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International audience; We report on the crystal structure and physical properties of Tl0.6Mo3S5, which belongs to a novel family of materials with large tunnels, reminiscent of those observed in the romanechite structure type. Tl cations are partially filling these tunnels delimited by the Mo-S cluster framework in which the Mo atoms form infinite zigzag chains. Single-crystal X-ray diffraction data indicate that this compound crystallizes in the monoclinic, non-centrosymmetric space group P21 (no. 4; a = 9.344(2) angstrom, b = 3.234(2) A, c = 11.669(2) A and beta = 113.09(2)° at 293 K). While electron diffraction performed on single crystals further evidences a commensurate modulation running solely along the b axis with a modulation wave vector further experiments carried out on a polycrystalline sample suggest a compositional dependence of q on the Tl content. Low-temperature transport properties measurements (5-300 K) reveal that Tl0.6Mo3S5 behaves as a narrow-band-gap n-type semiconductor. The strongly non-linear temperature dependence of the thermopower further suggests the presence of several electronic bands contributing to the electrical transport. Consistent with the extended electronic distribution in the tunnels that requires a description using two adjacent Tl sites, specific heat data measured down to 0.35 K evidence the presence of a Schottky-type anomaly that may be associated with the tunneling of the Tl cations between several equivalent, off-centered equilibrium sites. This inherent disorder contributes to suppression of the low-temperature Umklapp peak in the lattice thermal conductivity kL, the temperature dependence of which mimics that observed in strongly-disordered compounds. In spite of this characteristic, high kL values of up to 3.8 W m-1K-1 are reached at 300 K, due to the covalent Mo-S network that contributes to maintaining high sound velocities.

A LuFe 2 O 4+ δ sample, previously characterized by X-ray synchrotron and neutron diffraction, has been studied by electron microscopy techniques, in order to get a precise description of its micro- and nanostructures at room temperature. The X-ray synchrotron data vs . temperature show that the monoclinic distortion is associated with the charge ordering; this distortion results in elongated twinning domains, which enhance the complexity of the microstructural state at room temperature. The structural modulation associated with oxygen excess is observed in large domains inside a non modulated matrix, in contrast with the modulations associated with the charge ordering of the Fe 2+ and Fe 3+ species, which are mostly short-range. The investigation of the nature and density of defects in the sample shows that they are nano-scaled, preserving the regularity of the layer stacking mode, and limited to the formation of one- or two-units large stacking faults, associated with gliding mechanisms. Based on these observations, an original description of the LuFe 2 O 4 ferrite structure, through puckered [LuO 4 ] ∞ sandwiching [Fe 2 ] ∞ layers, is proposed.

A new form of cristobalite, collected in the form of spheroidal particles as the result of hot corrosion of clear fused silica ampoules, has been studied by X-ray diffraction, scanning and transmission electron microscopy. The transparent or translucent spherulites have a round but flattened shape, one vortex and a core made of nanosheets, perpendicular to the surface, organized in a nearly radial pattern. The electron diffraction study evidenced a single phase material with a pseudo-cubic supercell ( a sp ≈ b sp ≈ c sp ≈14.1 A and a F-type centred lattice) corresponding to an eight-fold multiplicity of the β cristobalite cell. The TEM study evidenced twinning domains and intergrown polytypes, similar to those usually observed in the β → α cristobalite-type transition and cristobalite→tridymite modifications. A formation mechanism of these cristobalite spherulites is proposed on the basis of a salt attack by a doping effect and the formation of droplets from a liquid state of silica.

A large solid solution Sr2(Fe3–xCox–yGay)O6.5−δ, isostructural with the modulated Fe-2201 and Co-2201-type structures, has been characterized using X-ray powder diffraction (XRPD), transmission ele...

The 4%Cu for Mn substitution in CuMnO2 decreases slightly the lattice parameters, reduces the Jahn−Teller distortion of the MnO6 octahedra, but does not change the temperature dependence of the structure, showing a C2/m to P1 structural transition (in the vicinity of the magnetic transition temperature). In contrast, the antiferromagnetic structure is strongly modified by the substitution, as a propagation vector k = (0 1/2 0) is evidenced for Cu1.04Mn0.96O2 compared to k = (0 1/2 1/2) for CuMnO2. Consequently, the interplane magnetic coupling (along the c axis) changes from antiferromagnetic in CuMnO2 to ferromagnetic in Cu1.04Mn0.96O2 without change in the antiferromagnetic arrangement of the ferromagnetic chains in the (a,b) plane. The nanostructural study points toward the existence of numerous defects at the nanoscale which justify the modeling of strains used in the refinement of the crystalline structure.

