Etude cristallographique des termes n = 4,5, 5 et 6 des séries (La,Ca)nTinO3n+2, (Nd,Ca)nTinO3n+2 et Can(Ti,Nb)nO3n+2

The homologous phases corresponding to the formula A_nB_nO_3n+2 observed in the systems: La₂Ti₂O₇CaTiO₃, Nd₂Ti₂O₇CaTiO₃ and Ca₂Nb₂O₇CaTiO₃, have a structure derived from the structure of perovskite by periodic crystallographic shears, which delimitate n octahedra thick sheets. In the [0 1 0]1 direction, the sheets are following each other with two types of arrangement.Crystals of terms n = 4,5, 5 and 6 were investigated by X-ray diffraction. According to the type of arrangement of the sheets, the phases studied have either an orthorhombic symmetry or a monoclinic symmetry, this last one giving rise to merohedral twins.     

Synthesis and characterization of the novel Nb3O5F5 niobium oxyfluoride: the term n=3 of the NbnO2n−1Fn+2 series

The solid-state synthesis of the oxyfluoride Nb₃O₅F₅, its crystal structure determined from X-ray powder diffraction data as well as some physical characterizations, are reported. Nb₃O₅F₅ constitutes the term n=3 of the Nb_nO_2n−1F_n+2 series related to the Dion-Jacobson phases. It crystallizes, at room temperature, in the tetragonal system (space group I4/mmm (no. 139); Z=4; a=3.9135(1) Å, c=24.2111(2) Å, and V=370.80(3) Å³). The crystal structure appears to be an in-between of the three-dimensional network of NbO₂F and the two-dimensional packing of NbOF₃ (term n=1 of the Nb_nO_2n−1F_n+2 series). This layered structure consists of slabs made of three Nb(O,F)₆ corner-linked octahedra in thickness (n=3) shifted one from another by a ()/translation. Oxygen and fluorine atoms are randomly distributed over all the ligand sites.     

Structural and magnetic characterization of BiFexMn2-xO5 oxides (x=0.5, 1.0)

The title compounds have been synthesized by a citrate technique followed by thermal treatments in air (BiFe_0.5Mn_1.5O₅) or under high oxygen pressure conditions (BiFeMnO₅), and characterized by X-ray diffraction (XRD), neutron powder diffraction (NPD) and magnetization measurements. The crystal structures have been refined from NPD data in the space group Pbam at 295 K. These phases are isostructural with RMn₂O₅ oxides (R=rare earths) and contain infinite chains of Mn⁴⁺O₆ octahedra sharing edges, linked together by (Fe,Mn)³⁺O₅ pyramids and BiO₈ units. These units are strongly distorted with respect to those observed in other RFeMnO₅ compounds, due to the presence of the electronic lone pair on Bi³⁺. It is noteworthy the certain level of antisite disorder exhibited in both samples, where the octahedral positions are partially occupied by Fe cations, and vice versa. BiFe_xMn_2−xO₅ (x=0.5, 1.0) are short-range magnetically ordered below 20 K for x=0.5 and at 40 K for x=1.0. The main magnetic interactions seem to be antiferromagnetic (AFM); however, the presence of a small hysteresis in the magnetization cycles indicates the presence of some weak ferromagnetic (FM) interactions.     

Structural and magnetic properties of Ba3La3Mn2W3O18

A quaternary phase, Ba₃La₃Mn₂W₃O₁₈, was synthesized in reduced atmosphere (5% H₂/Ar) at 1200 °C and characterized by using powder X-ray diffraction, electron diffraction and high resolution TEM. Ba₃La₃Mn₂W₃O₁₈ crystallizes in rhombohedral space group with the cell parameters, and , and can be attributed to the n=6 member in the B-site deficient perovskite family, A_nB_n−1O_3n. The structure can be described as close-packed [La/BaO₃] arrays in the sequence of (hcccch)₃, wherein the B-site cations, W and Mn, occupy five octahedral layers in every six octahedral layers, which leave a vacant octahedral layers separating the 5-layer perovskite blocks. The B-cation layers in the perovskite block alternate along the c-axis in a sequence of W⁶⁺-Mn²⁺-W⁵⁺-Mn²⁺-W⁶⁺. The bond valence calculation and optical reflection spectrum confirm the presence of W⁵⁺. This compound behaves paramagnetically in wide temperature range and weak antiferromagnetic interaction only occurs at low temperatures.     

