Etude par diffraction X et microscopie electronique du système CaTiO3---Ca2Fe2O5

In order to check the extended defect model previously proposed for the nonstoichiometric perovskite A_nM_nO_3n−1 (n 2), the Ca₂Ti_2−2xFe_2xO_6−x solid solution has been studied by means of X-ray diffraction and electron microscope investigations. The results show the existence of a continuous evolution (0 x 1) with two different regions. Close to the CaTiO₃ composition (0 x 0.40) the oxygen vacancies seem to be randomly distributed and the symmetry remains pseudocubic. In the composition range 0.55 x 1, the electron microscope patterns show an ordering of the defects: the oxygen vacancies are ordered in the (0k0) planes in strings parallel to the [101] direction. It is concluded from these observations that perpendicular to the Oy direction isolated planes of tetrahedra arranged in rows separate planes of corner-linked octahedra. The n = 2; 2.5; 3, et 4 terms of the A_nM_nO_3n−1 series have been identified. C.S. planes characteristic of classical nonstoichiometric oxides with small cations do not appear: instead, tetrahedra planes succeed to the previous octahedra planes as the degree of nonstoichiometry increases.

Résumé

Afin de vérifier le modèle de non-stoechiométrie proposé dans un travail précédent pour les perovskites lacunaires A_nM_nO_3n−1 (n 2), la solution solide Ca₂Ti_2−2xFe_2xO_6−x a été, étudiée par diffraction X et microscopie électronique. Les résultats montrent l'existence d'une évolution continue (0 x 1) avec deux domaines différents. Pour les compositions proches de CaTiO₃ (0 x 0.40), les lacunes oxygénées semblent être distribuées statistiquement et la symétrie demeure pseudocubique. Dans le domaine de compositions 0.55 x 1, les images de microscopie électronique montrent un ordre des défauts: les lacunes d'oxygène sont ordonnées dans les plans (0k0) en rangées parallèles à la direction [101]. Ces observations ont permis de conclure que perpendiculairement à la direction Oy des plans isolés de files de tétraèdres séparent des plans d'octaèdres liés par leurs sommets. Les termes n = 2; 2.5; 3 et 4 des séries A_nM_nO_3n−1 ont été identifiés. Des plans de cisaillement rencontrés dans les oxydes non-stoechiométriques classiques de petits cations n'ont pas été mis en évidence: par contre, des plans de tétraèdres se substituent aux plans d'octaèdres initiaux lorsque le degré de non-stoechiométrie augmente.     

Lithium intercalation into PbNb2S5, PbNbS3, SnNb2Se5, BiVS3, SnVSe3, and PbNb2Se5 misfit layer chalcogenides

The intercalation of lithium into various misfit layer chalcogenides of two different stoichiometries was performed by using n-butyl lithium on powders. The reaction was found to proceed topochemically, and a greater expansion in the c direction and higher lithium contents were observed in the lithiated phases with “MM′₂X₅” approximate stoichiometries compared to “MM′X₃” stoichiometries. This behaviour difference is assigned to the different stacking sequence of the slices of the two sublattices formed by double layers of MX and sandwiches of M′X₂. Lattice distortions are induced during lithiation, leading to changes in the relative orientation of MS-type bilayers and to complete amorphization after long reaction times. The synthesis and partial characterization of a new misfit layer selenide of nominal composition “PbNb₂Se₅” is also reported. The value of the c-dimension (c = 37.3₇ Å) suggests a stacking sequence PbSe---NbSe₂---NbSe₂---PbSe---NbSe₂---NbSe₂, etc. This material becomes highly unstable on lithium intercalation and decomposes to its constituents after a few hours of lithiation.     

[{(NiOH)2Mo10O36(PO4)Ti2}n]: a novel chainlike trimetal heteropolyanion based on pseudo-keggin fragments

A new mixed Mo/Ni/Ti heteropoly compound [C₅H₅NH]₅ [(NiOH)₂Mo₁₀O₃₆(PO₄)Ti₂] has been hydrothermally synthesized and structurally determined by the single-crystal X-ray diffraction. Black prismatic crystals crystallize in the monoclinic system, space group P2(1)/n, a=11.2075(2), b=37.8328(5) c=13.0888(1) Å, β=101.4580(10)°, M=2276.13, V=5439.19(13) ų, Z=4. Data were collected on a Siemens SMART CCD diffractometer at 293(2) K in the range of 1.68<θ<25.09° using the ω-scan technique (λ=0.71073 Å R(F)=0.0872 for 9621 reflections). The title compound contains a trimetal heteropolyanion polymer and “trans-titanium”-bridging pseudo-Keggin fragments linked to a chain.     