Bismuth calcium aluminate, Bi2Ca6Al12O27, has been prepared as a ceramic and a single crystal. Analysis of reciprocal space using both electron and X-ray diffraction show an R-centred hexagonal unit cell: a = b = 17.3892 (4), c = 6.986 (1) Å. Additional weak reflections are observed; they require the introduction of a modulation wavevector q = 0.0453 (2)c* for indexing. The modulated structure has been solved using the superspace formalism [superspace group X\overline{3}(00\gamma)0]. A framework of corner-sharing AlO4 tetrahedra forms corrugated sixfold rings and uncommon triple rings. The Ca2+ cations exhibit an eightfold coordination sphere; edge-sharing CaO8 polyhedra form intertwinned zigzagging rows along c creating a three-dimensional net. Bi atoms are located in large hexagonal tunnels parallel to c and form Bi2O3 pairs, which adopt a trigonal bipyramidal configuration. The 6s 2 lone-electron pairs (Lp) point along c, in the opposite direction to the three Bi—O strong bonds to form two BiO3Lp tetrahedra with a common base. Different orientations of the Bi2O3Lp2 pairs, rotated by 60° around c, are observed. Their stacking modes in each of the hexagonal tunnels are described. The sequence of the stacking varies along c in each of the tunnels.

A new Tl-doped strontium ferrite Fe 2 (Sr 2-∈ Tl ∈ )Sr 3 Fe 4 O 14.65, with an original structure, has been synthesized and structurally characterized by powder X-ray diffraction and transmission electron microscopy. The TGA and Mossbauer studies evidence a mixed valence of iron. The structure exhibits a commensurate modulation, with a F-type subcell a ≈ b ≈ 5.4 A (≈ a p √2), c ≈ 42 A with a modulation vector q = αa* with a = 0.4. The supercell parameters have been refined as a= 27.1101(8) A, b= 5.5187(2) A and c= 42.0513(9) A, in the space group Fmmm. The electron diffraction and electron microscopy data of this novel ferrite show that it can be described as a Fe Tl -2234-type structure corresponding to the intergrowth of a quadruple perovskite slice [(SrFeO 2.8 ) 4 ], with a complex rock salt related slice [Fe2(Sr 2-∈ Tl ∈ )O 3.4 ]∞, built up of one double iron layer [Fe 2 O 2.4 ] sandwiched between two [SrO] layers. The HRTEM images show that the oxygen atoms and vacancies are randomly distributed in the perovskite layers while the HAADF STEM images evidence the absence of Tl segregation in the matrix. Fe2(Sr 2-∈ Tl ∈ )Sr 3 Fe 4 O 14.65 exhibits a very large value of x (llemu/mol) at 5 K, which remains large at 400 K; the M(H) loop presents a shape characteristic of ferrimagnetism, with a large coercive field of 0.3 T. The value of magnetization saturates at 400 K at 0.68 μ B /Fe. At 10 K, the value of magnetization reaches a maximum of 2 μ B /Fe. The resistivity presents a semiconducting-like behavior, with p ∼ 800 Ω·cm at 300 K.

The crystal structure of the copper vanadate Cu 3 VO 4 , determined by X-ray powder diffraction, shows tetrahedral coordination of Cu(I) which is particularly unusual in oxides. Cu 3 VO 4 presents a tetragonal unit-cell, isotypic to the famatinite Cu 3 SbS 4 , with the following parameters: a = 4.57531(9) A; c = 8.9918(3) A; space group I -42 m . Its structure can be described as a three dimensional framework of corner shared CuO 4 and VO 4 tetrahedra, distributed in an ordered way and forming pure copper-oxide layers [Cu 2 O 4 ] ∞ alternating with mixed copper/vanadium oxide layers [CuVO 4 ] ∞ . In the latter one observes a 1:1 ordering of the CuO 4 and VO 4 tetrahedra forming [CuVO 3 ] ∞ chains running along the [110] direction.

The electrochemical study of the empty tunnel structure Ti2Nb2O9 shows that it can intercalate lithium reversibly, through a solid−solution process, leading to the phase Li4Ti2Nb2O9. The high-resol...

The perovskite manganites CaMn1-xWxO3 are studied up to the solubility limit by using transmission electron microscopy, X-ray powder diffraction, and magnetic and transport measurements. The phase diagram shows three compositional regions at low temperatures. As x increases, (i) regular Pnma perovskite structure and G-type antiferromagnetism (AFM) combined with a weak ferromagnetic (FM) component, (ii) strongly distorted monoclinic P21/m structure and C-type AFM associated with orbital ordering, and (iii) strongly distorted Pnma structure, associated with charge and orbital ordering and AFM arrangement, are observed. Our measurements demonstrate that each W6+ ion creates two extra electron carriers in the Mn4+ matrix and appears as a defect in the Mn sublattice. This B-site disorder, together with the lower itinerancy of the doped carriers, make favorable conditions for orbital and/or charge ordering at unusually high critical temperatures (up to 380 K).