HRTEM study of cation-deficient perovskite-related AnBn−δO3n (n?4δ) microphases in the Ba5Nb4O15-BaTiO3 system

The occurrence of coherent intergrowths of cation-deficient perovskites in the Ba₅Nb₄O₁₅-BaTiO₃ system has been examined by high-resolution transmission electron microscopy and selected area electron diffraction. Because of their structural similarity, the simple members Ba₅Nb₄O₁₅ (n=5) and Ba₆TiNb₄O₁₈ (n=6) form coherent intergrowths—noted 5^P6¹—by the juxtaposition along the c-axis of P perovskite-like blocks n=5 and one perovskite-like block n=6, with P=1, 2 and 3. More generally, the ability to form intergrowths in the hexagonal perovskite systems is discussed considering the structural characteristics of the simple members. Examples taken from various systems show that the formation of such intergrowths is highly dependent on the size of the A cation present in simple members.     

Transmission electron microscopy study of Ruddlesden-Popper Can+1MnnO3n+1n=2 and 3 compounds

Two Ruddlesden-Popper compounds Ca_n+1Mn_nO_3n+1 with n=2 and 3 synthesized by a citrate gel technique have been studied by TEM. The structure of Ca₄Mn₃O₁₀ is consistent with the previously determined structure having the space group Pbca and a⁻ a⁻ c⁺/a⁻ a⁻ c⁺ tilt system. The presence of defects suggests the possible high-temperature phase transition from untilted I4/mmm to Pbca. The structure of Ca₃Mn₂O₇ was found to be different from the previously suggested I4/mmm symmetry. Ca₃Mn₂O₇ forms with an orthorhombic structure with either Cmcm or Cmc2₁ space group. A structural model for Cmc2₁ based on the tilting of almost-rigid octahedra with a⁺ c⁻ c⁻/a⁺ c⁻ c⁻ tilt system is proposed. The lamellar defects were shown to be twin variants of the Cmc2₁ structure with the (001)_t interfaces, which suggests the possible tilting phase transition from the ideal I4/mmm to Cmc2₁ following the maximal group-subgroup symmetry tree: I4/mmm→Fmmm→Bbmm(Cmcm)→Bb2₁m(Cmc2₁).     

Double (n=2) and triple (n=3) [M4Bi2n−2O2n] polycationic ribbons in the new Bi∼3Cd∼3.72M∼1.28O5(PO4)3 oxyphosphate (M=Co, Cu, Zn)

The crystal structure of the new Bi_∼3Cd_∼3.72Co_∼1.28O₅(PO₄)₃ has been refined from single crystal XRD data, R₁=5.37%, space group Abmm, a=11.5322(28) Å, b=5.4760(13) Å, c=23.2446(56) Å, Z=4. Compared to Bi_∼1.2M_∼1.2O_1.5(PO₄) and Bi_∼6.2Cu_∼6.2O₈(PO₄)₅, this compound is an additional example of disordered Bi³⁺/M²⁺ oxyphosphate and is well described from the arrangement of double [Bi₄Cd₄O₆]⁸⁺ (=D) and triple [Bi₂Cd_3.44Co_0.56O₄]⁶⁺ (=T) polycationic ribbons formed of edge-sharing O(Bi,M)₄ tetrahedra surrounded by PO₄ groups. According to the nomenclature defined in this work, the sequence is TT/DtDt, where t stands for the tunnels created by PO₄ between two subsequent double ribbons and occupied by Co²⁺. The HREM study allows a clear visualization of the announced sequence by comparison with the refined crystal structure. The Bi³⁺/M²⁺ statistic disorder at the edges of T and D entities is responsible for the PO₄ multi-configuration disorder around a central P atom. Infrared spectroscopy and neutron diffraction of similar compounds (without the highly absorbing Cadmium) even suggests the long range ordering loss for phosphates. Therefore, electron diffraction shows the existence of a modulation vector q^*=1/2a^*+(1/3+ε)b^* which pictures cationic ordering in the (001) plane, at the crystallite scale. This ordering is largely lost at the single crystal scale. The existence of mixed Bi³⁺/M²⁺ positions also enables a partial filling of the tunnels by Co²⁺ and yields a composition range checked by solid state reaction. The title compound can be prepared as a single phase and also the M=Zn²⁺ term can be obtained in a biphasic mixture. For M=Cu²⁺, a monoclinic distortion has been evidenced from XRD and HREM patterns but surprisingly, the orthorhombic ideal form can also be obtained in similar conditions.     

