D. Grebille

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.

Contribution of powder diffraction for structure refinements of aperiodic misfit cobalt oxides

Two families of misfit cobalt oxides have been reinvestigated using neutron and synchrotron powder diffraction data and Rietveld structure refinements in the four-dimensional formalism for aperiodic structures. In the first system [Ca2CoO3][CoO2]1.62, the structure-specific features of two complex polytypic phases previously described from single-crystal diffraction data have been confirmed for the main term of the series and are described with the C2/m(1,δ,0)s0 superspace symmetry (Rp = 0.026, Rwp = 0.031, RB = 0.014). The high-resolution synchrotron diffraction experiments have been interpreted with a triclinic distortion of the structure, probably related to an intrinsic modulation of the [Ca2CoO3] sublattice in relation with Co and O split atomic sites. In the second system Ca0.82(Cu0.65Co0.35O2), a new occupation modulation of the metallic site is shown with the Fmmm(α11)0ss superspace symmetry (Rp = 0.031, Rwp = 0.039, RB = 0.027) and is interpreted in relation with the modulated interatomic distances of the square coordination of the cationic site. In both cases, recently developed tools for structure refinements of aperiodic systems using powder diffraction data (in the JANA2000 package) have proved their efficiency, in particular as specific complements to previous single-crystal structure refinements.

Crystal structure of the antiferroelectric perovskite Pb2MgWO6

Lead magnesium tungstate, Pb2MgWO6, Mr = 718.54. Phase I: cubic, Z = 4, Fm3m, a = 8.0058 (4)A, V = 513.1 (2) A 3, Dx = 9.30 Mg m -3 at 350 K, final Rwp = 4.5 and 7.7%, RBragg = 2.9 and 5.7% for neutron and Xray powder data, respectively. Phase II: orthorhombic, Pmcn (Pnma), Z = 4, a = 7.9440(4) and 7.9041 (3), b = 5.6866(3) and 5.7035(2), c = 11.4059(5) and 11.4442 (4) A, V= 515.3 (1) and 515.9 (1) A 3 at 294 and 80 K, respectively, Dx = 9.26 Mg m -3 at 294 K. Final Rwp = 4.0 and 8.5%, RBragg = 4.0 and 9.2% at 294 K and Rwp = 4.0 and 7.4%, RBragg = 2.9 and 8.4% at 80 K for neutron and X-ray powder data, respectively. To achieve the determination of the structures, X-ray and neutron powder diffraction data were refined together using the Rietveld profile method. The Pb main displacement in the orthorhombic phase from the ideal cubic positions is almost along the [0121o direction. The O displacements correspond to a weak distortion of the octahedra.

Influence of pulsed laser deposition growth conditions on the thermoelectric properties of Ca3Co4O9 thin films

Thin films of the misfit cobaltite Ca3Co4O9 were grown on (0001)-oriented (c cut) sapphire substrates, using pulsed-laser deposition techniques. The dependence of the thermoelectric/transport properties on the film growth conditions was investigated. The optimal conditions (for low resistivities) were found to be 600°C, 0.1–0.2mbar of oxygen pressure, and 1.7J∕cm2. These films exhibited slightly metallic behavior, consistent with in-plane resistivity curves of single crystals and c-axis magnetically aligned samples. Hall effect measurements showed the density of the holelike carriers was 6.8×1020∕cm3. The in-plane epitaxial relationship between the thin film and the sapphire substrate is investigated.

Disordered misfit [Ca2CoO3][CoO2]1.62 structure revisited via a new intrinsic modulation

The structure of the thermoelectric lamellar misfit cobalt oxide [Ca(2)CoO(3)][CoO(2)](1.62) was refined again using single-crystal X-ray diffraction data. A new commensurate intrinsic modulation was observed involving a modulation vector orthogonal to the misfit direction ((2/3),0,-(1/3)). The five-dimensional superspace group is C2/m(1delta0)(alpha0gamma)gm and the structure was solved using a commensurate approximation. A new model is given involving an occupation modulation of the split sites of the [CoO] layer. This [CoO] layer can be described by triple chains running along b. The residual disorder along b can then be explained by the assumption of a local ordering with two types of clusters: CoO(2) and Co(5)O(4). A powder neutron diffraction experiment confirmed the ordering evidenced by the single-crystal X-ray diffraction study, but was not sufficient by itself to deal with this double modulated scheme. The new intrinsic modulation is destroyed by partial metal substitutions in the [CoO] layer. The structural modifications of this layer directly influence the physical properties which are related to the electronic structure of the [CoO(2)] layers.

