Long-range ordering in the Bi1−xAexFeO3−x/2 perovskites: Bi1/3Sr2/3FeO2.67 and Bi1/2Ca1/2FeO2.75

Two-ordered perovskites, Bi_1/3Sr_2/3FeO_2.67 and Bi_1/2Ca_1/2FeO_2.75, have been stabilized and characterized by transmission electron microscopy, Mössbauer spectroscopy and X-ray powder diffraction techniques. They both exhibit orthorhombic superstructures, one with a≈b≈2a_p and c≈3a_p (S.G.: Pb2n or Pbmn) for the Sr-based compound and one with a≈b≈2a_p and c≈8a_p (S.G.: B222, Bmm2, B2mm or Bmmm) for the Ca-based one. The high-resolution transmission electron microscopy (HRTEM) images evidence the existence of one deficient [FeO_x]_∞ layer, suggesting that Bi_1/3Sr_2/3FeO_2.67 and Bi_1/2Ca_1/2FeO_2.75 behave differently compared to their Ln-based homolog. The HAADF-STEM images allow to propose a model of cation ordering on the A sites of the perovskite. The Mössbauer analyses confirm the trivalent state of iron and its complex environment with three types of coordination. Both compounds exhibit a high value of resistivity and the inverse molar susceptibility versus temperature curves evidence a magnetic transition at about 730 K for the Bi_1/3Sr_2/3FeO_2.67 and a smooth reversible transition between 590 and 650 K for Bi_1/2Ca_1/2FeO_2.75.     

Microwave dielectric properties of BaO−2CeO2−nTiO2 ceramics

The BaO-2CeO₂-nTiO₂ ceramics with n=3, 4 and 5 have been prepared with CeO₂ as starting material. The ceramics have been characterized using scanning electron microscopy, X-ray diffraction, Raman and X-ray photoelectron spectroscopy techniques. The microwave dielectric properties have been measured using standard dielectric resonator techniques. BaO-2CeO₂-3TiO₂ (123), BaO-2CeO₂-4TiO₂ (124) and BaO-2CeO₂-5TiO₂ (125) ceramics showed dielectric constants of 38, 27 and 32, respectively. All the ceramics showed fairly good unloaded Q-factors. 124 and 125 compounds exhibited low τ_f values, while 123 showed a high τ_f value.     

The Crystal Structure of Sr2TiSi2O8

Sr₂TiSi₂O₈ single crystals were grown by Czochralski pulling and from a high-temperature solution. X-ray diffractometry revealed the modulated crystal structure of Sr₂TiSi₂O₈ to belong to the 5D superspace group P4bm (−α, α, 1/2; α, α, 1/2) with α=0.3. Atomic positions, anisotropic displacement factors and positional modulation parameters for Sr₂TiSi₂O₈ are determined and discussed. The positional modulation is further investigated by electron diffraction and high-resolution transmission electron microscopy. In the latter experiments, the 2D modulation appears to be superimposed by some 1D modulation waves. This effect is discussed in terms of growth conditions.     

Structural and electrical properties of the 2Bi2O3·3ZrO2 system

Powder mixtures of α-Bi₂O₃ (bismite) and monoclinic m-ZrO₂ (baddeleyite) in the molar ratio 2:3 were mechanochemically and thermally treated with the goal to examine the phases, which may appear during such procedures. The prepared samples were characterized by X-ray powder diffraction, differential scanning calorimetry (DSC), electrical measurements, as well as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The mechanochemical reaction leads to the gradual formation of a nanocrystalline phase, which resembles δ-Bi₂O₃, a high-temperature Bi₂O₃ polymorph. Isothermal sintering in air at a temperature of 820 °C for 24 h followed by quenching to room temperature yielded a mixture of ZrO₂-stabilized β-Bi₂O₃ and m-ZrO₂ phases, whereas in slowly cooled products, the complete separation of the initial α-Bi₂O₃ and m-ZrO₂ constituents was observed. The dielectric permittivity of the sintered samples significantly depended on the temperature. The sintered and quenched samples exhibited a hysteresis dependence of the dielectric shift, showing that the ZrO₂-doped β-Bi₂O₃ phase possess ferroelectric properties, which were detected for the first time. This fact, together with Rietveld refinement of the β-Bi₂O₃/m-ZrO₂ mixture based on neutron powder diffraction data showed that ZrO₂-doped β-Bi₂O₃ has a non-centrosymmetric structure with as the true space group. The ZrO₂ content in the doped β-Bi₂O₃ and the crystal chemical reasons for the stabilization of the β-Bi₂O₃ phase by the addition of m-ZrO₂ are discussed.     

