J. Chenavas

Synthèse et caractérisation d'une série de titanates pérowskites isotypes de [CaCu3](Mn4)O12

Abstract A series of titanates which have perovskite-like arrangements and are isostructural with [CaCu3](Mn4)O12 have been synthesized. The total charge of the A sites can be modified (1) by substituting the Ca2+ cations with monovalent ones and the tetravalent manganese cations of the B sites by a mixture of (Ti4+ + M5+) in which M = Ta, Nb, Sb, or (2) by substituting the Ca2+ cations by a combination of cations plus vacancies. In this case, if the total charge of the A sites is 2, one obtains compounds such as [ Th 4+ 1 2 □ 1 2 Cu 3 ](Ti 4 )O 12 and [T 3+ 2 3 □ 1 3 Cu 3 ](Ti 4 O 12 (T = rare earth ) ; on the contrary, if the charge is less than 2, then one has to compensate it by changing that of the B sites. This leads to compounds such as [□Cu3](Ti2M2)O12 (M = Ta, Nb, Sb).

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.

Two-phase structural refinement of La2CuO4.032 at 15 K

Abstract A two-phase refinement of a crystal for the compound La2CuO4+δ (δ=0.032), based on the neutron diffraction data collected at 15 K with the D9 diffractometer at ILL, using ǂ=0.48 A , has been carried out. One phase (30%) consists of stoichiometric La2CuO4 domains, while the other (70%) of oxygen-rich La2CuO4.048 domains. The percentages of each phase, which have been refined together with the other parameters (scale factor, positional parameters, thermal factors and occupancy factors of the oxygen atoms), agree very well with the value determined from χAC measurements. The La2CuO4.048 structure is essentially the same as the average structure reported in ref. [1], the only difference being the oxygen content which is found to be δ= δ 0.70 . The extra oxygen, O(4), is found to be in between two LaO layers in a similar position as the oxygen atoms located between two Nd layers in the N2CuO4 structure. The insertion of extra oxygen causes the displacement of some of the oxygen O(1) towards the O(3) positions. Different models are proposed for the distortion induced by this insertion according to the experimental value, 3.3 [6], found for the ratio of the amount of O(3) to that of O(4). If this ratio is assumed to be 3 and the O(3) atoms are localized about the insertion, then the formation of a short O(4)-O(3) bond would occur. The models not requiring the formation of the short bond correspond to ratios of 2 or 4. In the latter case there would be four displaced O(1), but due to the rigidity of the oxygen octahedra only two O(3) would be bonded to O(4), the other two being the apically opposite oxygen atoms of the same octahedra.

Oxygen vacancy ordering in Ba2YCu3O7−x around x= 0.5

Abstract Electron diffraction photographs of the title samples show extra diffuse scattering in the reciprocal a * b * plane, which can be attributed to the partial ordering of the oxygen vacancies. A unit cell 2a c x a c x 3a c (a c being the basic perovskite subcell parameter; a c ≅3.8A is proposed together with a structural model.

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.

A family of non-stoichiometric phases based on Ba2YCu3O7−δ (0≤δ≤1)

Abstract In the light of recent electron diffraction evidence a reappraisal is made of the problem of non-stoichiometry by oxygen deficiency in high-temperature superconducting Ba 2 YCu 3 O 7−δ . On the basis of the ordered states of the oxygen vacancies detected by that technique, a family of phases is proposed as a better description of oxygen stoichiometry and compositional variations in that material. Five phases appear to exist within the usually considered range 0≤δ≤1. All of these phases, which can be considered as consecutive members of a homologous series differing in oxygen content along the CuO rows running along the b axis, are themselves non-stoichiometric and appear to correspond to well differentiated regions in the characteristic T c versus composition phase diagram found by several authors.

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.

High pressure synthesis and crystal structure of NaMn7O12

A new compound, NaMn7O12, with the perovskite-like arrangement has been synthesized at 80 kbar and 1000°C. This compound is cubic, a = 7.3036 A, space group Im3 with four formula weights per unit cell. The structure has been solved by Patterson and Fourier synthesis and refined by least-squares based on 142 reflections. The final R and wR factors were 0.025 and 0.033, respectively. The A sites of the perovskite structure are occupied by sodium and manganese atoms in an ordered fashion. The sodium atoms are each surrounded by a 12-oxygen polyhedron whereas the manganese atoms have four nearest oxygens at 1.909 A forming a square and four more at 2.688 A forming a rectangle perpendicular to the square. The distortion of the oxygen network from the ideal perovskite structure is similar to that found for In(OH)3 and Sc(OH)3.

High pressure synthesis and crystal structure of a new series of perovskite-like compounds CMn7O12 (C = Na, Ca, Cd, Sr, La, Nd)

Abstract A new series of perovskite-like compounds CMn7O12 have been synthesized under high pressure and high temperature conditions. C is a large divalent or trivalent cation such as Ca, Cd, Sr, La and Nd. The structures of the quenched materials have been determined from powder X-ray data. They are distortions of the NaMn7O12 cubic structure. The [C2+Mn3+3](Mn3+3Mn4+)O12 compounds are trigonal ( R 3 ). The C2+ and Mn3+ as well as the Mn3+ and Mn4+ cations are ordered on the corresponding A and B sites of the perovskite structure, respectively. The [C3+Mn3+3] (Mn3+4)O12 compounds are monoclinic ( I 2 m ). In these compounds the order exists only in the A sites. It is shown that the lower symmetry may be the result of a cooperative Jahn-Teller effect of the Mn3+ cations occupying the B sites.

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.