J.L. Hodeau

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.

Probing the structure of heterogeneous diluted materials by diffraction tomography

The advent of nanosciences calls for the development of local structural probes, in particular to characterize ill-ordered or heterogeneous materials. Furthermore, because materials properties are often related to their heterogeneity and the hierarchical arrangement of their structure, different structural probes covering a wide range of scales are required. X-ray diffraction is one of the prime structural methods but suffers from a relatively poor detection limit, whereas transmission electron analysis involves destructive sample preparation. Here we show the potential of coupling pencil-beam tomography with X-ray diffraction to examine unidentified phases in nanomaterials and polycrystalline materials. The demonstration is carried out on a high-pressure pellet containing several carbon phases and on a heterogeneous powder containing chalcedony and iron pigments. The present method enables a non-invasive structural refinement with a weight sensitivity of one part per thousand. It enables the extraction of the scattering patterns of amorphous and crystalline compounds with similar atomic densities and compositions. Furthermore, such a diffraction-tomography experiment can be carried out simultaneously with X-ray fluorescence, Compton and absorption tomographies, enabling a multimodal analysis of prime importance in materials science, chemistry, geology, environmental science, medical science, palaeontology and cultural heritage.

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 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.

The crystal structure of SnYb3Rh4Sn12, a new ternary superconducting stannide

Abstract The crystal structure of superconducting SnYb3Rh4Sn12 has been determined from single-crystal X-ray diffraction data. This compound is cubic, space group Pm3n, a o = 9.676 A (1) and has two formulae per unit cell. The structure was solved from Patterson and subsequent Fourier synthesis. The least squares refinement was based on 375 independent reflections. The final R and wR factors were 0.015 and 0.014, respectively. The two Sn(1) atoms occupy the 2a (000) positions, the six Yb atoms the 6d ( 1 4 1 2 0) positions, the eight Rh atoms the 8e ( 1 4 1 4 1 4 ) positions and the twenty-four Sn(2) atoms the 24k (Oyz) positions (y ∼ 0.31, z ∼ 0.15). The Sn(2) atoms form a tridimensional array of corner-sharing trigonal prisms whose centers are occupied by the rhodium atoms. The Sn(1) and the Yb atoms occupy the icosahedral and cuboctahedral holes of this array, respectively. They form a sublattice which has the arrangement found in the structure of the A15 compounds. The structure of SnYb3Rh4Sn12 can be described as containing two interpenetrated structures, namely Yb3Sn and RhSn3, or as having an A15 arrangement of clusters of atoms such as (SnSn12) and (YbSn12). These clusters are bound together by face-sharing among them; and by the rhodium atoms. An analogy is drawn between SnYb3Rh4Sn12 and the perovskite-like ternary oxides A′A″3B4O12.

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.

Structural distortion in the primitive cubic phase of the superconducting/magnetic ternary rare-earth rhodium stannides

Abstract A structural distortion in the primitive cubic phase of the SnM3Rh4Sn12 compounds with M = La, Ce, Pr, Nd, Sm, and Gd has been detected by electron diffraction and studied by X-ray diffraction. On the contrary, no distortion has been detected for the compounds with M = Eu, Yb, Ca, Sr, and Th. The La, Yb, Ca, Sr, and Th compounds become superconductors with Tc ranging between 8.7 K and 1.9 K, whereas those of Eu and Gd have a magnetic transition at about 11 K. Long exposure (200 hrs.) precession photographs revealed the existence of superstructure spots which can be indexed either on a body-centered cubic cell with aI = 2acp or on a tetragonal cell with aT = ∫2 acp and cT = ccp. In this latter case the sample must be considered as twinned and the extra spots would come from three different individuals each having the tetragonal axis along one of the three cubic fourfold axes. However, electron microscope photographs have failed so far to reveal the existence of domains. From the systematic absences it has been determined that the cubic distortion belongs to space group I213 while the tetragonal one to space group P4222. The main effect of the distortion is the lowering of the site symmetries which favors the disorder between the cationic tin and the M atoms. The size of these latter atoms does not seem to be an important factor for the distortion. The only feature which separates the distorted compounds from the undistorted ones is the valence of the M atoms. However, no explanation can be offered why the valence plays an important role for the distortion.

Synthesis and crystal structure of BaSrCuO2+x·CO3

Abstract A new oxycarbonate BaSrCuO 2.22 ·CO 3 has been synthesised by heat treating a mixture of Ba(OH) 2 ·8H 2 O, SrCO 3 , and CuO at 820°C in O 2 for 60–70 h. The structural analysis was based on powder X-ray and neutron diffraction data. The compound was found to be tetragonal, a =5.5899(2), c =7.7153(3) A. The structure can be described in the P4/mbm space group with 2 formulae per unit cell. The structural refinement was carried out by the Rietveld method. The atoms are in the following positions: Ba (0, 0, 0.2115), Sr (0, 0, 0.2423), Cu (0.5, 0, 0), C (0.5, 0, 0.5), O(1) (0.3434, 0.8434, 0.4553), O(2) (0.2509, 0.7509, 0.0187), O(3) (0.4672, 0.9672, 0.3286), O(4) (0.4140, 0.9140, 0.3771). The large cations are not long-range ordered, however; since the oxygen arrangement around them must be different, most of the atoms are highly disordered. The structure constains the following sequenc along the c -axis: CuO 2 , Ba (or Sr) O, CO x , Sr (or Ba) O, CuO 2 . The Cu cations have a square coordination with a fifth oxygen at 2.549 A. The Ba and Sr cations are surrounded by ten atoms: 2 O(1), 4 O(2), and 4 O(3) (or O(4)). The C cations are three-coordinated for x =0, a fourth oxyegn atom occupies the empty O(1) positions and in this case the C cation are squarely coordinated. Because of the disorder between the Ba and Sr cation, most of the anion positions are found to be split over two positions. Because of the disorder between the Ba and the Sr cation, most of the anion positions are found to be split over two positions. The valence sum calculations allows one to propose different models for local ordering.

Twinning in Ba2YCu3O6+x single crystals

Abstract Twinning occuring in tetragonal Ba 2 YCu 3 O 6 when it transforms to orthorhombic Ba 2 YCu 3 O 6+x , has been studied by X-ray and electron diffraction. For small values of x the symmetry is tetragonal. To minimize copper atoms with three fold coordination, the extra oxygen atoms are inserted two by two as to four fold coordinated Cu atoms and small blocks having different crystallographic orientation. For larger values of x, when the symmetry becomes orthorhombic, the blocks coalesce to form larger individuals twinned by the (110) twin law. We propose a model in which the infinite corner-sharing square chains are broken at the walls, the stoichiometry at the twin walls is the same as that of the crystal, and the Cu1 cations at the walls are surrounded by 4 oxygen atoms arranged as a distorted tetrahedron.