M. Marezio

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 study of the Cu-O chains in the high Tc superconductor YBa2Cu3O7 by high resolution neutron powder diffraction

Abstract The structure of orthorhombic YBa2Cu3O7 was refined in the temperature interval 5–320K, to an accuracy higher by a factor of about two compared to previous refinements. No major structural change is observed at the onset of superconductivity but the lattice dimensions and some of the structural parameters show small anomalies near 90K and 240K. All vibrational amplitudes are normal except for those of the 04 atoms across the Cu-O chain, which are large. Good agreement with the data is obtained by assuming that the 04 atoms are located on potential minima away by at least 0.08A from the chain axis at all temperatures. Annealing near 240K to look for a possible order-disorder transition revealed no structural changes.

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.

Pressure-Induced Enhancement of Tc Above 150 K in Hg-1223

The recently discovered homologous series HgBa2Can-1 Cun O2n+2+δ possesses remarkable properties. A superconducting transition temperature, Tc, as high as 133 kelvin has been measured in a multiphase Hg-Ba-Ca-Cu-O sample and found to be attributable to the Hg-1223 compound. Temperature-dependent electrical resistivity measurements under pressure on a (> 95%) pure Hg-1223 phase are reported. These data show that Tc increases steadily with pressure at a rate of about 1 kelvin per gigapascal up to 15 gigapascals, then more slowly and reaches a Tc = 150 kelvin, with the onset of the transition at 157 kelvin, for 23.5 gigapascals. This large pressure variation (as compared to the small effects observed in similar compounds with the optimal Tc) strongly suggests that higher critical temperatures could be obtained at atmospheric pressure.

The synthesis and crystal structure of CaCu3Mn4O12: A new ferromagnetic-perovskite-like compound

Abstract Single crystals of CaCu 3 Mn 4 O 12 , a new ferromagnetic perovskite-like compound ( T c − 160°C), have been synthesized at 50 kbar and 1000°C. By X-ray analysis it was found to be cubic ( a = 7.241A), space group Im 3 with two molecules per unit cell. The two Ca 2+ and six Cu 2+ cations occupy the A sites of the ideal perovskite structure, while the eight Mn 4+ cations occupy the B sites. In the Im 3 space group the sites occupied by the calcium and copper cations have different point symmetry and therefore the 12-oxygen polyhedra have different distortions. The Ca cations are surrounded by slightly distorted icosahedra, the Ca O distance is 2.562A. The twelve oxygens around the copper cations are arranged as three mutually perpendicular rectangles of different size, the smallest and the largest of which are almost squares. The three sets of Cu 0 distances are 1.942, 2.707, and 3.181A, respectively. The octahedral Mn 0 distance is 1.915A. This arrangement is similar to that found in NaMn 3 Mn 4 O 12 . A comparison between the two structures and a discussion of their thermal data are given.

Bismuth valence order−disorder study in BaBiO3 by powder neutron diffraction

Abstract Two structural arrangements have been shown to exist for the Perovskite-like compound BaBi 0.5 3+ Bi 0.5 5+ O 3 by powder neutron diffraction data. The first is characterized by a partial (75%) order of the Bi 3+ and Bi 5+ cations on the two crystallographically independent sites, while in the second arrangement the two cations are almost 100% disordered. The structure of eleven differently-prepared BaBiO 3 samples showed that the deciding factor for obtaining one or the other arrangement is the temperature at which the sample is prepared or subsequently heat-treated. On the contrary, the cooling rate and the atmosphere (air or oxygen) do not seem to be important parameters. DTA measurements indicated that BaBiO 3 undergoes an additional phase transition at 860°C on heating and 801°C on cooling. This transition probably corresponds to the disproportionation of the Bi into Bi 3+ and Bi 5+ cations. This conjecture explains why the heat-treatment at 800°C yields compounds containing partial cation ordering. Since the symmetry is cubic above and below the 801°C transition, the 75% ordered BaBiO 3 stable between 801°C and 480°C would contain a trigonal dynamical distortion which would be the precursor of the static trigonal distortion which takes place at 480°C. In every compound the partial ordering is always 75%. This could be interpreted as an indication that an additional ordering of Au 3 Cu type exists on the two Bi sublattices.

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.

Crystal structure dimensionality, and 4d electron distribution in K0.30MoO3 and Rb0.30MoO3

Abstract Single crystals of K0.30MoO3 and Rb0.30MoO3 were synthesized by electrolytic reduction of MoO3/ A2MoO4 melts. The crystal structures were refined from X-ray diffraction data (3265 and 1280 independent reflections, respectively). The finalR andwR factors were 0.037 and 0.047 for the K bronze and 0.031 and 0.033 for the Rb bronze. The lattice parameters of the body-centered cells used in the present refinements were: K0.30Mo03,a = 16.2311(7),b = 7.5502(4),c = 9.8614(4)A˚,β = 94.895(4)o; Rb0.30MoO3,a = 16.361(3),b = 7.555(1),c = 10.094(2)A˚,β = 93.87(5)o. The 4d electron distribution over the 20 Mo sites [4Mo(1), 8Mo(2), 8Mo(3)] of the unit cell are 10, 45, and 45% for K0.30Mo03 and 14, 43, and 43% for Rb0.30MoO3, respectively. In both cases about 90% of the 4d electrons are situated on those sites which contribute to the electrical conductivity. The variations of the lattice parameters versus temperature are reported. The thermal linear-expansion coefficient is highly anisotropic. The structural dimensionality depends upon the sublattice under consideration. The K, Mo, and O sublattices are mono-, two-, and three-dimensional, respectively. The relationship between the structural dimensionality of K0.30MoO3 and the physical properties is discussed.

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 aspects of the metal-insulator transitions in V0.985Al0.015O2

Abstract The crystal structure of V 0.985 Al 0.015 O 2 has been refined from single-crystal X-ray data at four temperatures. At 373°K it has the tetragonal rutile structure. At 323°K, which is below the first metal-insulator transition, it has the monoclinic M 2 structure, wh...