Attaining novel structures in compounds displaying remarkable properties is the goal of numerous solid-state chemists and physicists. Several approaches for predicting new structures exist but, whatever they are, one of the essential steps is to understand their architecture. It needs to collect the maximum information for deciphering the building code. In this paper, we give a personal account and focus on the powerful synergy of transmission electron microscopy and X-ray diffraction techniques for achieving it. We have chosen the example of the misfit-layered cobaltites, which appear more and more complex as we attain each new level in the hierarchy of their structures. We have limited our illustration to the first period of the misfit cobaltites “research story”, from the discovery of highly complex oxides to the understanding of the architecture building, without reporting on the numerous further papers devoted to iso-structural compounds and to their remarkable properties of thermoelectric power.

The (3 + 2)D modulated structure of the oxygen deficient compound LaSrCuO3.52 was determined using a combination of electron diffraction and neutron diffraction. The cell parameters are a = b = 3.7587(3) A, c = 13.0074(9) A, α = β = γ = 90°, superspace group I4/mmm (αα0, α−α0)00 mg, α = 0.2222(3), Rp = 4.5%, R0 = 2.1%, R1 = 4.9%. The oxygen vacancies are located in the CuO2−δ plane and are ordered with the oxygen atoms, creating three different oxygen environments for the Cu atoms: CuO4 squares, CuO5 pyramids and CuO6 octahedra.

Branded for its potential electronic ferroelectricity, charge-ordered $\mathrm{LuF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ has a layered Fe triangular framework, whose topology is a source of degeneracy, both at the charge and spin levels. Here we present an in-depth characterization of $\mathrm{LuF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}\text{\ensuremath{-}}hp$, the high-pressure $(hp)$ polymorph of $\mathrm{LuF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$, using electron, x-ray, and neutron diffraction, combined with transport and magnetization measurements. We show that $\mathrm{LuF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}\text{\ensuremath{-}}hp$ is characterized by a misfit-related monoclinic structure, accommodating a buckled triangular ${[\mathrm{Lu}]}_{\ensuremath{\infty}}$ layer and two shifted adjacent rectangular ${[\mathrm{Fe}]}_{\ensuremath{\infty}}$ planes belonging to a distorted rock salt--type layer. The release of the geometric frustration of the Fe magnetic lattice in the $hp$ form leads to collinear antiferromagnetic ordering at ${T}_{N}=380\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Possible coexistence of charge and magnetic orders in this material opens research pathways for the design of tunable multifunctional devices using high-pressure techniques.

International audience; This work is a compendium of the structural properties of layered LnFe2O4±x ferrites, focussed on recently discovered polymorphs and their transitions through five complex commensurate and incommensurate modulated structures. These phases are obtained varying the oxygen content (reversibly from O4.0 to O4.5) or applying high pressure (up to 15 GPa). The relationships between the different structures are discussed and models are proposed. These ferrites, which contain magnetic cations and exhibit remarkable 3D flexibility, open the road for magnetic, electric, catalytic or energy materials. © 2015 Elsevier Masson SAS. All rights reserved.

The structure of the Fe2(Bi0.69Sr2.31)Fe2O9.5−1/2δ has been carried out by single crystal X-ray diffraction. Its structure can be considered as commensurately modulated with a basic structure of Fmmm symmetry and with a modulation vector q ⇒ = α a ⇒ * + γ c ⇒ * whose α and γ values are, respectively, equal to 1/3 and 1. The ferrite is described as a three-fold superstructure with a ≈ 3 a p 2 , b ≈ a p 2 and c ≈ 26 A (space group Bbmb). It is described using a disorder model in agreement with diffuse scattering observations. The structure is built up from the intergrowth sequence between [(Bi, Sr)Fe2O3.5±1/2δ]∞ complex layers, related to the rock salt (RS)-type blocks, and double perovskite layers. Large analogies are found between the present structure and the so-called 2212 modulated structures related to the high Tc superconductor copper oxides or to the corresponding Fe or Co oxides; they are discussed here. A statistical disorder characterizes the [Fe2O2.5±1/2δ]∞ double layer. Taking advantage of similar structural arrangements in related oxide compounds, a structural model is proposed. A description of this double layer is then given in terms of sequence of polyhedra (tetragonal pyramids, mono-caped tetrahedra and trigonal bi-pyramids), in comparison with other related modulated structures with different periodicities. The powder sample exhibits a statistical distribution of α around an average value of 0.35; the oxygen non-stoichiometry appears as the driving force for the α value, i.e. for the modulation, commensurate or incommensurate. The Mossbauer study has been performed and interpreted on the basis of the refined structure.