The compound Cr2TiO5 in the system Cr2O3TiO2

The compound Cr₂TiO₅ could be synthesized as a stoichiometric single phase above 1660°C in air. Application of selected area electron diffraction, high resolution electron microscopy and powder X-ray diffraction studies showed that Cr₂TiO₅ is isomorphous with CrFeTiO₅, with V₃O₅ type structure. It is monoclinic, a = 7.020(1)Å, b = 5.025(1)Å, c = 9.945(2)Å and β = 111.43(2)°. It was found that Cr₂TiO₅ is unstable relative to a mixture of Cr₂O₃ (ss) and a so-called “E” phase, below 1660°C.     

Evolution and characterization of fluorite-like nano-SrBi2Nb2O9 phase in the SrO-Bi2O3-Nb2O5-Li2B4O7 glass system

Transparent glasses of various compositions in the system (100−x)Li₂B₄O₇−x(SrO-Bi₂O₃-Nb₂O₅) (where x=10, 20, 30, 40, 50 and 60, in molar ratio) were fabricated via splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses. X-ray powder diffraction (XRD) and transmission electron microscopic studies confirmed the amorphous nature of the as-quenched and crystallinity in the heat-treated samples. Fluorite phase formation prior to the perovskite SrBi₂Nb₂O₉ phase was analyzed by both the XRD and high-resolution transmission electron microscopy. Dielectric and the optical properties (transmission, optical band gap and Urbach energy) of these samples have been found to be compositional dependent. Refractive index was measured and compared with the values predicted by Wemple-Didomemenico and Gladstone-Dale relations. The glass nanocomposites comprising nanometer-sized crystallites of fluorite phase were found to be nonlinear optic active.     

New perovskite-based manganite Pb2Mn2O5

A new perovskite based compound Pb₂Mn₂O₅ has been synthesized using a high pressure high temperature technique. The structure model of Pb₂Mn₂O₅ is proposed based on electron diffraction, high angle annular dark field scanning transmission electron microscopy and high resolution transmission electron microscopy. The compound crystallizes in an orthorhombic unit cell with parameters a=5.736(1) Å≈√2a_p, b=3.800(1) Å≈a_p, c=21.562(6) Å≈4√2a_p (a_p—the parameter of the perovskite subcell) and space group Pnma. The Pb₂Mn₂O₅ structure consists of quasi two-dimensional perovskite blocks separated by 1/2[110]_p(1?01)_p crystallographic shear planes. The blocks are connected to each other by chains of edge-sharing MnO₅ distorted tetragonal pyramids. The chains of MnO₅ pyramids and the MnO₆ octahedra of the perovskite blocks delimit six-sided tunnels accommodating double chains of Pb atoms. The tunnels and pyramidal chains adopt two mirror-related configurations (“left” L and “right” R) and layers consisting of chains and tunnels of the same configuration alternate in the structure according to an -L-R-L-R-sequence. The sequence is sometimes locally violated by the appearance of -L-L- or -R-R-fragments. A scheme is proposed with a Jahn-Teller distortion of the MnO₆ octahedra with two long and two short bonds lying in the a-c plane, along two perpendicular orientations within this plane, forming a d-type pattern.     

Structural studies on W and Nd substituted La2Mo2O9 materials

The structure of a series of new ionic conductors based in lanthanum molybdate (La₂Mo₂O₉) has been investigated using transmission electron microscopy (TEM), high-resolution X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The superstructure 2a_c×3a_c×4a_c of the low temperature α-polymorph relative to the β-polymorph was confirmed by HRTEM imaging and electron diffraction. Furthermore, the effects of partial cation substitution in the La_2−xNd_xMo₂O₉ and La₂Mo_2−yW_yO₉ series have been also evaluated in the search of new clues to understand the structure and stabilisation of the high temperature and better conductor β-polymorph. The thermal analysis studies show that Nd-substitution does not stabilise completely the β-polymorph at room temperature, although no superstructure ordering was observed by both XRD and HRTEM. On the other hand, W-substitution stabilises the cubic β-polymorph for y>0.25, although, electron diffraction indicates a slight distortion from the cubic symmetry for low W-content. This distortion disappears as the W content increases and the Rietveld refinements gradually render better results.     

