S.N. Putilin

The synthesis and characterization of the HgBa2Ca2Cu3O8+δ and HgBa2Ca3Cu4O10+δ phases

Abstract The third (Hg-1223) and the fourth (Hg-1234) members of the recently-discovered homologous series HgBa2Can−1CunO2n+2+δ have been synthesized by solid state reaction, carried out at 950°C under 50 kbar at different annealing times. These phases have a tetragonal cell with lattice parameters: a = 3.8532 (6) A , c = 15.818(2) A , and a = 3.8540(3) A , c = 19.006 (3) A , respectively. The c parameters are in agreement with the formula c ≌ 9.5 + 3.2 (n − 1). Electron microscopy study showed similar lattice parameters as well as the occurence of different intergrowths and stacking faults. A periodicity of 22 A has also been detected, which may be attributed to the existence of the Hg-1245 phase. EDS analysis data of several grains of Hg-1223 and Hg-1234 are in agreement with the proposed chemical formulae. AC susceptibility measurements show that an increase of the superconducting transition temperature with n in the HgBa2Can−1CunO2n+2+δ series occurs till the third member, after which a saturation seems to be achieved.

Neutron powder diffraction study at room temperature and at 10 K of the crystal structure of the 133 K superconductor HgBa2Ca2Cu3O8+δ

Abstract The crystal structure of the superconductor HgBa 2 Ca 2 Cu 3 O 8+δ has been determined by neutron powder diffraction techniques at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a =3.8501(1), c=15.7837(9) A . The structure is described by the sequence of layers: …[(CuO 2 )(Ca)(CuO 2 )(BaO)(HgO δ (BaO)(CuO 2 )(Ca)]… The compound, prepared by a solid-state reaction between HgO and the precursor Ba 2 Ca 2 Cu 3 O x at a pressure of 18 kbar and 880°C, has an oxygen composition corresponding to δ =0.41 (2). This extra oxygen is located on the layer of the mercury atoms at the sites 1 2 , 1 2 , 1 2 . The superconducting transition temperature for the as-prepared sample is T c =133 K . No evidence has been found of orthorhombic distortions, nor for substitution of some of the Hg atoms by Cu. The copper and oxygen atoms of the layers (CuO 2 ) are coplanar for the Z =0 (CuO 2 plane and almost perfectly coplanar for the others. No phase transition occurs down to 10 K.

Neutron powder diffraction study of the crystal structures of HgBa2CuO4+δ and HgBaO2

Abstract The crystal structures of HgBa 2 CuO 4+δ and HgBaO 2 have been refined at room temperature using neutron powder diffraction data obtained from a sample containing 44% of the first phase, 42% of the second and 14% of Ba 2 Cu 3 O 5+δ . the compound HgBa 2 CuO 4+δ crystallizes with the symmetry of space group P4/mmm and lattice parameters a = 3.8829 (6), c = 9.5129 (14) A . The unit cell contains only one CuO 2 layer and the material is a superconductor with a value of T c of 94 K. The oxygen in excess of the O 4 stoichiometry ( δ = 0.063 in our sample) is located in interstitial positions on the HgO δ layer, and it is the only extra oxygen present in the structure. No mixing of the cations has been detected in this study. The compound HgBaO 2 crystallizes with the symmetry of space group R 3 m and lattice parameters (hexagonal axes) a = 4.0991 (6), c = 19.355 (3) A . In this new polymorph of HgBaO 2 the Ba atoms are octahedrally coordinated while Hg has two-fold coordination.

New complex copper oxides: HgBa2RCu2O7 (R = La, Nd, Eu, Gd, Dy, Y)

Abstract A new family of complex copper oxides, HgBa 2 R Cu 2 O 7 ( R = La, Nd, Eu, Gd, Dy, Y), has been synthesized and studied by X-ray powder diffraction, transmission electron microscopy and EDS analysis. The crystal structure of one of these compounds was determined. The Hg-atoms are in rock salt like blocks alternating with perovskite like ones where the copper atoms are located. This family can be considered as a first example of Hg-containing complex copper oxides with intergrowth structures (“1212” structural type). Magnetic measurements down to 12 K did not show noticeable diamagnetic effects.

Synthesis and neutron powder diffraction study of the superconductor HgBa2CaCu2O6+δ before and after heat treatment

Abstract The crystal structure of the superconductor HgBa2CaCu2O6+δ has been analyzed by neutron powder diffraction techniques at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a=3.8526(2), c=12.6367(8) A. The structure is made of the sequence of layers …[(BaO)(CuO2)(Ca)(CuO2)(BaO)(HgOδ)]… The compound, prepared by a solid-state reaction between HgO and the precursor Ba2CaCu2Ox at 18 kbar and 880°C, has an oxygen content corresponding to δ=0.35(2) and a transition temperature of T c ⋍ 104 K . Annealing in O2 (300°C for 20 h) reduces the oxygen content to δ=0.28(2). A sample annealed under the same conditions or in N2 (250°C for 12 h) was found to have a Tc of 123 K. Additional annealing in N2 at 400°C for 18 h lowers Tc to 110 K. Our results indicate that the material with δ=0.35 is overdoped. The oxygen above the O6 stoichiometry is located on the Hg layers of the structure, at the positions 1 2 , 1 2 , 0. No evidence has been found of the substitution of mercury by copper. The copper and oxygen atoms of the (CuO2) layers are almost exactly coplanar in this compound.

