A novel layered titanoniobate LiTiNbO5: topotactic synthesis and electrochemistry versus lithium

A new layered titanoniobate, LiTiNbO5, an n = 2 member of the A(x)M(2n)O(4n+2) family, has been synthesized using a molten salt reaction between HTiNbO5 and an eutectic "LiOH/LiNO3". This compound crystallizes in the P2(1)/m space group with a = 6.41 A, b = 3.77 A, c = 8.08 A, and beta = 92 degrees . It exhibits |TiNbO5|(infinity) layers similar to HTiNbO5, but differs from the latter by a "parallel configuration" of its |TiNbO6|(infinity) ribbons between the two successive layers. The topotactic character of the reaction suggests that exfoliation plays a prominent role in the synthesis of this new form. This new phase intercalates reversibly 0.8 lithium through a first-order transformation leading to a capacity of 94 mAh/g at a potential of 1.67 V vs Li/Li+.     

Couplages a longue distance dans des structures rigides. Stereochlmie d'alkyudene cyclopentenones

A study of the neighbouring group effects on the ¹H and ¹³C chemical shifts and the results of NOE experiments have allowed the identification of the stereoisomers of alkylidene cylopentenones. Various long range coupling constants have been measured and their signs determined by selective decoupling. The mechanisms and angular dependences of these couplings have been discussed. The Π contributions are predominant and the absolute value of ⁴J_transoid associated with a dihedral angle of 30° is higher than the corresponding value of ⁴J_cisoid.     

New aluminate with a tetrahedral structure closely related to the c84 fullerene

A new aluminate Sr33Bi(24+delta)Al48O(141+3delta/2), having an F3m cubic structure (a = 25.090 angstroms, Z = 4) and forming a close packed face centered cubic array of "Al84" fullerene geometry, has been discovered. This original structure consists of corner-sharing AlO4 tetrahedra forming "Al84O106" cubic units whose assemblage delimits five types of cages, three of them being empty, one being occupied by strontium, and the fifth one forming the huge spheric fullerene-type cavity. In the latter, strontium, oxygen, and bismuth ions form concentric spheres, with an onion-skin-like configuration. The latter ions are disposed into a compact "Bi16O(52-n[]n)" anion whose the exceptional geometry is characterized by a strong stereoactivity of the 6s2 lone pair of Bi3+.     

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 Sr₂Bi₂Cu₂O₇₊ has 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.     

Crystallographic study of tetragonal,superconducting YBa2(Cu0.93Fe0.05)3O7

Single crystal- and powder X-ray diffraction, powder neutron diffraction at room temperature and 3 K, high resolution electron microscopy and electron diffraction were used to study the crystal- and defect-structure of YBa₂(Cu_0.93Fe_0.05)₃O₇. Crystals of this compound are superconducting (T _c 80 K) and appeart to be tetragonal down to at least 3 K. The structure resembles that of the undoped YBa₂Cu₃O₇ phase with the oxygen content being very close to 7.0 and the Fe atoms preferentially occupying the Cu(1) site. A copper deficiency is shown to be present on this site too, leading to the approximate formula YBa₂(Cu_0.97Fe_0.03)₂(Cu_0.86Fe_0.10.04)O₇. High resolution electron microscopy reveals the existence of orthorhombic micro-domains, 20–30 Å in diameter, probably due to short range ordering of oxygen atoms on the O(1) site. In view of these results the structure has to be regarded as being tetragonal only in a statistical sense.     

The oxycarbonates Sr4(Fe2−xMnx)1+y(CO3)1−3yO6(1+y): nano, micro and average structural approach

The possibility to synthesize layered oxycarbonates, with nominal composition Sr₄Fe_2−xMn_xO₆CO₃ involving trivalent manganese, with 0≤x≤1.5, is reported for the first time. The structural study of Sr₄FeMnO₆CO₃ using NPD, HREM, Mössbauer and XANES, shows that this phase is closely related to n=3 member of the Ruddlesden–Popper family. It derives from the latter by replacing the middle layer of transition metal octahedra by triangular CO₃ groups, with two different “flag” and “coat hanger” configurations. The magnetic order is antiferromagnetic and fundamentally different from the magnetic behavior of Sr₄Fe₂O₆CO₃.     

A new 2212-type stair like structure: Bi14Sr21Fe12O61, m=5 member of the generic [Bi2Sr3Fe2O9]m[Bi4Sr6Fe2O16] family

A new oxide, Bi₁₄Sr₂₁Fe₁₂O_61, with a layered structure derived from the 2212 modulated type structure Bi₂Sr₃Fe₂O₉, was isolated. It crystallizes in the I2 space group, with the following parameters: a=16.58(3) Å, b=5.496(1) Å, c=35.27(2) Å and β=90.62°. The single crystal X-ray structure determination, coupled with electron microscopy, shows that this ferrite is the m=5 member of the [Bi₂Sr₃Fe₂O₉]_m[Bi₄Sr₆Fe₂O₁₆] collapsed family. This new collapsed structure can be described as slices of 2212 structure of five bismuth polyhedra thick along , shifted with respect to each other and interconnected by means of [Bi₄Sr₆Fe₂O₁₆] slices. The latter are the place of numerous defects like iron or strontium for bismuth substitution; they can be correlated to intergrowth defects with other members of the family.