C. Michel

Les oxydes A2BaCuO5 (A = Y, Sm, Eu, Gd, Dy, Ho, Er, Yb)

Abstract A series of new phases, A 2 BaCuO 5 ( A = Y, Sm, Eu, Gd, Dy, Ho, Er, Yb), has been isolated. These compounds are orthorhombic, with a ⋍ 7.1, b ⋍ 12.2 , and c ⋍ 5.6 A. The probable space groups deduced from the electron diffraction patterns are Pbnm and Pbn 2 1 . The structure has been resolved from X-ray powder patterns. The framework can be considered as built up from distorted monocapped trigonal prisms A O 7 which share one triangular face forming A 2 O 11 blocks. The edge-sharing A 2 O 11 blocks form a three-dimensional network which delimits cavities where Ba 2+ and Cu 2+ are located. Barium is coordinated to 11 oxygen atoms, while the coordination polyhedron of copper is a distorted tetragonal pyramid CuO 5 .

Intercroissance de feuillets “perovskites lacunaires” et de feuillets type chlorure de sodium: Les oxydes La2−xA1+xCu2O6−x2 (A = Ca, Sr)

New compounds La2−xA1+xCu2O6−x2 have been isolated with 0≤x≤0, 14 for A=Sr and x = 0.10 for A=Ca. These compounds crystallize in a tetragonal cell with a ⋍ 3.90 A and c ⋍ 20 A; the possible space group is I4/mmm. The structure of these compounds is derived from that of Sr3Ti2O7: it can be described as an intergrowth of ”oxygen defect double perovskite“ layers and of SrO — type layers. The coordination of the metallic ions is discussed.

YBaCuFeO5+δ: A novel oxygen-deficient perovskite with a layer structure

A novel ordered oxygen-deficient perovskite with a layer structure, YBaCuFeO/sub 5+delta/, was isolated. It was characterized by chemical analysis, Moessbauer spectroscopy, and magnetic susceptibility measurements versus temperature. Its structure was determined by powder neutron diffraction. It crystallizes in the tetragonal system with a = 3.867 A approx. a/sub p/ and c = 7.656 A approx. 2a/sub p/. The structure can be described as formed of (CuFeO/sub 5/)infinity double layers of corner-sharing CuO/sub 5/ and FeO/sub 5/ pyramids, perpendicular to c. The cohesion between the layers is ensured by yttrium planes whereas the barium ions are located inside the layers. The relationships of this structure with those of YBa/sub 2/Cu/sub 3/O/sub 7-delta/ and LaBa/sub 2/Cu/sub 3/O/sub 7-delta/ are discussed.

A series of oxygen-defect perovskites containing CuII and CuIII: The oxides La3−xLnxBa3 [CuII5−2y CuIII1+2y] O14+y

Abstract A series of oxygen-defect perovskites, containing Cu II and Cu III , La 3 Ba 3 [Cu II 5−2 y Cu III 1+2 y ] O 14+ y , has been synthesized at 1000°C, for 0.05 ≤ y ≤ 0.43. The substitution of lanthanum for yttrium and lanthanides has been studied. These oxides are tetragonal: a = a p 2 1 2 and c = 3 a p . The structural study of La 3 Ba 3 Cu 6 O 14.10 shows that oxygen vacancies are ordered, involving for copper three sorts of coordination: square, pyramidal (4 + 1), and distorted octahedral (4 + 2). The distribution of Cu III , as well as the lanthanide ions on the different sites, is discussed.

The oxygen defect perovskite BaLa4Cu5O13.4, a metallic conductor

A new oxygen defect perovskite BaLa4Cu5O13.4, characterized by a mixed valence of copper has been isolated; the parameters of the tetragonal cell are closely related to that of the cubic perovskite:a = 8.644(4)A = ap 5√ and c = 3.867(3) A = ap. The X-ray diffraction study shows that the atoms are displaced from their ideal positions in the cubic cell, owing to the presence of ordered oxygen vacancies. The study of conductivity, magnetic susceptibility and thermoelectric power versus temperature shows that this oxide is a very good metallic conductor.

