W.T. Fu

Superconductivity in (Pb, Bi)2Sr2−xLaxCu2O6+δ

Abstract Pb 2 Sr 2− x La x Cu 2 O 6+δ has a structure, which can be derived from that of Pb 2 Sr 2 Ca 1− x Y x Cu 3 O 8+δ , by removing a CuO 2 and (Ca, Y) layer. The until cell parameters for the composition with x = 1.0 are a = 0.5333 (2), b = 0.5421 (2), c = 1.2609 (6) nm with space group P22 1 2. The onset of the superconducting transition temperature is 32 K with zero resistance at 26 K for x = 1.2, whereas for other compositions a broader transition is observed. Partial replacement of Pb by Bi leads to a decrease in T c . Heating in air at 500°C leads to a transformation to tetragonal symmetry, which can be reversed by heating in nitrogen at 500°C.

High-temperature superconductivity in LaBaCaCu3O6.85

Abstract A new superconducting compound with composition LaBaCaCu3O6.85 has been prepared. It crystallizes in a pseudo-tetragonal structure related to YBa2Cu3O7. The onset superconducting temperature is 78 K. Electron diffraction clearly shows the presence of a superstructure in this material. Conductivity and superconductivity data are presented and possible structures of the unit cell are discussed.

Iridium-based double perovskites for efficient water oxidation in acid media

The development of active, cost-effective and stable oxygen-evolving catalysts is one of the major challenges for solar-to-fuel conversion towards sustainable energy generation. Iridium oxide exhibits the best available compromise between catalytic activity and stability in acid media, but it is prohibitively expensive for large-scale applications. Therefore, preparing oxygen-evolving catalysts with lower amounts of the scarce but active and stable iridium is an attractive avenue to overcome this economical constraint. Here we report on a class of oxygen-evolving catalysts based on iridium double perovskites which contain 32 wt% less iridium than IrO2 and yet exhibit a more than threefold higher activity in acid media. According to recently suggested benchmarking criteria, the iridium double perovskites are the most active catalysts for oxygen evolution in acid media reported until now, to the best of our knowledge, and exhibit similar stability to IrO2.

Structural and transport properties of the triple-layer compounds Ba4(Pb1−xBix)3O10 (0≤ x<0.3)

A series of new compounds Ba4(Pb1−xBix)3O10 with 0≤x<0.3 has been prepared and characterized by X-ray and electron diffraction. The d.c. resistivity vs. temperature data for different x values are presented and structural features are compared to those of the superconducting phase Ba(Pb1−xBix)O3. The “mother-compound” Ba4Pb3O10 is tetragonal, with lattice constants a=0.4280(1) and c=3.017(1) nm. Its structure can be regarded as a stacking of Ba(PbBi)O3 triple-layers, separated by single BaO layers. Electron diffraction reveals the presence of a weak superstructure with the a-axis related to that of the perovskite by a=√2ap. Frequent intergrowth of this phase with the perovskite structure is observed. The conductivity of the samples Ba4Pb3O10 is nearly independent of temperature, while Bi-doped samples exhibit semiconductor-like behaviour at low temperature. No superconducting transition is observed down to 2K.

Preparation and crystal structure of Pb2Ba2YCu3O8+δ

Abstract The structure of Pb2Ba2YCu3O8+δ has been determined by electron diffraction and neutron powder diffraction. This compound adopts the Pb2Sr2YCu3O8+δ structure, having orthorhombic symmetry, space group P2212, with unit cell parameters a=0.54654(3) b=0.54973(3) and c=1.61729(9) nm. The orthorhombic deviation from the tetragonal symmetry is caused by a shift of the oxygen atoms in the PbO layer, leading to a distorted PbO5 pyramid with three short and two large Pb-O distances. Pb2Ba2YCu3O8+δ shows a semiconductor-like behavior.

