H. Leligny

A five-dimensional structural investigation of the misfit layer compound [Bi0.87SrO2]2[CoO2]1.82

The structure of the misfit layer compound [Bi0.87SrO2]2[CoO2]1.82, bismuth strontium cobaltite, was determined by single-crystal X-ray diffraction using the five-dimensional superspace-group formalism. This composite crystal, of monoclinic symmetry, is composed of two subsystems exhibiting incommensurate periodicities along b, the binary axis direction. The first composite part [Bi0.87SrO2] displays an intrinsic modulation of planar monoclinic type characterized by the wavevector q* = 0.293a* + 0.915c*. The second composite part [CoO2] shows two different centered lattice variants. The structure of the misfit layer crystal can be described as an alternation along c of distorted rock-salt-type slabs, formed from [BiO] and [SrO] layers (first subsystem), and of [CoO2] layers (second subsystem) displaying a distorted CdI2-type structure. Two main structural results are obtained. First, as a consequence of the intrinsic modulation, disordered zones, characterized by Bi vacancies, are regularly distributed in the [BiO] layers. Second, strong chemical bonds are implied between the strontium atoms of the first subsystem and the oxygen atoms of the second one.

X-ray structural determination of the quasi-commensurate phase of barium sodium niobate

In order to provide an accurate basis for the interpretation of the physical properties of this complex incommensurate system, the structure of the room temperature quasi-commensurate phase of barium sodium niobate is determined by taking into account, for the first time, the satellite reflections present in the diffraction spectrum of this material. The quasi-commensurate phase considered is assimilated to a strictly commensurate phase, the satellite reflections being considered as superlattice reflections and referred to a crystalline supercell with a=35.187 AA, b=17.620 AA and c=7.987 AA. This approximation gives an excellent agreement index R=0.031 for the fit to the intensities of the 2432 basic reflections and the 1691 satellite reflections collected at 294 K ( lambda Mo K alpha =0.71069 AA, mu 1=111.47 cm-1, space group Bbm2). The structural results show that the commensurate modulation consists of a tilting and a shearing of all the NbO6 octahedra in the structure in contrast to previous inferences which had only suggested the occurrence of a shear. The framework of the structure may be described on the basis of the existence of two different Nb5O24 units of five corner-sharing octahedra each. The structure obtained by spatially averaging the modulation is consistent with previous structural studies of the basic reflections of this material. Physical characteristics pertaining to the polar character, or the domain structure, are also deduced.

X-ray investigation of the incommensurate modulated structure of Bi2+xSr3-xFe2O9+ delta

A single crystal of the Bi 2212 iron oxide Bi2+xSr3-xFe2O9+ delta , exhibiting an incommensurate modulation, was characterized. The modulated structure was refined from X-ray diffraction data including satellite reflections up to the second order and using the four-dimensional formalism. A substitutional modulation (Bi for Sr3-x) occurs over the Sr3-x sites neighbouring the BiO layers whereas the other Sr3-x sites, between the FeO2 layers, are only occupied by strontium atoms. This leads to a bismuth-rich ferrite of chemical formula Bi2.4Sr2.6Fe2O9+ delta Different types of coordination are observed both for the iron atoms and for the strontium atoms on the substituted sites. Evidence for anomalies in the Bi positions was found; these anomalies were located in the BiO layers, suggesting a possible static disorder of bismuth atoms. Moreover. This effect would be closely related to the variation of the iron coordination. Finally, a comparison with the related superconductor compound Bi2Sr2CaCu2O8+ delta is given.

Oxygen disorder phenomena in the modulated Bi2Sr2CoO6-type structure

The incommensurate modulated structure of Bi1.91Sr2CoO5.93 (a = 5.4614(6) A, b = 5.4471(5) A, c = 23.415(2) A) with * = 0.240(2)*+* has been refined in the superspace group F2mm(01)00s using the four-dimensional formalism for modulated structures and single-crystal x-ray diffraction data. Two different regions with different sites for Bi and O atoms are evident along the modulation direction (called M and I regions). The oxygen disorder observed in the M region leads to different possible configurations for [BiO] layers. In particular, a double-chain configuration comparable to the one observed in related compounds is outlined. Therefore, a comparison with related compounds (BiCu or BiFe) is performed.

A Sr-rich 1223 cuprate, Tl1+xBa23Sr43Ca2−xCu3O9, with a Tc of 110 K: A single-crystal study

Abstract Single crystals of the Sr-rich 1223 thallium cuprate Tl 1+ x Ba 2 3 Sr 4 3 Ca 2− x Cu 3 O 9 have been grown for the first time. The structure of this phase was determined from a single-crystal X-ray diffraction study in the space group P4/mmm with a =3.8274(4) A, c =15.524(3) A. The atomic positions are similar to those found in the pure barium 1223 cuprates. Nevertheless, the present cuprate differs from the pure barium phase by the much smaller distance between two adjacent O(2) and O(3) planes. Consequently, the CuO 5 pyramids are significantly flattered with respect to TlBa 2 Ca 2 Cu 3 O 9 , with Cu-O apical distances of 2.47 A for this (Ba, Sr) cuprate instead of 2.73 A for the pure Ba cuprate. The susceptibility measurements performed on a single crystal show that T c can be increased from 90 K for the as-synthesized crystal to 110 K by Ar/H 2 annealing at 300°C; moreover, this effect is reversible: by oxygen annealing, T c decreases again to 90 K. Comparison with the pure barium 1223 cuprate that exhibits T c values ranging from 110 K to 122 K demonstrates that the latter are not correlated with the Cu-O apical distance of the CuO 5 pyramids.