The Pb for Sr substitution in Sr4Fe6O13±δ evidenced a limited existence range of solid solution Sr4−xPbxFe6O13−δ, with x ⩽ 0.7 . For higher lead content, an original phase has been discovered for the composition Pb4Sr13Fe24O53 with a terrace-like structure. The cell is monoclinic and the parameters refined from the powder X-ray diffraction data are a T = 35.039 ( 2 ) A , b T = 5.5897 ( 2 ) A , c T = 25.662 ( 2 ) A and β = 98 ° 102 ( 2 ) , space group Cc or C2/c. The reconstruction of the reciprocal space allowed describing the structure from a monoclinic sub-cell with different sets of modulation vectors. The high resolution electron microscopy showed that the new phase has a terrace-like structure corresponding to the formation of two types of ribbons parallel to the [302]sub direction. One ribbon is made of 2212-type bricks, 3 / 2 a → sub large, and the second is made of 2201-type bricks, 2 a → sub large. The new structure is therefore described as the intergrowth of 2201-type slices, m octahedra large, and 2212-type slices, n octahedra large. It is therefore the m = 4 and n = 3 member of a large new structural family [Fe2(Sr2)FeO6.5±1/2δ]2201m[Fe2(Sr3−xA2+x)Fe2O9.5±1/2δ]2212n. The magnetic measurements evidence an anti-ferromagnetic-like magnetic transition at T ≈ 730 K showing that the samples, as the Sr4−xPbxFe6O13−δ, ones are magnetic at room temperature.

The synthesis and crystal structure of a mixed valent manganite CaMn3O6, equivalent to Ca2/3Mn2O4, are reported, along with the magnetic properties. The structure was determined using electron diffraction and high-resolution transmission electron microscopy and refined from X-ray and neutron powder diffraction data (a = 10.6940(3) A, b = 11.3258(3) A, c = 8.4881(2) A, = 122.358(2), space group P21/a, RI = 0.037, RP = 0.118). The structure is based on a framework of double chains of edge-sharing MnO6 octahedra. The corner-sharing chains form a framework with six-sided tunnels, identical to that of the CaFe2O4 structure. The Ca2+ cations are located in the tunnels. Compared to CaFe2O4, one-third of the Ca positions in the tunnels remain vacant, with an ordered distribution of vacant and occupied sites. The empty sites in neighboring Ca chains are shifted relative to each other along the c-axis by one period of the CaFe2O4 subcell, which results in a symmetry decrease from orthorhombic to monoclinic. Based on the interatomic Mn-O distances, the charge ordering in CaMn3O6 is discussed. The compound exhibits a strong anti-ferromagnetic character, and differences between the zfc and fc magnetization curves at low temperature suggest ferro- or ferrimagnetic interactions.

A new layered titanoniobate, LiTiNbO5, an n = 2 member of the A(x)M(2n)O(4n+2) family, has been synthesized using a molten salt reaction between HTiNbO5 and an eutectic "LiOH/LiNO3". This compound crystallizes in the P2(1)/m space group with a = 6.41 A, b = 3.77 A, c = 8.08 A, and beta = 92 degrees . It exhibits |TiNbO5|(infinity) layers similar to HTiNbO5, but differs from the latter by a "parallel configuration" of its |TiNbO6|(infinity) ribbons between the two successive layers. The topotactic character of the reaction suggests that exfoliation plays a prominent role in the synthesis of this new form. This new phase intercalates reversibly 0.8 lithium through a first-order transformation leading to a capacity of 94 mAh/g at a potential of 1.67 V vs Li/Li+.

(Sr1−x Ca x )33Bi24+δ Al48O141+3δ/2 fullerenoid solid solutions have been synthesized and the effect of partial substitution of Sr by Ca has been characterized by (scanning) transmission electron microscopy, applying different imaging methods. Most of the defects commonly observed in face centered cubic compounds, have also been observed in (Sr1−x Ca x )33Bi24+δ Al48O141+3δ/2. Based on purely geometrical and topological models, structural presentations for the coherent twin boundaries and stacking faults have been constructed on the basis of complex spherical “Al84O210” units. The results are compared to defects observed in the crystallite fullerite C60.

Financial support by the MEC (MAT2003-07483-C02-02) and Generalitat de Catalunya (2005SGR-0059, PICS2005-14) is thanked. C.F. acknowledges financial support from MEC (Spain). We thank ILL, the CRG-D1B and ESRF for the provision of beam time. The LNCMP in Toulouse is also acknowledged.

The ceramics of misfit cobaltites form a large family of layer oxides which exhibit large values of Seebeck coefficient (S 300K ≥ 90μV.K -1 ). They can be divided in two categories depending on the number of the rocksalt-type (RS) layers, n = 3 or n = 4, separating the [CoO 2 ] conducting layers. From the comparison of their structural features, it is found that the S values are governed by the incommensurability ratio and/or the charge of the RS block. This is clearly evidenced by making chemical substitutions at the level of the RS block: isovalent but with cations of different ionic radius (r Ba 2+ > r Sr 2+ > r Ca 2+ ) modifying the incommensurability ratio or aliovalent substitutions for cobalt (Pb 4+ , Ti 4+ ) which keep unchanged the structural incommensurability but make the RS charge changing.