Thermal decomposition mechanisms of MgCl2·6H2O and MgCl2·H2O

The thermal decomposition mechanisms and the intermediate morphology of MgCl₂·6H₂O and MgCl₂·H₂O were studied using integrated thermal analysis, X-ray diffraction, scanning electron microscope and chemical analysis. The results showed that there were six steps in the thermal decomposition of MgCl₂·6H₂O: producing MgCl₂·4H₂O at 69 °C, MgCl₂·2H₂O at 129 °C, MgCl₂·nH₂O (1 ≤ n ≤ 2) and MgOHCl at 167 °C, the conversion of MgCl₂·nH₂O (1 ≤ n ≤ 2) to Mg(OH)Cl·0.3H₂O by simultaneous dehydration and hydrolysis at 203 °C, the dehydration of Mg(OH)Cl·0.3H₂O to MgOHCl at 235 °C, and finally the direct conversion of MgOHCl to the cylindrical particles of MgO at 415 °C. To restrain the sample hydrolysis and to obtain MgCl₂·H₂O, MgCl₂·6H₂O was first calcined in HCl atmosphere until 203 °C when MgCl₂·H₂O was obtained; HCl gas was then turned off and the calcination process continued, producing Mg₃Cl₂(OH)₄·2H₂O calcined at 203 °C, Mg₃(OH)₄Cl₂ at 220 °C and MgO at 360 °C. The temperature of producing MgO from calcination of MgCl₂·H₂O was lower (360 °C) than that from MgCl₂·6H₂O (415 °C) because of its more reactive intermediate products: the irregular shape and tiny needle-like Mg₃Cl₂(OH)₄·2H₂O particles and the uneven surface porous Mg₃(OH)₄Cl₂ particles. The MgO particles obtained at 360 °C had a flake structure.     

Crystal structure and magnetic properties of Co2TeO3Cl2 and Co2TeO3Br2

The crystal structures of the two new synthetic compounds Co₂TeO₃Cl₂ and Co₂TeO₃Br₂ are described together with their magnetic properties. Co₂TeO₃Cl₂ crystallize in the monoclinic space group P2₁/m with unit cell parameters a=5.0472(6) Å, b=6.6325(9) Å, c=8.3452(10) Å, β=105.43(1)°, Z=2. Co₂TeO₃Br₂ crystallize in the orthorhombic space group Pccn with unit cell parameters a=10.5180(7) Å, b=15.8629(9) Å, c=7.7732(5) Å, Z=8. The crystal structures were solved from single crystal data, R=0.0328 and 0.0412, respectively. Both compounds are layered with only weak interactions in between the layers. The compound Co₂TeO₃Cl₂ has [CoO₄Cl₂] and [CoO₃Cl₃] octahedra while Co₂TeO₃Br₂ has [CoO₂Br₂] tetrahedra and [CoO₄Br₂] octahedra. The Te(IV) atoms are tetrahedrally [TeO₃E] coordinated in both compounds taking the 5s² lone electron pair E into account. The magnetic properties of the compounds are characterized predominantly by long-range antiferromagnetic ordering below 30 K.     

Microstructure and electrical transport in nano-grain sized Ce0.9Gd0.1O2−δ ceramics

An enhancement of the electrical conductivity has been found in nano-grain sized Ce_0.9Gd_0.1O_2−δ ceramics when measured in N₂ (p_O2=3.5×10⁻⁶ atm) in comparison with the most commonly accepted values of bulk ionic conductivity. We first present the synthesis and characterisation of the nanoparticles later used for the preparation of dense nanoceramics of Gd-doped CeO₂. The nanoparticles were characterised by X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The good sintering properties of these nanopowders allowed us to obtain very dense ceramics (>90% theoretical density) while keeping the grain size close to 100 nm. The microstructure of these nanoceramics was analysed by AFM and scanning electron microscopy (SEM) while the electrical characterisation was performed by the 4-point dc technique between 500 and 950 °C in air or N₂ and ac impedance between 150 and 400 °C in air and or argon. We briefly discuss the possibilities of electron vs. oxygen ion conduction and grain boundary vs. bulk conductivity. The features exhibited by these ceramics represent an increased potential to process solid electroceramics materials with specific levels of electronic and/or ionic conductivities for a variety of electrochemical devices.     

