P. Bordet

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 and electron diffraction study of YBa2Cu22Cu1.77Fe.23O7.13

Abstract The compound of formula YBa 2 Cu 2.7 Fe 0.3 O 7.13 has been analyzed by neutron and electron diffraction techniques. The material is tetragonal with lattice parameters a = b = 3.8674(1), c = 11.6687(2) A and space group P4/mmm. The Fe cations substitute only the Cu cations located on the basal plane of the structure and can adopt three different types of coordination (tetrahedral, pyramidal and octahedral) depending upon the content and distribution of the extra oxygen atoms on the plane. Calculations of the effective valence of iron cations seem to indicate that Fe 3+ is present in tetrahedral coordination and Fe 4+ in pyramidal and octahedral coordination, while values of Cu 2.2+ and Cu 2.47+ were found for the copper cations located at (00z) and (000), respectively. The electron diffraction experiments show diffuse scattering planes parallel to (110) ∗ and (1 1 0) ∗ . Crosses of strong intensity are visible at reciprocal nodes located between the reciprocal lattice layers. This diffuse scattering is interpreted in terms of linear clusters of iron cations axtending along the [110] and [1 1 0] directions having a width of a few cations. The clusters are separated by domains of orthorhombic YBa 2 Cu 3 O 6+x having the same orientation or rotated of 90° one with respect to the other.

A note on the symmetry and Bi valence of the superconductor Bi2Sr2Ca1Cu2O8

Abstract The sub-structure of Bi 2 Sr 2 Ca 1 Cu 2 O 8 can best be described on a non-centric orthorhombic A2aa cell, which permits the oxygen in the BiO plane to move off the center of the Bi square to approach to within 2.2 A of a pair of Bi atoms. Each Bi then has two close oxygens within the BiO plane, and a third at 2.12 A connecting to the CuO layer. The new structure permits the apparent Bi valence to approach 3+ with a more reasonable Bi-O co-ordination than for earlier approximate structural models. This model satisfies the high resolution neutron data of Bordet et al., while agreeing in part with the co-ordination proposed on chemical grounds by von Schnering et al.

Preparation and neutron diffraction of superconducting “tetragonal” and non-superconducting orthorhombic Tl2Ba2Cu1O6

Abstract Many different samples of Tl 2 Ba 2 Cu 1 O 6 have been prepared by different heat treatments at relatively low temperature. The material as first prepared is clearly orthorhombic and non-superconducting, but on annealing and quenching, the samples become almost tetragonal with high T c . Various T c values between 3+ O 2- atoms are missing, creating the electron holes apparently necessary for superconductivity. The main difference between the various samples appears to be that the non-superconducting material is orthorhombic, with a well ordered superstructure, while the superconducting material is pseudo-tetragonal, with disordered oxygen within the TlO plane, as for the higher superconducting members of the Tl 2 Ca n Ba 2 Cu n +1 O 2( n +3 ) series. Superconductivity then appears to depend on the precise structural arrangement, and not just on stoichiometry and the number of electron holes.

Powder neutron diffraction study of ZrTiO4, Zr5Ti7O24, and FeNb2O6

Abstract The zirconium titanates ZrTiO 4 and Zr 5 Ti 7 O 24 and the iron niobate FeNb 2 O 6 have been investigated with the neutron diffraction powder technique and the Rietveld method. All three compounds crystallize with the symmetry of space group Pbcn and have lattice parameters a = 4.8042(2), b = 5.4825(3), c = 5.0313(2) A for ZrTiO 4 ; a = 14.3574(6), b = 5.3247(3), c = 5.0200(2) A for Zr 5 Ti 7 O 24 ; and a = 14.2661(2), b = 5.7334(1), c = 5.0495(1) A for FeNb 2 O 6 . Pure zirconium titanate, ZrTiO 4 , has the α-PbO 2 -type structure with a random distribution of the two cations. The compounds Zr 5 Ti 7 O 24 and FeNb 2 O 6 are ordered superstructures of α-PbO 2 and in these two cases the observed distortions depend on the differences between ionic radii, leading to a fersmite-type structure in the case of Zr 5 Ti 7 O 24 and to a columbite-type structure in the case of FeNb 2 O 6 .The zirconium titanates ZrTiO/sub 4/ and Zr/sub 5/Ti/sub 7/O/sub 24/ and the iron niobate FeNb/sub 2/O/sub 6/ have been investigated with the neutron diffraction powder technique and the Rietveld method. All three compounds crystallize with the symmetry of space group Pbcn and have lattice parameters a = 4.8042(2),b = 5.4825(3), c = 5.0313(2) A for ZrTiO/sub 4/; a = 14.3574(6), b = 5.3247(3), c = 5.0200(2) A for Zr/sub 5/Ti/sub 7/O/sub 24/; and a = 14.2661(2), b = 5.7334(1), c = 5.0495(1) A for FeNb/sub 2/O/sub 6/. Pure zirconium titanate, ZrTiO/sub 4/, has the ..cap alpha..- PbO/sub 2/-type structure with a random distribution of the two cations. The compounds Zr/sub 5/Ti/sub 7/O/sub 24/ and FeNb/sub 2/O/sub 6/ are ordered superstructures of ..cap alpha..-PbO/sub 2/ and in these two cases the observed distortions depend on the differences between ionic radii, leading to a fersmite-type structure in the case of Zr/sub 5/Ti/sub 7/O/sub 24/ and to a columbite-type structure in the case of FeNb/sub 2/O/sub 6/.