Tunneled Intergrowth Structures in the Ga2O3–In2O3–SnO2 System

The structures of several Ga₂O₃–In₂O₃–SnO₂ phases were investigated using high-resolution electron microscopy, X-ray diffraction, and Rietveld analysis of time-of-flight neutron diffraction data. The phases, expressed as Ga_4−4xIn_4xSn_n−4O_2n−2 (n=6 and 7–17, odd), are intergrowths between the β-gallia structure of (Ga,In)₂O₃ and the rutile structure of SnO₂. Samples prepared with n≥9 crystallize in C2/m and are isostructural with intergrowths in the Ga₂O₃–TiO₂ system. Samples prepared with n=6 and n=7 are members of an alternative intergrowth series that crystallizes in P2/m. Both intergrowth series are similar in that their members possess 1-D tunnels along the b axis. The difference between the two series is described in terms of different crystallographic shear plane operations (CSP) on the parent rutile structure.     

Phase Relations at 1500°C in the Ternary System ZrO2–Gd2O3–TiO2

Phase relations at 1500°C in the ternary system ZrO₂–Gd₂O₃–TiO₂ have been determined by the powder X-ray diffraction of samples prepared by standard solid state reaction. A large area of this ternary oxide system centered on the Gd₂Ti₂O₇–Gd₂Zr₂O₇ join was shown to exhibit the pyrochlore and defect fluorite structures. The pyrochlore structure was observed for stoichiometries as far from the ideal M₄O₇ as M₄O_6.7 and M₄O_7.4, although the degree of disorder seemed much higher at these stoichiometries. On further deviation from the ideal M₄O₇ stoichiometry a smooth transition to fluorite average structure was observed for Zr-rich compositions. None of the other binary phases were observed to show significant extent of solid solution into the ternary region.     

New complex oxides of the A3n+3mA′nB3m+nO9m+6n family: Ba6A′Mn4O15 (A′=Mg, Ni)

Complex oxides Ba₆AMn₄O₁₅, where A=Mg (I) and Ni (II), belonging to the homologous series A_3n+3mA′_nB_3m+nO_9m+6n (n=1, m=1) were obtained by solid state reaction method from Ba carbonate and oxides MgO, NiO, MnO₂. Both new oxides are incommensurate. Their crystal structures were interpreted as composite ones with two subcells: a=10.042(3) Å, c₁=4.318(2) Å, c₂=2.565(1) Å, c₁/c₂=1.6834 for (I) and a=10.044(3) Å, c₁=4.308(2) Å, c₂=2.551(1) Å, c₁/c₂=1.6887 for (II). Magnetic susceptibility measurements in the range 2–850 K revealed antiferromagnetic correlations in Ba₆MgMn₄O₁₅ (T_N=7 K) and a pseudo-square-planar environment of some Ni²⁺ cations in Ba₆NiMn₄O₁₅.     

Layer structure: The oxides A3Ti5MO14

Five new oxides, K₃Ti₅MO₁₄, Rb₃Ti₅MO₁₄ (M = Ta, Nb), and Tl₃Ti₅NbO₁₄, have been synthesized. The structure of these oxides consists of octahedral layers similar to those observed for Na₂Ti₃O₇ and held together by monovalent ions; the sheets consist of blocks of 2 × 3 edge-sharing octahedra, which are then joined to each other by the corners of the octahedra. The relative disposition of the layers is similar to that observed for Tl₂Ti₄O₉. These oxides can be considered as the member n = 3 of a series of closely related structures with formula A_nB_2nO_4n+2, where n indicates the number of octahedra which determines the width of the blocks of 2 × n octahedra.     