Modulated Misfit Structure of the Thermoelectric (Bi0.84CaO2)2(CoO2)1.69 Cobalt Oxide

The structure of the thermoelectric lamellar misfit cobalt oxide [Bi0.84CaO2]2[CoO2]1.69 has been refined using single crystal X-ray diffraction data. Using the four dimensional superspace formalism for aperiodic structures, the superspace group is confirmed P2/m(0delta1/2) (a1 = 4.9069(4), b1 = 4.7135(7), b2 = 2.8256(4), c1 = 14.668(5) A, beta1 = 93.32(1) degrees). The modulated displacements and site occupancies have been refined and are both compatible with the misfit character of the structure, and with a longitudinal modulation of the Bi-O layers of the structure. This modulation is similar to the corresponding one in the related Sr phase [Bi0.87SrO2]2[CoO2]1.82, but now oriented in the orthogonal direction. Because its incommensurate wavelength is locked with the aperiodicity of the misfit structure, it is possible to distinguish between the modulation parameters induced by the accommodation of both subsystems and those related to the longitudinal modulation of the Bi-O layers. In this original structure, two independent aperiodic phenomena coexist in an single crystallographic direction. A particular attention has been paid to the structural configuration of the CoO2 layer, in relation with other similar phases. The thermoelectric properties are probably directly related to the specific distortion of the compressed layer, but the different measured values for the Seebeck coefficient cannot be related to a significant modification of the CoO6 octahedra.

Neutron Rietveld refinement of the incommensurate phase of the ordered perovskite Pb2CoWO6

The incommensurate structure of lead cobalt tungstate has been refined by the Rietveld method on neutron data collected at 250 K. The space group is planar monoclinic 12/m(alpha0gamma)0s [a = 7.9602 (4), b = 5.6779 (3), c = 5.6967 (3) A, beta = 90.047 (5)degrees, q(inc) = 0.9000 (9)a* + 0.1735 (6)c*]. The use of powder diffraction techniques to investigate ferroelastic modulated phases is discussed and compared with a previous polydomain single-crystal structural analysis. The modulated displacements of light atoms have been determined, allowing an accurate description of the modulation of both the cations and the O-atom framework. The refinement suggests a displacive model for the phase transition, involving significant atomic shifts for Pb atoms and a quite complex mixing of tilt and deformation of the oxygen octahedra. The average character of this modulated structure is antiferroelectric.

X-ray diffraction study of the damage induced in yttria-stabilized zirconia by swift heavy ion irradiations

The lattice damage was investigated by x-ray diffraction techniques in yttria-stabilized zirconia single crystals with the (100) or (110) orientation upon irradiation with swift heavy ions (from 100-MeV C to 2.6-GeV U) in a broad electronic stopping power range (from about 0.3 to 48 keV nm−1). The θ-2θ scans show that no amorphization or change to a new crystalline phase occurs regardless of the ion and crystal features. However, the rocking curves (ω scans) and reciprocal space mappings show evidence of the mosaicity of the crystals, which is produced above a threshold electronic stopping power between 18 and 27 keV nm−1. This threshold is in agreement with our previous Rutherford backscattering spectroscopy/channeling spectroscopy data. Two kinds of damage phenomena are found: (i) nuclear-collision induced clusters of point defects which generate Bragg peak shifts and broadening in the 2θ-ω and θ-2θ scans, and (ii) electronic-excitation induced lattice damage yielding broad peaks in the ω scans above th...

Original disorder–order transition related to electronic and magnetic properties in the thermoelectric misfit phase [Ca2CoO3][CoO2]1.62

A structural phase transition is shown around 400 K for the thermoelectric lamellar misfit cobalt oxide [Ca2CoO3][CoO2]1.62. This transition is related to a rearrangement of the central [CoO] layer of the [Ca2CoO3] slab of this structure, characterized by a commensurate intrinsic modulation q2 = 2/3a*-1/3c*. The partial residual disorder related to split Co and O atomic sites along the misfit b direction disappears and one can describe this layer with its triple chains as a modulated configuration with a regular and not distorted periodicity along b. This phase transition is associated with small changes observed in the transport and magnetic properties as a function of temperature.

Quantitative texture analysis of grain-aligned [Ca2CoO3]0.62[CoO2] ceramics processed by the reactive-templated grain growth method

In this study, the texture analysis of highly grain-aligned ceramics of thermoelectric [Ca2CoO3]0.62[CoO2] prepared by the reactive-templated grain growth process was examined. The direct quantitative texture analysis and the combined approach, two advanced methods of x-ray diffraction analysis, were used to analyze fiber texture strength and crystallite size. The calculation of distribution density, correlated to the representation of normal and inverse pole figures, highlights the influence of the texture development on the thermoelectric properties. The electrical conductivity was significantly increased by the improvement of grain alignment during the final uniaxial-pressing treatment. An increase in orientation degree enhances thermoelectric properties, even for highly textured ceramics. The quantitative texture analysis appears to be essential for the development of textured thermoelectric ceramics with enhanced properties.