Lanthanum pyrochlores and the effect of yttrium addition in the systems La2−xYxZr2O7 and La2−xYxHf2O7

The crystal structures of the compounds La_2−xY_xZr₂O₇ and La_2−xY_xHf₂O₇ with x=0.0, 0.4, 0.8, 1.2, 1.6, and 2.0 have been studied using neutron powder diffraction and electron microscopy to determine the stability fields of the pyrochlore and fluorite solid solutions. The limits of pyrochlore stability in these solid solutions are found to be close to La_0.8Y_1.2Zr₂O₇ and La_0.4Y_1.6Hf₂O₇, respectively. In both systems the unit cell parameter is found to vary linearly with Y content across those compositions where the pyrochlore phase is stable, as does the x-coordinate of the oxygen atoms on the 48f (x,,) sites. In both systems, linear extrapolations of the pyrochlore data suggest that the disordering is accompanied by a small decrease in the lattice parameter of approximately 0.4%. After the pyrochlore solid solution limit is reached, a sharp change is observed from x∼0.41 to 0.375 as the disordered defect fluorite structure is favoured. Electron diffraction patterns illustrate that some short-range order remains in the disordered defect fluorite phases.     

Reaction of some tertiary phosphines and phosphites with [Co(η5-C5H5)(S2C2{CN}2)]

Reaction of the 16 electron monomer [Co(η⁵-C₅H₅)(S₂C₂{CN}₂)] with various tertiary phosphines and phosphites (L) gives readily the 18 electron monomers [Co(η⁵-C₅H₅)(S₂C₂{CN}₂)L] which for L = P(OR)₃ have J($\text{P}$C₅$\text{H}$₅) ca. 6 Hz but J($\text{P}$C₅$\text{H}$₅) = 0 for L = PR₃.     

The Crystal Structures, Microstructure and Ionic Conductivity of Ba2In2O5 and Ba(InxZr1−x)O3−x/2

This paper describes the results of electron microscopy, high-temperature powder neutron diffraction, and impedance spectroscopy studies of brownmillerite-structured Ba₂In₂O₅ and perovskite structured Ba(In_xZr_1−x)O_3−x/2. The ambient temperature structure of Ba₂In₂O₅ is found to adopt Icmm symmetry, with disorder of the tetrahedrally coordinated (In³⁺) ions of the type observed previously in Sr₂Fe₂O₅. Ba₂In₂O₅ undergoes a ∼6-fold increase in its ionic conductivity over the narrow temperature range from ∼1140 K to ∼1230 K, in broad agreement with previous studies. This transition corresponds to a change from the brownmillerite structure to a cubic perovskite arrangement with disordered anions. Electron microscopy investigations showed the presence of extended defects in all the crystals analyzed. Ba(In_xZr_1−x)O_3−x/2 samples with x=0.1 to 0.9 adopt the cubic perovskite structure, with the lattice parameter increasing with x.     

Electronic parameters of Sr2Nb2O7 and chemical bonding

X-ray photoelectron spectroscopy (XPS) measurements were carried out on a strontium pyroniobate (Sr₂Nb₂O₇) powder sample, which was synthesized using standard solid-state method. The binding energy (BE) differences between the O 1s and cation core levels, Δ(O-Nb)=BE(O 1s)-BE(Nb 3d_5/2) and Δ(O-Sr)=BE(O 1s)-BE(Sr 3d_5/2), were used to characterize the valence electron transfer on the formation of the Nb-O and Sr-O bonds. The chemical bonding effects were considered on the basis of our XPS results for Sr₂Nb₂O₇ and earlier published structural and XPS data for other Sr- or Nb-containing oxide compounds. The new data point for Sr₂Nb₂O₇ is consistent with the previously derived relationship for a set of Nb⁵⁺-niobates that Δ(O-Nb) increases with increasing mean Nb-O bond distance, L(Nb-O). A new empirical relationship between Δ(O-Sr) and L(Sr-O) was also obtained. Interestingly, the correlation between Δ(O-Sr) and L(Sr-O) was found to differ from that between Δ(O-Nb) and L(Nb-O). Possible cause for the difference is discussed.     

