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.     

Two polymorphous lithium stannoferrites LiFeSnO4: A ramsdellite-type and a hexagonal close-packed structure

The stannoferrite LiFeSnO₄ has been synthesize and investigated by X-ray diffraction and electron microscopy. Two forms have been characterized. High-temperature LiFeSnO₄ is orthorhombic, isotypic with MnO₂ ramsdellite: a = 3.066 Å, b = 5.066 Å, c = 9.874 Å; low-temperature LiFeSnO₄ is hexagonal with a = 6.012Z Å, c = 9.776 Å and isotypic with Li_1.6Zn_1.6Sn_2.8O₄. Both structures have been refined. This polymorphous transition and relationships between the two forms and spinel structure are discussed.     

Hexagonal-spinel transitions in antimonates Li2Cr3−xMIIIxSbO8 (MIII = Al,Fe, Ga)

A new family of antimonates Li₂Cr_3?xM^III_xSbO₈ (M^III = Al, Fe, Ga) was synthesized and studied by X-ray diffraction and ir spectroscopy. The Al-containing compounds exhibit a hexagonal close-packed structure similar to that of LiFeSnO₄ ($\text{a ? 5.8, c ? 9.5}$ Å. For M = Fe or Ga, two structural forms are isolated: a low-temperature hexagonal form which is isotypic with LiFeSnO₄ and a high-temperature cubic form isotypic with the spinel structure. The hexagonal spinel transformation was observed for the first time, while the reverse transformation cannot be obtained.     

Ba0.15WO3: A bronze with an original pentagonal tunnel structure

The structure of a new barium tungsten bronze, Ba_0.15WO₃, has been established by X-ray diffraction and high-resolution microscopy studies. This bronze is orthorhombic, space group Pbm2 or Pbmm, with a = 8.859(3) Å, b = 10.039(8) Å, and c = 3.808(2)Å. The “WO₃” framework is built up from corner-sharing WO₆ octahedra forming pentagonal tunnels where the barium ions are located. Structural relationships with hexagonal tungsten bronze and tetragonal tungsten bronze structures are discussed.     

Spectres infrarouges et structure des anions 2-methylallyle, CH3-C(CH2)2- et triméthylèneméthane, C(CH2)3

The infrared spectra of solid samples of C₄H₇K and C₄D₇K have been investigated in the 4000 to 30 cm⁻¹ range. A complete assignment of intramolecular fundamentals of C₄H₇⁻ and C₄D₇⁻ ions and of potassium-allyl vibrations is proposed and the intramolecular force constants are calculated. The C(CH₂)₃²⁻ anion has been identified spectroscopically. Structures of C₃H₅⁻, C₄H₇⁻ and C(CH₃)₃²⁻ are discussed and compared with those optimised by the MINDO/3 method.     

Oxygen defect K2NiF4-type oxides: The compounds La2−xSrxCuO4−x2+δ

Oxygen defect K₂NiF₄-type oxides $\text{La}{}_{\mathrm{2?x}}\text{Sr}{}_{\mathrm{x}}\text{CuO}{}_{\mathrm{<mtext>4?</mtext><mtext>x</mtext><mtext>2</mtext><mtext>+\delta </mtext>}}$ have been synthesized for a wide composition range: 0 ≤ x ≤ 1.34. From the X-ray and electron diffraction study three domains have been characterized: orthorhombic compounds with La₂CuO₄ structure for 0 ≤ x < 0.10, tetragonal oxides similar to LaSrCuO₄ for 0.10 ≤ x < 1 and several superstructures derived from the tetragonal cell ( with n = 3, 4, 4.5, 5, 6) for 1 ≤ x ≤ 1.34. The compounds corresponding to 0 < x < 1 differ from the other oxides in that they are characterized by the presence of copper with two oxidation states: + 2 and + 3. A model structure for La_0.8Sr_1.2CuλO_3.4, in which copper has only the + 2 oxidation state, and for which the actual cell is tegragonal—a = 18.80₄ Å and c = 12.94 Å—has been established. The particular structural evolution of these compounds is discussed in terms of a competition between the capability of Cu(II) to be oxidized to Cu(III) and the ordering of oxygen vacancies.     

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.     