The superstructure of Bi 1-x Ca x MnO 3 (x = 0.4 or 0.45), which exhibits a charge/orbital-ordered state at room temperature (Tco ≈ 330 K), has been successfully determined at different temperatures using single-crystal X-ray diffraction data and the superspace formalism. This peculiar approach provides a very convenient way to analyze all possible 3D symmetry (P2 1 /m, Pm, and Pnm2 1 ) of the superstructure. The structural model of the ordered state has been confirmed thanks to complementary data from several other techniques (electron diffraction, high-resolution electron microscopy (HREM), and synchrotron high-resolution X-ray powder diffraction, all vs T). It is compatible with an orthorhombic cell (a = 11.002(2) A, b = 7.588(1) A, c = 5.425(4) A at 150 K) and the space group Pnm2 1 . The structural model is based on the alternation of one double band of Mn(1)O 6 octahedra and one double band of Mn(2)O 6 octahedra, the two octahedron types exhibiting almost similar distortion. HREM images were then simulated from the refined atomic positions; the good agreement with the experimental images confirms the model. Finally, the relationships between structural characterizations and magnetic properties have been investigated for these compounds.

The investigation of the system Cs−Ga−P−O by hydrothermal synthesis has allowed a new hydrated hydroxygallophosphate with an intersecting tunnel structure, Cs2Ga6(OH)2(PO4)6·1.55H2O, to be synthesi...

The incommensurate modulated structure of Sr4Fe6O13±δ refined in super space formalism, from single crystal X-ray diffraction data. The analysis of this structure shows that the geometry of the iron polyhedra, their respective positions and arrangement are fundamentally different from those previously described for this oxide. Three types of iron environment, namely trigonal bipyramids, tetragonal pyramids and monocaped tetrahedra are observed in the double iron oxygen layers. The peculiar role of a structural unit built up from double trigonal bipyramids is outlined and the notion of disorder is detailed. The structural mechanism of this modulation character of the double iron layer is analyzed, compared to other related phases built of one single perovskite layer inter-grown with a complex slice made of one modulated double layer sandwiched between two [AO]∞ layers.

Recently, the ferroelectromagnet YMnO3 has been the focus of interest because it exhibits both antiferromagnetism (Neel temperature ∼ 80 K ) and ferroelectricity (Curie temperature ∼ 914 K ). There have been no reports of complete YMn 1 − x M x O 3 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 + n Cu n M O 3 + 3 n ( n = 1 ( M = Ti ), 2 ( M = V ) and 3 ( M = Mo ); Ln: lanthanide). Several YMn1−x(Cu3/4Mo1/4)xO3 compounds have been synthesized. The solid solution, from YMnO3 ( x = 0 ) to YCu3/4Mo1/4O3 ( 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 x ⩾ 0.5 compositions.

Iron oxides, oxyhydroxydes and oxycarbonates derived from the layered Ruddlesden-Popper (RP) structure form a large family of layered compounds. Besides the classical RP oxides Srn+1FenO3n+1, single intergrowths with the generic formulation (A,Sr)n+2FenO3n+2 and (A,Sr)n+3FenO3n+3 (A = Tl, Pb, Bi…) can be generated by increasing the multiplicity of the rock salt layers, and multiple intergrowths of these single intergrowths can be synthesized. Starting from oxygen deficient RP oxides such as n = 3 member Sr3NdFe3O9−δ, oxyhydroxydes hydrates and oxyhydroxydes such as Sr3NdFe3O7.5(OH)2·H2O and Sr3NdFe3O7.5(OH)2 can be created topotactically. Carbonate groups can also replace FeO6 octahedra in the n = 3 member Sr4Fe3O10, leading to layered oxycarbonates Sr4Fe3−x(CO3)xO10−4x with 0 < × ≤ 1. Shearing mechanism applied transversally to the layers allows collapsed structures to be generated such as the [Bi2Sr3Fe2O9]n [Bi4Sr6Fe2O16] family and the ferrite Bi13Ba2Sr25Fe13O66. Finally the replacement of rock salt SrO layers in the intergrowth Sr2FeO4 allows a new series of modulated structures [Sr8Fe12O26]·[Sr3Fe2O6]n to be generated, built up of layers of FeO5 bipyramids and tetragonal pyramids intergrown with perovskite layers.

Combining low-temperature electron (ED) and synchrotron powder diffraction (SPD) techniques, we have precisely determined the phase-separated crystal structure of Pr{sub 0.5}Ca{sub 0.5}Mn{sub 0.97}Ga{sub 0.03}O{sub 3}. We demonstrate that the phase separation is associated with the onset of charge/orbital ordering at {approx}230K and that two ordered phases coexist at low temperature. In order to account for the significant anisotropic linewidth broadening observed in the SPD patterns, we had to include a specific strain model in the refinements. One of the phases, the most severely strained, is suggested to result from an imperfect charge ordering.