Powder X-ray and neutron diffraction study of the superconductor Bi2Sr2CaCu2O8

A sample of the new BiO-perovskite superconductor has been synthesized. X-ray diffraction has been used to locate the cations while neutron diffraction has been used to determine the precise oxygen co-ordination. The stricture contains CuO 2 planes with Cu coordinated to four oxygens at 1.92A within the plane, plus more distant oxygen at 2.65A perpendicular to the plane. Two such copper oxide (perovskite) planes are intercalated with planes of BiO. The stricture is described in an Fmmm subcell (5.4A, 5.4A, 30.8A), except for the oxygen within the BiO layers, and to a lesser extent the Bi itself, which require a x5 larger b-axis. The X-ray refinement in Bbmm indicates that this Bi displacement is 0.27A.

Structural and electrochemical study of lithium insertion into γ-Fe2O3

Abstract Lithium insertion into the defect spinel γ-Fe 2 O 3 was investigated both chemically and electrochemically. A combination of slow potential-controlled insertion and in situ X-ray diffraction shows three successive reactions: (i) filling the octahedral vacancies up to x (Li) ≈ 0.25, (ii) a spinel→rocksalt-type first-order transition at increasing x (Li) (equilibrium potential 1.60 V versus Li + /Li), (iii) filling the remaining octahedral vacancies in the rocksalt phase. Structure refinements at x (Li)=0.86±0.01 from both X-ray and neutron diffraction data showed a remaining occupation of tetrahedral sites equal to ≈0.10.

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).

Two new bulk superconducting phases in the Y-Ba-Cu-O system: YBa2Cu3.5O7 + x (Tc ≈ 40 K) and YBa2Cu4O8 + x (Tc ≈ 80 K)

Abstract As a result of our P-T-x phase diagram studies under high oxygen pressure in the Y-Ba-Cu-O system, we have discovered two bulk superconducting phases YBa2Cu3.5O7 + x (Tc ≈ 40 K) and YBa2Cu4O8 + x (Tc ≈ 80 K) and have roughly determined their thermodynamic fields of stability. Both phases are metastable under normal conditions and can be synthesized only under high oxygen pressure. YBa2Cu4O8 + x has been observed in the past by HREM as a defect in “123” decomposed powders and as an ordered phase coexisting in “123” films. The approximate conditions of thermodynamic stability of the “124” phase have been determined in the course of our P-T-x studies, so that it was possible to synthesize bulk quantities of the material. We present here a thermodynamic, structural and physical characterization of these compounds. Although it is metastable under normal pressure, the “124” phase shows a high thermal stability of oxygen up to 850 °C.

MgB2 single crystals: high pressure growth and physical properties

Single crystals of MgB2 with a size up to 1.5 × 0.9 × 0.2 mm3 have been grown with a high pressure cubic anvil technique. The crystal growth process is very peculiar and involves an intermediate nitride, namely MgNB9. Single crystals of BN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9. Magnetic measurements with SQUID magnetometry in fields of 1–5 Oe show sharp transitions to the superconducting state at 37–38.6 K with a width of ~0.5 K. The high quality of the crystals allowed the accurate determination of magnetic, transport (electric and heat) and optical properties as well as scanning tunnelling spectroscopy (STS) and decoration studies. Investigations of crystals with torque magnetometry show that H//cc2 for high quality crystals is very low (24 kOe at 15 K) and saturates with decreasing temperature, while H//abc2 increases up to 140 kOe at 15 K. The upper critical field anisotropy γ = H//abc2/H//cc2 was found to be temperature dependent (decreasing from γ 6 at 15 K to 2.8 at 35 K). The effective anisotropy γeff, as calculated from reversible torque data near Tc, is field dependent (increasing roughly linearly from γeff 2 in zero field to 3.7 in 10 kOe). The temperature and field dependence of the anisotropy can be related to the double gap structure of MgB2 with a large two-dimensional gap and small three-dimensional gap, the latter of which is rapidly suppressed in a magnetic field. Torque magnetometry investigations also show a pronounced peak effect, which indicates an order–disorder phase transition of vortex matter. Decoration experiments and STS visualize a hexagonal vortex lattice. STS spectra in zero field evidence two gaps 3 meV and 6 meV with a weight depending on the tunnelling direction. Magneto-optic investigations in the far-infrared region with H//c show a clear signature of the smaller of the two superconducting gaps, completely disappearing only in fields higher than H//cc2.