Original close-packed structure and magnetic properties of the Pb4Mn9O20 manganite

The crystal structure of the Pb₄Mn₉O₂₀ compound (previously known as “Pb_0.43MnO_2.18”) was solved from powder X-ray diffraction, electron diffraction, and high resolution electron microscopy data (S.G. Pnma, a=13.8888(2) Å, b=11.2665(2) Å, c=9.9867(1) Å, R_I=0.016, R_P=0.047). The structure is based on a 6H (cch)₂ close packing of pure oxygen “h”-type (O₁₆) layers alternating with mixed “c”-type (Pb₄O₁₂) layers. The Mn atoms occupy octahedral interstices formed by the oxygen atoms of the close-packed layers. The MnO₆ octahedra share edges within the layers, whereas the octahedra in neighboring layers are linked through corner sharing. The relationship with the closely related Pb₃Mn₇O₁₅ structure is discussed. Magnetization measurements reveal a peculiar magnetic behavior with a phase transition at 52 K, a small net magnetization below the transition temperature, and a tendency towards spin freezing.     

Structural and magnetic properties of the solid solution () YMn1−x(Cu3/4Mo1/4)xO3

Recently, the ferroelectromagnet YMnO₃ has been the focus of interest because it exhibits both antiferromagnetism (Néel temperature 80 K) and ferroelectricity (Curie temperature 914 K). There have been no reports of complete YMn_1−xM_xO₃ solid solutions in which substitution of the foreign M cation preserves the hexagonal P6₃cm structure. In contrast there exist several homeotypic phases with the general formula, Ln_1+nCu_nMO_3+3n (n=1 (M=Ti), 2 (M=V) and 3 (M=Mo); Ln: lanthanide). Several YMn_1−x(Cu_3/4Mo_1/4)_xO₃ compounds have been synthesized. The solid solution, from YMnO₃ (x=0) to YCu_3/4Mo_1/4O₃ (x=1) has been characterized by X-ray diffraction and transmission electron microscopy study. For 0<x<0.9, the compounds are found to crystallize in the non-centrosymmetric structure, space group P6₃cm, of YMnO₃. The Mn-free end member, x=1, crystallizes in a complex multiple cell, the superstructure being associated to Cu³⁺/Mo⁶⁺ cationic ordering. Dilution of the Mn³⁺ magnetic array by the paramagnetic (Cu²⁺) and diamagnetic (Mo⁶⁺) cations is found to decrease the antiferromagnetic ordering temperature and it becomes undetectable for x0.5 compositions.     

Heterometallic clusters arising from cubic Ni3M′O4 (M′=K and Na) entity: Solvothermal synthesis with/without the assistance of microwave

Solvothermal reaction assisted with microwave leads to the formation of two unique heterometallic cubic clusters [Ni₃M′(L)₃(OH)(CH₃CN)₃]₂·CH₃CN (M′=K for 1 and M′=Na for 2, where L is an anion of 2-[(2-hydroxy-3-methoxy-benzylidene)-amino]-ethanesulfonate) with higher efficiency, yields and purity than those without it. The 6-metallacrown-3 [Ni₃(OH)(L)₃]⁻ groups exhibit interesting ion trapping and self-assembly of size-different Na⁺ and K⁺ through form recognition and coordination activity in 1 and 2. The magnetic studies for 1 and 2 suggest that the {Ni₃M′O₄} (M′=K and Na) cores both display dominant ferromagnetic interactions from the nature of the binding modes of μ₃-O (oxidophenyl) and μ₃-OH.     

Reinvestigation and Structural Approach of the Ba–Pt–O System for <Y=Ba/Pt<

A reinvestigation of the Ba-rich region of the Ba–Pt–O system has been undertaken. The existence of a solid solution, already mentioned, has been confirmed, and its true formulation has been determined: Ba₁₂[Ba_xPt_3−x]Pt₆O₂₇with 0≤x≤3. The structure of these phases can be schematically considered as being built up from the periodic stacking along thecdirection of one close-packing [Ba₃O₉] layer, similar to those of the hexagonal perovskite, and three [Ba₃A′O₆] layers, withA′=Ba, Pt. The stacking of these mixed layers is of hexagonal type, and then columns parallel tocare formed. They consist of octahedra and trigonal prisms, sharing faces, which are occupied byA′ cations. For 0≤x≤1, the structure appears to be incommensurate and should be described as a composite crystal based on two subsystems: [Ba] and [(Ba, Pt)O₃]. Forx>1, the X-ray diffraction pattern can be explained using a three-dimensional cell, but the structure refinement attempts as well as the electron diffraction patterns seem to indicate these phases are commensurate modulated structures.     