A new n = 2-member of the Ruddlesden−Popper (RP) cobaltite series, Sr3-δCo1.9Nb0.1O6.65-δ, has been stabilized by niobium doping. This oxide reacts topotactically with atmospheric water through hydrolysis/hydration and reduction of cobalt, leading to an oxyhydroxide hydrate derivative, Sr3-δCo1.9Nb0.1O4.86-δ(OH)3.04·0.4H2O. The reaction with water at room temperature induces a magnetic transformation from a spin glass behavior with a freezing temperature Tg ≈ 50 K for the pristine RP oxide to weak ferromagnetism with an apparent Curie temperature TC ≈ 200 K for the oxyhydroxide hydrate derivative. The spectacular effect of water upon the magnetic properties of this layered oxide is explained by the partial reduction of Co4+ into Co3+ species, concomitantly inducing a coordination change for cobalt cations that form antiferromagnetic coupling between high-spin Co3+. A generalization of this phenomenon to other layered cobaltites and even iron oxides is considered.

A new ferrite Bi4Sr14Fe24O56 (i.e., Bi4/3Sr14/3Fe8O18.66) has been synthesized. The high-resolution electron microscopy study of this oxide shows that its modulated structure consists of an origina...

Different samples of the ferrite Sr 4 Fe 6 O 13 , synthesized in air, nitrogen, and argon, have been studied by high-resolution electron microscopy. It is shown that this oxide is oxygen-deficient, i.e., Sr 4 Fe 6 O 13-δ , and that its structure is modulated. The electron diffraction study shows that the modulation vector is pa* + qc*, with p = (1 - δ)/2 varying in the range [0.4-0.5] and q close to 1. It is shown that the component of the modulation vector along c induces complex modulated structures. It is also demonstrated that oxygen nonstoichiometry corresponds to the generation of microphases [Sr 8 Fe 12 O 26 ] [Sr 2 Fe 3 O 6 ] n , whose first integral member n = 1, which corresponds to p = 0.4, is isolated here for the first time by synthesis under argon. Structural models involving intergrowths of single perovskite [Sr 2 Fe 2 O 6 ]∞ layers with complex [Sr 2 Fe 4 O 7-δ ]∞ layers involving two kinds of pyramidal ribbons, FeO 5 trigonal bi-pyramids and FeO 5 distorted tetragonal pyramids, explain the oxygen deficiency.

Magnetic and crystal structures of the manganite Pr 0.8 Ca 0.2 MnO 3 have been studied by neutron powder and single-crystal X-ray diffraction. Structure refinements using single crystal data [orthorhombic system, Pnma , (No. 62), a RT =5.5534(3) A, b RT =7.6548(8) A, c RT =5.4400(5) A, D x =6.422 g cm −3 , R RT =0.029, R w RT =0.038] are consistent with a single domain sample. Structure and atomic displacement parameters exclude any electronic localization, even in a disordered way at 300 and 100 K. Low temperature electron diffraction observations do not show any trace of charge ordering. A Pr contribution to the magnetic structure has been shown with a maximum moment of 0.79 μ B and spins alignments roughly along [101] orientations, at a lower temperature than the ferromagnetic transition observed at 130 K, due to Mn spins ordering.

The oxycarbonate Sr 4 Fe 2 O 6 CO 3 has been revisited, using a new synthesis route. Its microstructure has been established by electron microscopy and evidences an orthorhombic-monoclinic distortion at 220 K. The latter is associated with a spin reorientation (7°) visible in magnetic susceptibility, heat capacity, and neutron diffraction studies. To understand the structural transition, particular attention has been paid for the carbonate groups: they adopt a specific orientation within a layer but different in successive layers, which involve disorder along c and order in the (a,b) plane and are at the origin of the unusual broadening of the Mossbauer lines.

The first member of the Ruddlesden–Popper family, Ca2MnO4, has been revisited. Coexistence of two structures has been shown from electron microscopy at room temperature and neutron diffraction data have evidenced two antiferromagnetic structures at low temperature. Two forms, with an orthorhombic Aba2 ( a≈b≈a p 2 and c≈12 A) and a tetragonal I41/cad ( a≈a p 2 and c≈24 A) symmetries, were found to coexist coherently within the same matrix.

We present a comparative investigation under field of three Bi1−xSrxMnO3 manganites in the low-doping regime (x⩽0.50). Powdered samples were investigated in pulsed fields up to 35, 50 or 130 T depending on the compound (x=0.25, 0.33, 0.50). Remarkable differences are found in the M(H) curves when comparing the response to the magnetic field of the two different charge-order phases.

Tetrahedral oxides, like silicates and aluminates, have attracted great interest due to their potential for numerous applications in various fields ranging from catalysis, ion exchange and molecular sieves, to thermo- and photoluminescence. In spite of their tetrahedral character, no effort has been made to date for establishing structural relationships between these tetrahedral oxides with different forms of carbon, for example, fullerenes. Here, we report for the first time an oxide that exhibits a three-dimensional framework of AlO4 tetrahedra forming huge 'Al84' spheres, similar to those of the D2d isomer of the C84 fullerenes. These Al84 spheres, displayed in a face-centred-cubic lattice, are easily identified by high-resolution electron microscopy. We also show that this Sr33Bi24+delta Al48O141+3 delta/2 aluminate exhibits an onion-skin-like subnanostructure of its Bi/Sr/O species located inside the Al84 spheres. The role of the original pseudo-spheric anion [Bi16O52-n empty square box n]-with n vacancies (empty square box)-in the stabilization of such a structure is discussed. This structure seems to be promising for the generation of a large family of fullerene-type (fullerenoid) oxides with various properties.