Modular construction of oxide structures--compositional control of transition metal coordination environments

The effects of reaction temperature and pO2 were investigated on a series of (Ba,Ca,Nd)FeO3-delta perovskite systems in order to isolate phases containing ordered arrangements of the distinct vacancy and cation ordering patterns identified in less compositionally complex iron oxide systems. Initial synthesis in air at high temperature yields cubic perovskite phases (I) with average iron oxidation states higher than 3; selected area electron diffraction together with diffuse features observed in the synchrotron X-ray diffraction (SXRD) patterns of these materials show evidence of small domains of short-range cation and vacancy order. Annealing these materials in nitrogen or in a sealed tube in the presence of an NiO/Ni buffer yielded the Fe(3+) phase Ca2Ba2Nd2Fe6O16 (II), closely related to Sr2LaFe3O8 but with partial cation order as well as anion order present the larger Ba cations are largely present in the 12-coordinate site between the octahedral iron layers, and Ca is largely present in 10-coordinate sites between octahedral and tetrahedral sites. Further reduction of Ca2Ba2Nd2Fe6O16 using a Zr getter yields the mixed-valence phase Ca2Ba2Nd2Fe6O15.6 (III). The structure of III was solved by maximum entropy analysis of XRD data coupled with analysis of high-temperature neutron diffraction data and refined against combined SXRD and high-Q ambient-temperature neutron data. This material crystallizes in a 20-fold perovskite super cell (Imma, a approximately square root(2 x a(p), b approximately 10 x a(p), c approximately square root(x 2a(p)) and can be visualized as an intergrowth between brownmillerite (Ca2Fe2O5) and the YBa2Fe3O8 structure. There are three distinct iron coordination environments, octahedral (O), square-pyramidal (Sp), and trigonal planar (Tp, formed by distorting the tetrahedral site in brownmillerite), which form a Sp-O-Tp-O-Sp repeat. Bond valence calculations indicate that Tp is an Fe(2+) site, while the O and Sp sites are Fe(3+). The A-site cations are also partially ordered over three distinct sites: 8-coordinate between the Sp layers, 10-coordinate between Tp and O layers, and 12-coordinate between Sp and O layers. Mossbauer spectroscopy, magnetometry, and variable-temperature neutron diffraction show that the material undergoes two magnetic transitions at approximately 700 and 255 K.     

Decrypting the TEM images for deciphering the microstructural code of complex oxides

The knowledge of the structure of the real solids is required for achieving the desired architectures in the research of new materials and/or optimizing the relationships between structure and properties. Understanding complex oxides needs accurate characterization at different length scales and the combined application of all solid-state techniques. Deciphering the relationships between all this information provides codes that allow the identification of the different structural levels, their roles and the way they interact. These step-by-step routes are illustrated through two basic mechanisms of solid-state chemistry: to determine the building units of one complex oxide in order to predict the existence of other arrangements on the one hand and to correlate complex ordering phenomena, such as those involving charges, orbitals and spins of manganese atoms in perovskite-type manganites on the other hand.     

Blood volume and red cell mass in children with moderate and severe malaria measured by chromium‐53 dilution and gas chromatography/mass spectrometric analysis

Understanding blood volume changes in children with malaria is important for managing fluid status. Traditionally, blood/red cell volume measurements have used radioactive chromium isotopes. We applied an alternative approach, using non‐radioactive chromium‐53 labelling and mass spectrometry to investigate red cell volume (RCV) in Gabonese children with malaria. Nineteen children with malaria participated (10 severe, 9 moderately severe; ages 15 months to 7 years). Blood labelled with ⁵³Cr‐chromate ex vivo was re‐injected, then sampled 30 min later. Pre‐ and post‐injection ⁵³Cr content were measured by gas chromatography/electron ionisation mass spectrometry of the chromium‐trifluoroacetylacetone (TFA) chelate, calibrated against ⁵⁰Cr standards. Blood and red cell volumes were calculated from isotopic dilution in 15 of 19 children (in four, insufficient signal mitigated analysis). In this small pilot study, there were no significant differences between moderate and severe cases. Including all subjects, the mean RCV was reduced compared with predicted values (184 vs. 269 mL; p = 0.016) but blood volume, 71 ± 33 mL/kg (normalised for weight), was close to predicted, ～77 mL/kg, commensurate with reduced haematocrit. Blood lactate concentration correlated negatively with RCV/weight (r = ?0.56, p = 0.028), consistent with anaemia. In one case, sequential samples over 42 days gave an estimated rate of ⁵³Cr disappearance of 1.4%/day (equivalent half‐life: 70 days). ⁵³Cr‐labelling of red cells may be used to estimate blood and red cell volumes and can be used as an investigative tool in situations such as childhood diseases and resource‐constrained settings. Although the red cell mass is depleted in malaria, the blood volume appears relatively well preserved. Copyright © 2009 John Wiley & Sons, Ltd.