Evgeny V. Antipov

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.

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.

On the stability region and structure of the Nd1+xBa2−xCu3Oy solid solution

Abstract We studied the low and upper stability boundaries and the structure evolution of the Nd 1+ x Ba 2− x Cu 3 O z solid solution along its single-phase field. It was found that in oxygen the limit of the solid solution extends up to Nd 1.9 Ba 1.1 Cu 3 O z at 950–1050°C while at lower and higher temperatures it becomes narrower. In nitrogen atmosphere the substitution range is much smaller and x does not exceed 0.3 at 800°C. The replacement of Ba 2+ by Nd 3+ is accompanied by decreasing both a and c lattice constants of quenched samples. At the same time in oxygen at x ≥ 0.6 ordering phenomena occur leading to the orthorhombic distortion of the tetragonal subcell. A model of the solid solution crystal structure with a B-centered unit cell and lattice parameters a = 2 a sub , b = b sub and c = 2 c sub was suggested and refined.

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).

BI-2201 PHASES - SYNTHESIS, STRUCTURES AND SUPERCONDUCTING PROPERTIES

Abstract The Bi2+xSr2−xCuO6+° (Bi-2201) phases and La substituted Bi-2201 compounds with general formulas Bi2Sr2−xLaxCuO6+δ and Bi2.3−xSr1.7LaxCuO6+δ were prepared and characterized by X-ray powder diffraction, electron diffraction, iodometric titration and resistance measurements. The monophasic samples of Bi2+xSr2−xCuO6+δ were obtained in air at 850°C for 0.15 ≤ x ≤ 0.4, while synthesis at 740°C in oxygen flow extended this range to the stoichiometric cation composition (0 ≤ x ≤ 0.4). The superconducting transitions were not detected for these monophasic samples in contrast to the La substituted phases which exhibited superconductivity, and the highest Tc,onset = 33 K was found for the Bi2Sr1.6La0.4Cu6.33 compound. Superconductivity in the Bi2Sr2−xLaxCuO6+δ series exists in overdoped and underdoped regions. Incommensurately modulated structures of nonsuperconducting Bi2.3Sr1.7CuO6.23 phase and superconducting Bi2Sr1.7La0.3CuO6.28 phase (Tc,onset = 26 K) were refined from X-ray powder data by a Rietveld technique using the four-dimensional space group P:A2/a:−11. The maximal Cu displacements from the average position in the first structure was found to be sufficiently larger than in the latter one. The distorted structural arrangement of the (CuO2) layers in the Bi2.3Sr1.7CuO6.23 structure can be a reason for the suppression of superconductivity in this phase, while their less corrugated configuration in the La containing Bi-2201 structure leads to the existence of superconductivity.

Target-aimed synthesis of anion-deficient perovskites.

The brownmillerite-type A 2B 2O 5 structure is considered as a parent one, giving rise to different derivatives: layered double perovskites, A-site and anion-vacancy-ordered perovskites, and the perovskite-like compounds with crystallographic shear planes. The structural relationships and synthesis pathways for these classes of materials are discussed with particular attention to the ordering at the A or B sublattices, anion vacancy ordering, and their mutual interaction.

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.

Synthesis and structural studies of Sr2Co2−xGaxO5, 0.3⩽x⩽0.8

The present thesis deals with the investigation of some perovskite related complex cobaltates. The phases Sr2Co2-xGaxO5 (0.3 ≤ x ≤ 0.7), Sr2Co2-xAlxO5 (0.3 ≤ x ≤ 0.5), Sr1-xBixCoO3-y (0.1 ≤ x ≤ 0.2), Sr0.75Y0.25Co1-xGaxO2.625 (0.125 ≤ x ≤ 0.375) and Sr0.75Y0.25Co1-xFexO2.625+δ (0.125 ≤ x ≤ 0.625) were synthesised and characterised. All these compounds crystallises with similar structures, they are all composed by altering layers of octahedra and tetrahedra although in the two former, the tetrahedra are organised in chains (the Brownmillerite structure), while in the latter three the tetrahedra arranges as segregated Co4O12 units (the 314 type structure). The techniques X-ray and neutron diffraction, transmission electron microscopy, thermal analysis and magnetic measurements were used to track structural and important physical properties.

Synthesis and crystal structure of the Sr2Al1.07Mn0.93O5 brownmillerite

A new brownmillerite-type compound Sr2Al1.07Mn0.93O5 was synthesized. The crystal structure was determined using electron diffraction and high resolution transmission electron microscopy and refined from X-ray powder diffraction data (space group Imma, a = 5.4358(1) A, b = 15.6230(4) A, c = 5.6075(1) A, RI = 0.036, RP = 0.023). The structure is characterized by a disordered distribution of the tetrahedral chains in L and R configuration and a partial occupation of the octahedral position by the Mn3+ and Al3+ cations. The relationships between the crystal structures of Sr2Al1.07Mn0.93O5 and its A2B′MnO5 analogues (A = Ca, Sr, B′ = Al, Ga) and the structural reasons for the different types of tetrahedral chain ordering in brownmillerites are discussed. The temperature dependences of the magnetic susceptibility and specific heat reveal that the compound is antiferromagnetically ordered below TN = 105 K.

Direct space structure solution from precession electron diffraction data: Resolving heavy and light scatterers in Pb13Mn9O25

The crystal structure of a novel compound Pb(13)Mn(9)O(25) has been determined through a direct space structure solution with a Monte-Carlo-based global optimization using precession electron diffraction data (a=14.177(3)A, c=3.9320(7)A, SG P4/m, R(F)=0.239) and compositional information obtained from energy dispersive X-ray analysis and electron energy loss spectroscopy. This allowed to obtain a reliable structural model even despite the simultaneous presence of both heavy (Pb) and light (O) scattering elements and to validate the accuracy of the electron diffraction-based structure refinement. This provides an important benchmark for further studies of complex structural problems with electron diffraction techniques. Pb(13)Mn(9)O(25) has an anion- and cation-deficient perovskite-based structure with the A-positions filled by the Pb atoms and 9/13 of the B positions filled by the Mn atoms in an ordered manner. MnO(6) octahedra and MnO(5) tetragonal pyramids form a network by sharing common corners. Tunnels are formed in the network due to an ordered arrangement of vacancies at the B-sublattice. These tunnels provide sufficient space for localization of the lone 6s(2) electron pairs of the Pb(2+) cations, suggested as the driving force for the structural difference between Pb(13)Mn(9)O(25) and the manganites of alkali-earth elements with similar compositions.