Chemistry and structure of Hg-based superconducting Cu mixed oxides

The most relevant aspects of thermodynamics, preparation, crystal structure and superconducting properties under ambient and high pressure of HgBa2Can−1CunO2n+2+δ superconductors are reviewed in this contribution. The ideas which inspired authors to search for the new superconductors among Hg-based complex cuprates are discussed as an example illustrating the benefit of the concept of intergrowth structure formation for the design of new layered materials. Recent results on solid–gas equilibriums useful for reproducible syntheses of different homologues are summarized. The crystal structures are discussed with main attention focused on features such as concentration and location of extra oxygen atoms, static atomic displacements due to partial occupation of oxygen site in a Hg-layer, substitution in a Hg position and a presence of stacking faults. It is shown that these factors have to be taken into account for a correct structure description. The relationships between Tc, doping level, amount and formal charge of extra anions and external pressure are outlined to predict possible paths for enhancement of the superconducting properties in complex cuprates.

Discovery of a second family of bismuth-oxide-based superconductors

The superconducting oxide BaPb1−xBixO3, discovered in 1975 (ref. 1), is an exotic system having an unusually high transition temperature (Tc) of ∼12 K, despite a relatively low density of states at the Fermi level. The subsequent prediction that dopingthe electronically inactive barium donor sites, instead ofthe bismuth sites, might induce superconductivity with a higher Tc led to the discovery in 1988 of superconductivity in the Ba1−xKxBiO3 system (Tc ∼30 K for x = 0.4). But it remains an open question why many of the superconducting properties of these materials are similar to those of the well-known copper oxide superconductors, despite their pronounced structural differences: the former have a three-dimensional bismuth–oxygen framework, whereas the structures of the latter are predominantly two-dimensional, consisting of copper–oxygen planes. Understanding of the copper oxide superconductors has gained immensely from the study of many different superconducting systems, and so it might be expected that the identification of bismuth oxide superconductors beyond the substituted BaBiO3 compounds will prove to be similarly fruitful. Here we report the synthesis of a second family of superconducting bismuth oxides, based on SrBiO3. We show that partial substitution of potassium or rubidium for strontium induces superconductivity with Tc values of ∼12 K for Sr1−xKxBiO3 (x = 0.45–0.6) and ∼13 K for Sr1−xRbxBiO3 (x = 0.5).

Inducing superconductivity and structural transformations by fluorination of reduced YBCO

Abstract Bulk superconductivity with Tc up to 94 K has been induced by fluorination of non-superconducting YBa2Cu3O6.11 using XeF2 as a fluorination agent. Strong changes on X-ray patterns were found after fluorination of reduced YBCO. High resolution electron microscopy of superconducting samples showed the presence of a new phase with c ≈ 13 A which exists as noticeable areas included within a matrix of the 123 structure or which occurs as isolated defects with a limited extension. All fluorinated compounds exhibited a strong disorder along the c-direction resulting in a ‘loss’ of c-parameter on X-ray patterns. The present results demonstrate that fluorine indeed enters the YBCO structure with a significant structural rearrangement for the high level of fluorination. The structure of the fully fluorinated YBa2Cu3O6F2 phase, possibly responsible for superconductivity, has been deduced from high resolution electron microscopy.

Investigation of the HgBa2CuO4+δ structure under external pressures up to 5 GPa by neutron powder diffraction

Abstract Neutron powder diffraction investigation of the changes in the structure of the mercury superconductor HgBa 2 CuO 4+δ has been carried out in the pressure range 0–5.07 GPa at room temperature. The compressibility values of the unit cell parameters and several bond distances in the structure have been determined. The relative reductions of the unit cell parameters are close to each other and are approximately 1.7% in the base plane and 2.3% along the c -axis. At the same time, the interatomic distance modulations are strongly inhomogeneous: approximately 4% for the apical CuO bond and only about 0.4% for the apical HgO bond.

Single crystal X-ray diffraction study of the HgBa2CuO4+δ superconducting compound

Abstract A high precision X-ray diffraction analysis up to sin θ / λ = 1.15 of a HgBa 2 cuO 4+δ single crystal having a T c of ≈ 90 K is presented. The cell parameters are a = 3.8815(4), c = 9.485 (7) A . The refinements indicate the existence of a split barium site due to the presence of excess oxygen in the mercury layer. The position of this excess oxygen might be slightly displaced from the (1/2 1/2 0) position. A 6% mercury deficiency is observed. Models, including mercury defects, substitution by copper cations, or carbonate groups, are compared. However, we obtain no definite evidence for either of the three models. A possible disorder of the Hg position, due to the formation of chemical bonds with neighbouring extra oxygen anions, could correlate to the refinements of mixed species at the Hg site. A low temperature single crystal x-ray diffraction study, and comparison of refinements for the same single crystal with different extra oxygen contents, are in progress to help clarify this problem.