 K and a volume fraction of 17% at 4.2 K is induced.

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.

Columnar defects and irreversibility lines in Tl-based superconductors

Abstract Tl-based superconducting ceramics of different phases (Tl-2212, Tl-1223, Tl-2223) have been irradiated by heavy ions (6 GeV Pb and U). It is shown by high resolution electron microscopy that Pb irradiations induce columnar defects of about the same diameter (95 ± 5) A in Tl-2223 and Tl-1223 whereas a larger dispersion of the diameter around the value of 95 A is observed for Tl-2212. The U irradiation leads to diameters slightly more scattered than in the Pb irradiation. These samples have been studied by SQUID magnetometry before and after irradiation. Drastic increases of the critical current induced by the irradiation have been observed and the ‘hierarchy’ existing between phases about the location of their irreversibility lines remains the same after irradiation at the same fluence.

The Mercury-based 1201-0201 intergrowth HgBa2La2Cu2O8+δ : a 53K superconductor

A new Hg-based cuprate, HgBa2La2Cu2O8+δ, has been isolated. It crystallises in a tetragonal 14/mmm space group with a = 3.8239 (2) A and c = 32.067 (2) A. High resolution electron microscopy reveals the intergrowth of the “1201” - type structure HgBa2CuO4+δ with the “0201” - structure La2CuO4. This phase represents the first member “m = n = 1” of the family [HgBa2CuO4+δ]m[La2CuO4]n, the “m = 2, n = 1” member being observed as an occasional defect. The as-synthesized phase is superconducting with a Tc (onset) of 28K. This critical temperature can be increased up to 53K, with a diamagnetic volume fraction of 12% at 5K, by annealing in an argon flow. The superconducting properties are compared to those of the homologous Tl-cuprate TlBa2−xLa2+xCu2O9 which exhibits a Tc of 42K.

Copper oxycarbonates and mercury-based cuprates: promising high Tc superconductors

A short overview of two recently discovered series of high-Tc superconductors is given. The first one deals with copper oxycarbonates which exhibitcs up to 82 K. Their structural principle is based on the fact that rows of CO3 groups can assure the connection between either octahedral or pyramidal copper layers forming new intergrowths withTcs superior to those of the mother structures. The second family concerns the mercury-based cuprates, characterized byTcs ranging from 27 to 132 K. The structure of the mercury compounds is closely related to those of the thallium cuprates TlBa2Cam−1CumO2m+3 which exhibit thallium monolayers; they differ from the Tl cuprates by a high oxygen deficiency at the level of the mercury layers due to the preference of this cation for two-fold coordination.