Single-crystal synthesis and structure refinement of the LiCoO2-LiAlO2 solid-solution compounds: LiAl0.32Co0.68O2 and LiAl0.71Co0.29O2

Single crystals of the LiCoO₂-LiAlO₂ solid solution compounds LiAl_0.32Co_0.68O₂ and LiAl_0.71Co_0.29O₂ were synthesized by a flux method using alumina crucibles. A single-crystal X-ray diffraction study confirmed the trigonal space group and the lattice parameters a=2.8056(11) Å, c=14.1079(15) Å, and c/a=5.028 for LiAl_0.32Co_0.68O₂, and a=2.8023(7) Å, c=14.184(4) Å, and c/a=5.061 for LiAl_0.71Co_0.29O₂. The crystal structures have been refined to the conventional values R=3.2% and wR=2.4% for LiAl_0.32Co_0.68O₂, and R=3.6% and wR=3.5% for LiAl_0.71Co_0.29O₂. The evidence of the location of Al atoms in the pseudotetragonal coordination (6c site), reported previously in LiAl_0.2Co_0.8O₂, could not be observed in the present electron density distribution maps in both LiAl_0.32Co_0.68O₂ and LiAl_0.71Co_0.29O₂. The octahedral distortion analysis indicated that the Al-substitution strongly affected the distortion of the LiO₆ octahedron in this solid-solution compound system, but hardly affected that of the (Al.Co)O₆ octahedron.     

A high-temperature structure for Ta2O5 with modulations by TiO2 substitution

Solid solutions in the series (1−x)Ta₂O₅−xTiO₂ with x=0.0-0.1 were prepared by high-temperature ceramic processing methods, and the crystal structure was determined at room temperature by transmission electron microscopy, electron diffraction and high-resolution lattice imaging. A structural model is proposed for the oxygen-deficient tantalum oxide (Ta₂O₅) phase with high TiO₂ doping level (x=0.08). The model is based on edge sharing of an oxygen octahedron-hexagonal bi-pyramid-octahedron molecular building block unit that repeats four times per unit cell. Electron diffraction reveals a monoclinic distortion from a pseudo-tetragonal model structure that is modulated primarily along 〈110〉. The modulation length varies with increasing TiO₂ content. Furthermore, by quantitative HREM analysis and matching of lattice images by simulation, it is shown that the modulation is associated with small ionic displacements in specific lattice planes that coincide with Ta ions in the model structure coordinated by oxygen hexagonal bi-pyramids. Based on this evidence, it is suggested that the modulation comes from a replacement of Ta with Ti ions, and the loss of inversion symmetry in the modulated structure is related to the dielectric properties of the material.     

Structural disorder in Ba0.6Sr0.4Al2O4

The structural disorder in Ba_0.6Sr_0.4Al₂O₄ (space group P6₃22) was investigated by X-ray powder diffraction and selected-area electron diffraction (SAED). The initial structural model was determined using direct methods, and it was further modified by the combined use of Rietveld method and maximum-entropy method (MEM). MEM-based pattern fitting method was subsequently applied, resulting in the final reliability indices of R_wp=9.61%, R_p=6.96%, R_B=1.40% and S=1.25. The electron density distribution was satisfactorily expressed by the split-atom model in which the strontium/barium and oxygen atoms were split to occupy the lower symmetry sites. The diffuse scattering in SAED was mainly attributable to the positional disorder of oxygen atoms.     

New effective reagent [Cp2ZrH2 · ClAlEt2]2 for alkene hydrometallation

New bimetallic complex [Cp₂ZrH₂ · ClAlEt₂]₂ (1) was synthesized, and its reactivity in hydrometallation reaction with the following alkenes was studied: hept-1-ene, okt-1-ene, α-methylstyrene, (1S)-β-pinene, (+)-camphene. Complex 1 shows the highest reactivity among the other known Al,Zr-bimetallic complexes: [Cp₂ZrH₂ · ClAlBuⁱ₂]₂ (2), [Cp₂ZrH₂ · AlEt₃]₂ (3), [Cp₂ZrH₂ · AlBuⁱ₃]₂ (4) and [Cp₂ZrH₂ · HAlBuⁱ₂] (5) as well as organoaluminium compounds (OAC): ⁱBu₂AlH, ⁱBu₃Al and ⁱBu₂AlCl in presence of Zr catalysts. Chlorine containing complexes 1 and 2 appear to be more effective in alkene hydrometallation, and relative hydrometallation rates are (1S)-β-pinene ? (+)-camphene < α-methylstyrene < oct-1-ene < hept-1-ene. Hydrometallation of (1S)-β-pinene and its subsequent oxidation with I₂ run with high diastereoselectivity and yield trans-myrtanol. However, the diastereoselectivity of (+)-camphene hydrometallation is less than that for (1S)-β-pinene, and the reaction gives predominately endo-camphanol.