Structural changes with lead substitution in Bi2-xPbxSr2CoO6

Structures refined for two members of the group of Bi2-xPbxSr2CoO6-type compounds are proposed. For x = 0.16, the structure is incommensurately modulated (a = 5.453(2) A, b = 5.426(2) A, c = 23.43(1) A, = 0.245(2) + , F2{mm}(0β1)00{s}). For x = 0.8, the modulation becomes very weak and has been neglected in the structural refinement (a = 5.3117(7) A, b = 5.4232(6) A, c = 23.590(2) A, Pnan). A new interpretation of the undoped modulated Bi2Sr2CoO6 phase is given. A new type of configuration for the [BiO] layers is confirmed and original modulation features are elucidated in comparison with the related superconducting cuprate phases.

Single-crystal study of the m = 2 tubular cobalt oxide, Bi4Sr12Co8O30−δ

The structure of the m = 2 tubular compound Bi4Sr12Co8O30−δ, bismuth strontium cobalt oxide, was determined by single-crystal X-ray diffraction. This phase of orthorhombic symmetry exhibits a very strong tetragonal pseudosymmetry. The structure consists of 90°-oriented Bi2Sr2CoO6+δ slices, four Co atoms wide, forming [Sr4Co4O13]∞ pillars at their intersection. The Co atoms in these pillars form four corner-sharing CoO5 bipyramids. In the resulting [Co4O13] cluster, an anionic disorder is evidenced and discussed. Then, an accurate description of the particular structure of the pillars is given. Finally, a comparison with the Mn tubular compound Bi3.6Sr12.4Mn8O30−δ is carried out.

Structural investigation of composite phases Ba1 + x [(NaxMn1 – x)O3] with x approx. 2/7, 5/17 and 1/3; exotic Mn4.5+ valence

Abstract Structural models are proposed for three composite compounds of chemical formula Ba1 + x [(NaxMn1 – x)O3] (x approx. 2/7, 5/17 and 1/3) by single crystal X-ray diffraction; superspace formalism is used to obtain an unified description of the three phases. The modulation affecting Ba atoms can be easily designed but the competition “occupational/displacive” modulations relating to the Mn/Na metallic columns were particularly difficult to modelize. However, the large amplitude of the displacive modulation affecting the oxygen atoms (approx. ±0.7 Å) in comparison with that observed for related compounds (approx. ±0.3 Å) makes it a direct consequence of the Na+ alkali insertion inside the trigonal prisms. Owing to this insertion, the Mn atoms exhibit, in the three phases, an “exotic” oxidation state of about +4.5. In addition, even if the sequence between face sharing MnO6 octahedra and NaO6 trigonal prisms can be analytically deduced from the x value, it is clear that the Na/Mn contrasts play in favour of its accurate determination through the XRD single crystal refinement.

Cluster configurations in modulated EuVxMo8±yO14 crystals

Three orthorhombic crystals of chemical formula Eu(x)V(y)Mo(8+/-z)O(14) were investigated by X-ray diffraction (Mo Kalpha radiation, lambda = 0.71073 Å). They have nearly the same lattice parameters (a approximately 11.3, b approximately 10.0, c approximately 9.2 Å), display one-dimensional incommensurate modulations of wavevector q* = gammac* and are characterized by the same superspace group Cmca(00gamma)s00. The crystals differ both in their compositions (namely Eu(0.976(6))V(1.13(5))Mo(7.10(5))O(14), Eu(0.986(4))V(1.10(3))Mo(7.30(1))O(14) and EuMo(7.96(1))O(14)) and in their gamma components [0.195 (2), 0.245 (2) and 0.286 (3), respectively]. The average structures of these crystals appear closely related to the structures of LaMo(7.7)O(14) (not modulated) and LaMo(8)O(14) (modulated); however, two main differences are outlined: first, the modulation direction is c in the Eu-containing crystals but b in the modulated La-containing crystal [q* = (1/3)b*], second, the Eu-containing crystals have centrosymmetric structures while the La-containing crystals have polar structures (space group C2ca). The Mo (or Mo and V) atoms are stacked to form (001) layers of metallic clusters. The density modulation of these structures implies the existence of the new types of clusters Mo(9), Mo(10), Mo(6)V(4), Mo(7)V(3) and Mo(8)V(2) besides the clusters M(8) (Mo(8), Mo(6)V(2) and Mo(7)V) and M(7) (Mo(7) and Mo(6)V) which are already known. Mo(8) units with cis and trans configurations and Mo(6)V(2) units with a trans configuration appear as the main cluster types in these crystals. The nature of the metallic clusters changes along c, but inside one (001) layer it is likely that only one cluster type with a given configuration is present. The main structural result is the formation, in some unit cells, of strong intercluster Mo-Mo, Mo-V or V-V bonds with distances close to 2.6 Å within a layer as well as between two neighbouring layers.