We have studied by neutron diffraction, resistivity and magnetization measurements the variation of the structural, magnetic and electric properties as a function of the cation size mismatch at the perovskite A-site. The four selected compounds RE0.5AE0.5MnO3, with extremely large A-site cation size 〈rA〉=1.25–1.28 A, correspond to two 〈rA〉 values and for each 〈rA〉, two mismatch values have been retained. This study shows that the decrease of σ2 is an essential factor in stabilization of the ferromagnetism. As a result, the Curie temperature is markedly increased for the samples with smaller σ2, even if they exhibit a complex ferromagnetic/antiferromagnetic phase-separated state at low temperature.

The perovskite manganites Pr 1-x Sr x MnO 3 in the region 0.45 ≤ x ≤ 0.75 exhibit two crystallographic modifications at room temperature-orthorhombic Ibmm and tetragonal I4/ mcm-with boundary situated at x ∼ 0.48. The high-temperature X-ray diffraction data show that for x ≥ 0.6 the tetragonal phase of I4/mcm symmetry transforms on heating directly to the cubic Pm3m symmetry. On the other hand, for x < 0.6 the I4/mcm phase transforms successively to orthorhombic Ibmm through a two-phase region, then to rhombohedral R3c, and finally to cubic Pm3m. The transient rhombohedral R3c phase is present also for orthorhombic compounds Pr 1-x Sr x MnO 3 with x < 0.48. High-temperature structural transitions are not associated with any observable anomalies in electrical transport and paramagnetic properties while the occurrence of three magnetically ordered phases (FM, A-type and C-type AFM) at low temperature influence resistivity and thermopower essentially.

The magnetic properties of a polycrystalline Sr3NiIrO6 sample have been investigated by means of susceptibility and magnetization measurements. On the one hand, it is found that this compound, made of magnetic chains on a triangular lattice, behaves very similarly to Ca3CoRhO6 and Ca3CoIrO6. In particular, at low temperature ( T < T 2 ( = 21 K)), a frozen state, characterized by a very slow spin dynamic, is evidenced. On the other hand, for T 2 < T < T 1 ( = 70 K), this phase exhibits magnetization values, smaller than the expected ones, with a 1/3 plateau reminiscent of the ferrimagnetic state of Ising spins on a triangular lattice. Nonetheless, the absence of saturation in 35 T and the low magnetization values are consistent with an antiferromagnetic intrachain coupling between Ir4+ (S = 1/2) and Ni2+ (S = 1). Sr3NiIrO6 can be viewed as made of antiferromagnetically coupled ferrimagnetic chains on a triangular lattice. On the basis of these results, a magnetic (H, T) phase diagram is proposed.

The synthesis and characterization of a polycrystalline sample of the n = 2 Ruddlesden−Popper phase Nd2BaLiRuO7 are reported. X-ray and neutron diffraction show that the 6-coordinate sites are occupied by a 1:1 ordered arrangement of Li and Ru, that the 12-coordinate sites within the perovskite-like blocks are occupied by Ba, and that the cation sites on the block edges are occupied by Nd. Transmission electron microscopy shows that this compound is less susceptible to stacking faults then many other n = 2 phases, and 7Li MAS NMR spectroscopy shows that the environment of the single Li site in the structure is well-defined. The compound is an antiferromagnet below 32 K, with ordered magnetic moments of 2.2(2) μB and 2.53(8) μB on the Ru and Nd cations respectively at 2 K.

The investigation of the manganites La 2/3− x Pr x Sr 1/3 MnO 3 , La 2/3 Sr 1/3− x Ca x MnO 3 and La 2/3+ x Ca 1/3−2 x Ag x MnO 3 , which all exhibit Mn 3+ :Mn 4+ =2, shows that it is possible to reach high magnetoresistance at room temperature, up to 21% under 1.2 T. These materials are compared to La 5/6 Ag 1/6 MnO 3 which corresponds to the same Mn 3+ :Mn 4+ ratio and exhibits a magnetoresistance of 25% in this field. An interesting feature deals with the value of the insulator–metal transition temperature T IM , often higher than T C , especially for Ag-based compounds. It is suggested that the latter results either from a better oxygenation of the surface of the grains or from a migration of silver toward the surface.