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.     

Synthesis and characterization of n=5, 6 members of the La4Srn−4TinO3n+2 series with layered structure based upon perovskite

Layered compounds have been synthesized and structurally characterized for the n=5 and 6 members of the perovskite-related family La₄Sr_n−4Ti_nO_3n+2 by combining X-ray diffraction and transmission electron microscopy. Their structure can be regarded as comprising [(La,Sr)₅Ti₅O₁₇] and [(La,Sr)₆Ti₆O₂₀] perovskite blocks joined by crystallographic shears along the a-axis, with consecutive blocks shifted by 1/2 [100]_p. The n=5 member is similar to the previously reported n=5 member of other A_nB_nO_3n+2-related series. The n=6 member, which has only been briefly reported in other systems previously, is also a well-behaved member of this A_nB_nO_3n+2 series.     

The magnetic structure of clinopyroxene-type LiFeGe2O6 and revised data on multiferroic LiFeSi2O6

The clinopyroxene compounds LiFeSi₂O₆ and LiFeGe₂O₆ have been investigated by constant wavelength neutron diffraction at low temperatures and by bulk magnetic measurements. Both compounds are monoclinic, space group P2₁/c and do not exhibit a change in nuclear symmetry down to 1.4 and 5 K respective. However, they transform to a magnetically ordered state below 20 K. LiFeSi₂O₆ shows a simple magnetic structure with no indication of an incommensurate modulation. The magnetic space group is P2₁/c′ and the structure is described by a ferromagnetic coupling of spins within the infinite M1 chains of edge-sharing octahedra, while the coupling between these M1 chains is antiferromagnetic. The magnetic phase transition is accompanied by magnetostriction of the lattice when passing through the magnetic phase transition. The magnetic structure of LiFeGe₂O₆ is different to the silicate: the space group is and the magnetic unit cell doubled along the a-direction. Within the M1 chains spins are coupled antiferromagnetically, while the chain to chain coupling is antiferromagnetic when coupling goes via the GeB tetrahedron and ferromagnetic when it goes via the GeA tetrahedron.     

Synthesis and characterization of sodium titanates Na2Ti3O7 and Na2Ti6O13

Na₂Ti₃O₇ and Na₂Ti₆O₁₃ were synthesized by sol-gel method in order to obtain pure phases. Different heat-treatments were applied on powders and pellets of these materials. The effects were studied by XRD, dilatometry, TGA-DTA, SEM and electrochemical impedance spectroscopy. Pure Na₂Ti₃O₇ was obtained at 973 K. Sintering at 1373 K caused a partial decomposition into Na₂Ti₆O₁₃. The Na₂Ti₃O₇ powder sintered at 1273 K showed polygonal microstructure. Na₂Ti₃O₇ pellets sintered at 1323 K for 10 h exhibited large structures. This latter microstructure decreased the electrical conductivity of Na₂Ti₃O₇. Pure Na₂Ti₆O₁₃ was obtained at 873 K. Sintering at 1073 K caused a partial decomposition into TiO₂ (rutile). Na₂Ti₆O₁₃ pellets sintered at 1323 K for 10 h exhibited common shrinkage behavior. This shrinkage process increased the electrical conductivity of this material. The presence of TiO₂ resulted in a oxygen partial pressure dependence of the electrical conductivity.     