Single crystal study of the one dimensional Ca \(\)Co \(\)O \(\) compound: five stable configurations for the Ising triangular lattice

Abstract:Single crystals of the one-dimensional phase Ca3Co2O6 of several mm length have been grown. The magnetic study of such a crystal confirms the previous observations on polycrystalline samples: it consists of a triangular lattice of ferromagnetic [Co2O6] chains ( K) antiferromagnetically coupled ( K). The dynamic of these chains array, probed by AC susceptibility, is very slow as shown from the large shift of the freezing temperature from 12 K to 16.5 K as the excitation frequency increases by three orders of magnitude (100 to 103 Hz). The origin of this effect is believed to be the result of different arrangements with close energies for the chain ferromagnetic moments on the triangular lattice. Five stable magnetic configurations have been evidenced by the magnetization as a function of applied field curves registered at 2 K. Their relative magnetizations correspond to m=1/4, 1/2, 1, 2, 3 where m=3 represents the ferromagnetic ordering of three chains on the same triangle, each chain having a m=1 magnetization. A magnetic phase diagram is finally proposed.

Superconductivity up to 100 K in lead cuprates: A new superconductor Pb0.5Sr2.5Y0.5Ca0.5Cu2O7−δ

Abstract A new superconductor Pb 0.5 Sr 2.5 Y 0.5 Ca 0.5 Cu 2 O 7−δ has been isolated whose superconducting properties depend upon the thermal treatments, i.e. are closely related to the oxygen stoichiometry. This material exhibits a T on c =100 K whereas the T off c ( R =0) ranges from 45 K to 53 K. The magnetic measurements using a SQUID magnetometer show superconducting volume fractions ranging from 14% to 72%. Anomalies appear in the magnetization curves at low field for some samples. This effect and the broad transition are interpreted in terms of an inhomogeneous distribution of oxygen in the matrix. The structure of this tetragonal phase ( a =3.81 A, c =11.908 A) belongs to the “1212” type observed for thallium cuprates, i.e. consists of an intergrowth of double rock salt-type layers involving lead and strontium and of double copper pyramidal layers interleaved with yttrium and calcium. This oxide differs also from the classical intergrowth structures by the existence of numerous oxygen vacancies in the rock salt type layers (δ≅0.7−0.5); moreoer satellites are observed on E.D. diffraction patterns. These two effects are explained by the presence of the 6s 2 lone pair of Pb(II).

Superconductivity in the Bi — Sr — Cu — O System

During the investigation of the system Bi—Sr—Cu—O a novel family of superconducting oxides, close to the composition Sr2Bi2Cu207+δhas been isolated, with a midpoint critical temperature ranging from 7 K to 22 K. The X-ray diffraction and high-resolution electron microscopy studies have shown its relationships with perovskite and demonstrated its bidimensional character. The possible relationships of this lamellar oxide with Aurivillius phases is discussed.

A new member of the thallium superconductive series, the “1212” oxide TlBa2CaCu2O8−y: Importance of oxygen content

Abstract A new superconductor was isolated and characterized by X-ray diffraction and electron microscopy. This oxide crystallizes in tetragonal system ( P4 mmm ) with the following parameters: a = 3.833 A and c = 12.680 A. A structural model was established from X-ray powder data. The structure is a new member of the family (AO)n(A′CuO3−y)n′ (A = Tl, Ba and A′ = Ba, Ca) with n = 2 and n′ = 2. It can be described as the intergrowth of double (AO) layers, rock salt-type, with double oxygen-deficient perovskite layers involving CuO5 pyramids. The optimization of the thermal treatment and the importance of oxygen content are discussed.

La8−xSrxCu8O20: An oxygen-deficient perovskite built of CuO6, CuO5, and CuO4 polyhedra

Abstract A new oxygen-deficient perovskite corresponding to the formulation La8−xSrxCu8O20−e (1.28 ≤ x ≤ 1.92) has been isolated. The results of chemical analysis show that copper exhibits a mixed valence in this oxide. This perovskite crystallizes in a tetragonal cell with parameters related to the ap parameter of the cubic perovskite by: a ⋍ a p 2√2, c ⋍ a p . The structure has been determined by neutron powder diffraction in the space group p4 mbm . This study shows that the oxygen vacancies are ordered, forming rows parallel to 〈001〉. The [Cu8O20] framework is built of corner-sharing CuO6 ocatahedra, CuO5 pyramids, and CuO4 square planar groups forming hexagonal tunnels in which are located the La3+ and Sr2+ ions. The slight oxygen substoichiometry with regard to the formulation A8Cu8O20 is discussed in terms of a superstructure along the c axis ( c ⋍ 2a p ), which was observed in some crystals by electron diffraction.