Two new compounds of the Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O systems have been prepared. Their structure is built up from the intergrowth of four rock-salt-type layers and one [CoO2] hexagonal layer. Both cobaltites exhibit large thermopower values (S-300 K similar to 140 muV K-1), low resistivity values (rho(300) (K) 40-60 mOmega cm) and small thermal conductivities (kappa(300) (K) similar to I W K-1 m(-1)). Furthermore, these compounds exhibit a negative magnetoresistance, (MR = rho(H)-rho(H=0)/rho(H=0)), reaching, at 2.5 K, -85% in 7 T for the Bi/Ca/Co/O misfit cobaltite. A large negative magnetothermopower is also found for these cobaltites in the same temperature range. A qualitative explanation of the observed behaviour is proposed.

The structural and magnetic transitions in the Pr0.5Sr0.5-xBaxMnO3 series have been investigated by neutron diffraction and electron microscopy, in connection with the transport and magnetic properties, allowing a new phase diagram (T, x, or ) to be drawn. Two important points emerge from this study. First, a phase separation is observed for a large part of the diagram, described as a mixture of ferromagnetic, antiferromagnetic, and poorly ordered magnetic phases at low temperature for x > 0.2. Second, A-type antiferromagnetism (AFM) develops in nearly regular perovskite lattice. The case of Pr0.5Ba0.5MnO3 is also discussed because its microstructure is not conventional with local cationic ordering in the random A-site matrix.

The study of the structural and magnetic phase diagram of the manganites La1−xAgxMnO3 shows similarity with the La1−x′Srx′MnO3 series, involving a metallic ferromagnetic domain at relatively high temperature (≈300 K). The Ag-system differs from the Sr-one by a much smaller homogeneity range (x≤1/6) and the absence of charge ordering. But the most important feature of the Ag-manganites deals with the exceptionally high magnetoresistance (−25%) at room temperature under 1.2 T, that appears for the composition x=1/6. The latter is interpreted as the coincidence of the optimal double exchange condition (Mn3+:Mn4+=2) with Tmax=300 K (maximum of the ρ(T) curve in zero field).

The possibility to synthesize layered oxycarbonates, with nominal composition Sr 4 Fe 2− x Mn x O 6 CO 3 involving trivalent manganese, with 0≤ x ≤1.5, is reported for the first time. The structural study of Sr 4 FeMnO 6 CO 3 using NPD, HREM, Mossbauer 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 CO 3 groups, with two different “flag” and “coat hanger” configurations. The magnetic order is antiferromagnetic and fundamentally different from the magnetic behavior of Sr 4 Fe 2 O 6 CO 3 .

A new type of charge ordering stable up to T 1 500 K has been evidenced by electron microscopy in the Mn 3+ -rich perovskite Bi 2/3 Sr 1/3 MnO 3 . This new superstructure, based on a Imma-type subcell is characterized by a double modulation with a = 2a p √2, b = 4a p , c = a p √2. X-ray powder diffraction evidences a second structural transition at T 2 580 K and suggests a displacive modulation of the oxygen atoms below T 2 . These results are interpreted as the correlated coexistence of charge-orbital ordering and modulated displacive ordering of oxygen atoms, in the form of double Mn stripes, below 500 K.

A new manganese oxide Ba7Ca2Mn5O20, with an original structure derived from the 21 L hexagonal perovskite, has been synthesized and studied by XRPD and HREM. This rhombohedral phase (R 3 m), with a=5.8207(2) A and c=51.359(1) A, consists of [BaO3] (h and c) layers and [BaO2] (c′) layers with a (hhccc′cc)3 stacking sequence. This structure is closely related to that of Ba8Ca1.8Mn6.2O23−δ, containing identical blocks “Ba5Ca2Mn3O14” built up of layers of corner-sharing CaO6 octahedra, MnO6 octahedra and MnO4 tetrahedra. Both oxides can be described as the members n=2 and 3 of the new structural family Ba5+nCa2Mn3+nO3n+14 , which differs from the hexagonal perovskite family by its bidimensional character, due to the disconnection of the MnO4 tetrahedra layers. The charge distribution of manganese, Mn3+/Mn4+ in octahedra and Mn5+ in tetrahedra is a particular feature of this structure.

Metal-transition oxides are potential materials for thermoelectric devices operating at high temperatures ( T ≫300 K). In particular, compounds exhibiting small resistivity and large Seebeck coefficient are required. Two kinds of oxides have been retained: the ‘misfit’ cobaltites which are p-doped metallic phases and the electron doped perovskite manganites. For the former, it is found that the Ca for Sr substitution enhances the thermopower values in the Pb/Sr/Co/O misfit cobaltite whereas it has only a moderate effect in the case of the Tl/Sr/Co/O phase. From this study, it appears that the oxidation state of Co in the conducting layers plays a crucial role on the physical properties. Moreover, for the manganites, we show that electron-doped SrMnO 3 can be prepared by a two-steps method. The doping, created via substitution at either the A-site (Pr 3+ for Sr 2+ ) or B-site (Mo 6+ for Mn 4+ ), leads to metallic compounds with large thermopower values.