Synthesis and structure analysis of tetragonal Li7La3Zr2O12 with the garnet-related type structure

We have successfully synthesized a high-purity polycrystalline sample of tetragonal Li₇La₃Zr₂O₁₂. Single crystals have been also grown by a flux method. The single-crystal X-ray diffraction analysis verifies that tetragonal Li₇La₃Zr₂O₁₂ has the garnet-related type structure with a space group of I4₁/acd (no. 142). The lattice constants are a=13.134(4) Å and c=12.663(8) Å. The garnet-type framework structure is composed of two types of dodecahedral LaO₈ and octahedral ZrO₆. Li atoms occupy three crystallographic sites in the interstices of this framework structure, where Li(1), Li(2), and Li(3) atoms are located at the tetrahedral 8a site and the distorted octahedral 16f and 32g sites, respectively. The structure is also investigated by the Rietveld method with X-ray and neutron powder diffraction data. These diffraction patterns are identified as the tetragonal Li₇La₃Zr₂O₁₂ structure determined from the single-crystal data. The present tetragonal Li₇La₃Zr₂O₁₂ sample exhibits a bulk Li-ion conductivity of σ_b=1.63×10⁻⁶ S cm⁻¹ and grain-boundary Li-ion conductivity of σ_gb=5.59×10⁻⁷ S cm⁻¹ at 300 K. The activation energy is estimated to be E_a=0.54 eV in the temperature range of 300–560 K.     

Preparation and Structure of Cerium Titanates Ce2TiO5, Ce2TiO7, and Ce4Ti9O24

Compounds Ce₂TiO₅, Ce₂Ti₂O₇, and Ce₄Ti₉O₂₄ were prepared by heating appropriate mixtures of solids containing Ce⁴⁺ and Ti³⁺ or Ti which were placed in a platinum-silica-ampoule combination at T = 1250°C (3d) under vacuum. The new compounds were characterized by powder patterns. We obtained Ce₂TiO₅ which is isotypic to La₂TiO₅ and crystallizes in the Y₂TiO₅-type (space group Pnma) with a = 10.877(6) Å, b = 3.893(1) Å, c = 11.389(8) Å, Z = 4. Ce₂Ti₂O₇ is isotypic to La₂Ti₂O₇ and crystallizes in the monoclinic Ca₂Nb₂O₇ type (space group P 2₁) with a = 7.776(6) Å, b = 5.515(4) Å, c = 12.999(6) Å, β = 98.36(5), Z = 4. The compound Ce₄Ti₉O₂₄ crystallizes orthorhombic with a = 14.082(4) Å, b = 35.419(8) Å, c = 14.516(4) Å, Z = 16. The new cerium titanate Ce₄Ti₉O₂₄ is isotypic to Nd₄Ti₉O₂₄ (space group Fddd (No. 70)) which represents a novel type of structure.     

Rotational analysis of the v7,v11 (a′,a″) pair of bands of the infrared spectrum of the deuterium substituted propynal molecule C2H-CDO

The mid-infrared spectrum of the v₇,v₁₁ (a′,a″) pair of bands of the deuterium substituted propynal molecule C₂H-CDO was recorded at a resolution of about 0.08 cm⁻¹. An analysis of the pair of bands was completed using the method of simulation of the observed bands with synthetic spectra taking into account the effects of second order Coriolis interactions between the energy levels of the two bands. Best fit values for the changes in the rotational constants (A″ − A′), (B″ − B′) and (C″ − C′), the second order Coriolis constant ζ_7,11 and the δ_7,11 = v₁₁ − v₇ constant have been derived.     

A comparative study of the Aurivillius phase ferroelectrics CaBi4Ti4O15 and BaBi4Ti4O15

The room temperature structures of the four-layer Aurivillius phase ferroelectrics CaBi₄Ti₄O₁₅ and BaBi₄Ti₄O₁₅ are determined by means of single crystal X-ray diffraction. Regarding the CaBi₄Ti₄O₁₅ phase, in agreement with the tolerance factor, a significant deformation of the perovskite blocks is observed. The rotation system of the octahedra is typical from even layer Aurivillius phases and leads to the use of the space group A2₁am. For the BaBi₄Ti₄O₁₅ phase, only a weak variation with respect to the F2mm space group can be suggested from single crystal X-ray diffraction. A significant presence of Ba atoms in the [M₂O₂] slabs is confirmed in agreement with the previous works but specific Ba²⁺ and Bi³⁺ sites have to be considered due to the large difference in bounding requirement of these cations. Possible origins for the ferroelectric relaxor behavior of the Ba-based compound are discussed in view of the presented structural analyses.