Crystal chemistry of bismuthate-based superconductors: the origin of (local) charge disproportionation

Abstract The crystal structures of the superconducting BaPb 1− x Bi x O 3 and Ba 1− x K x BiO 3 systems are summarized and the role of lead or potassium substitution is discussed. It is pointed out that the metal-insulator transition in these systems can readily be explained with a model based on the competition between (conduction) bandwidth and charge disproportionation. In the context of this model, the origin of the charge disproportionation of Bi (whether long-range or local ordering of Bi 3+ and Bi 5+ ) can be understood. This qualitative model may also explain the negative results of other bismuth-doped systems, providing a useful test for experimental challenges.

Structural and physical properties of the tetragonal superconductors LaBaACu3O7-δ (A = Ca and Sr)

Abstract The compounds LaBaACu 3 O 7-δ (A = Ca and Sr) have been prepared and characterized by X-ray diffraction and electron microscopy. Their structure is similar to that of YBa 2 Cu 3 O 7 , except that they are tetragonal. Electron diffraction of LaCaBaCu 3 O 7-δ reveals the presence of a weak superstructure having doubled, a , b and c axes. The oxygen content and superconducting transition temperature were found to vary significantly, depending on the post-annealing conditions. Oxygen- and air-annealed samples have T c values of 76 K and 60 K for A = Ca and 50 K and 33 K for A = Sr respectively. Air-quenched samples exhibit semiconductor-like behaviour in both cases. The d.c. resistivity and a.c. susceptibility vs. temperature data for LaBaSrCu 3 O 7-δ are presented. The absence of an orthorhombic unit cell for these compounds is discussed and a model for the oxygen-vacancy ordering is proposed to explain the observed superstructure in LaBaCaCu 3 O 7-δ .

High resolution electron microscopy and electron diffraction on LaCaBaCu3O6.85

Abstract High resolution electron microscopy has been carried out on LaCaBaCu3O6.85. This compound has a structure very similar to that of YBa2Cu3O7, except that this compound is tetragonal according to X-ray diffraction. Electron diffraction on materials annealed in oxygen shows the presence of a weak superstructure having a doubled a and c axis with a B centering, and which is orthorhombic, whereas in air annealed samples diffuse streaks are observed along a ∗ and b ∗ , corresponding to a doubled c axis. HREM suggests that the Y site is for approximately 75% occupied by La, when it is assumed that all Ba is located at the Ba positions. It is argued that the absence of an orthorhombic structure similar to that of YBa2Cu3O7 is caused by twinning on a very fine scale due to twin boundary pinning of small regions which exhibit an ordering of La, Ca and Ba, or to La on the Ba site requiring a different surrounding viz. extra oxygen in the CuO layer at z=0.

Crystal structure of the ordered perovskite: BaBi0.5Sb0.5O3

The structure of BaBi{sub 0.5}Sb{sub 0.5}O{sub 3} is determined by single crystal X-ray diffraction. It is an ordered perovskite, having rhombohedral symmetry, space group R {bar 3}m (No. 166), with lattice parameters a = 6.043(1) {angstrom} and {alpha} = 60.05(2)(degree). The final agreement factor R based on F{sup 2} for 6420 reflections was 0.052. An important structural feature of BaBi{sub 0.5}Sb{sub 0.5}O{sub 3} is the displacements of oxygen atoms from their ideal positions within the pseudo-cubic [110]{sub p} planes, creating an alternative arrangement of (larger) BiO{sub 6} and (smaller) SbO{sub 6} octahedra. Therefore, the valence states of bismuth and antimony can be readily assigned as Bi(III) and Sb(V).

A comparative study on the structure of APbO3 (A=Ba,Sr)

Abstract A comparative structural study of the tetravalent plumbates, BaPbO3 and SrPbO3, has been carried out by using neutron powder diffraction technique. Both compounds are an orthorhombic distortion of the perovskite structure, with the lattice parameters related to that of the ideal cubic-perovskite (ap) by a≈√2ap, b≈√2ap and c≈2ap. The structure of BaPbO3 (Ibmm) consists of the PbO6 octahedra tilted around the primitive [110]p axis, whereas the tilts of the PbO6 octahedra in SrPbO3 (Pbnm) are about the [110]p and [001]p axis respectively. These different tilts, necessary to keep the stable perovskite structure, are discussed and the influence of octahedral distortions on the physical properties of the relevant compounds are considered.