Metal-insulator transition in VnO2n−1

Research on phase relationships and structure studies by electron diffraction confirm V_nO_2n?1 (n = 3–9) phases between V₂O₃ and VO₂. Metal-insulator phase transitions have been found in all phases but V₃O₅ and V₇O₁₃. Electrical, magnetic and thermodynamic properties associated with the transitions are reported for sintered samples or for single crystals prepared by a vapor-transport method. The results are collated and reviewed in summarized form.     

High n-value phases in the complex bismuth oxides with layered structure,Bi2CaNan−2NbnO3n+3

Complex bismuth oxides with layered structure are prepared with a series of compositions in the system Bi₂CaNb₂O₉-NaNbO₃. It is found by X-ray powder diffraction that each compound is composed of more than two phases, which are described by a formula Bi₂CaNa_n?2Nb_nO_3n+3, e.g., in the sample with the nominal composition Bi₂CaNb₂O₉ · 8NaNbO₃, the phases with n = 6 to 8 appear predominantly. These phases are closely intergrown to each other. Moreover, high-resolution electron microscopy reveals that microsyntactic intergrowth frequently occurs in the phases with n > 5. The occurrence of the latter intergrowth is explained in terms of the bond length obtained.     

Phases and phase transitions of compounds (CnH2n+1NH3)2MnCl4 with n = 1, 2, 3

Phases and structural phase transitions of the compounds (CH₃NH₃)₂MnCl₄, (C₂H₅NH₃)₂MnCl₄ and (C₃H₇NH₃)₂MnCl₄ have been studied by means of thermoanalytical methods (DSC) and X-ray single crystal and powder diffraction data in the temperature range of 85–480°K at normal pressure. All phases show perovskite-like layer structures. The high temperature phases (α phase) correspond to the K₂NiF₄ type and may be regarded as the aristotype of each polymorphic compound. All transitions are reversible. Transition patterns are:

Based on the DSC peak-shape analysis and diffraction data a model of a tilting system of the MnCl₆-octahedra layer is introduced in order to understand essential features of structures of different phases and their transition behavior. Single crystal film data of (C₃H₇NH₃)₂MnCl₄ phases reveal some disorder phenomena. The ε phase exhibits a superstructure along [010] with a triplication of the shortest axis corresponding to the δ phase. The γ phase of this compound shows strong satellite reflections, due to a transverse distortion wave along the [100] lattice direction.     

Mise en évidence de CaF2e4O6 et détermination des structures cristallines des ferrites de calcium CaFe2+nO4+n(n = 1, 2, 3): nouvel exemple d'intercroissance

The synthesis of a new calcium ferrite CaFe₄O₆ has been carried out at 1125°C under a controlled atmosphere of H₂H₂O. The existence of this compound modifies a part of the diagram FeCaO. The crystal structures of the ferrites CaFe_2+nO_4+n (n = 1, 2, 3) have been resolved on a series of single crystals; these ferrites crystallize in the orthorhombic system, space group Cmcm, with the average parameters a = 3.04 Å, b = 10 Å, c = 10 + 2.65 nÅ. The three structures derive from each other through an intergrowth process, in the direction of the c axis, with CaFe₂O₄ blocks between the FeO blocks. The coordination of the iron atoms is slightly changed by the nature of the neighboring blocks during stacking.     

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.     

Investigation on double perovskite Ba4Ca2Ta2O11

The structure, conductivity and water uptake of the oxygen-deficient perovskite-type compound Ba₄Ca₂Ta₂O₁₁ have been investigated. Ba₄Ca₂Ta₂O₁₁ crystallizes in the cryolite structure (cubic, Fm3m SG) with a = 8.4508(2) Å, under dry air. The compound can be partially hydrated up to a maximum water content of approximately 0.52 mol H₂O per mol Ba₄Ca₂Ta₂O₁₁. In moist air, the structure symmetry becomes monoclinic (C2/m) and the temperature dependence of total conductivity shows a different behavior because of changes in transport mechanism. Three regions can be observed as a function of temperature. For the low temperature range 200–400 °C, the protonic conduction is prevailing with an activation energy E_A = 0.85 eV. In the intermediate temperature range (400–600 °C), O²⁻ anionic and protonic conductions are mixed with an activation energy E_A = 0.45 eV and in the third region, for temperatures above 600 °C, O²⁻conduction is prevailing with an activation energy E_A = 0.85 eV.     

Crystal and Magnetic Structures of Ba4Mn3O10

A polycrystalline sample of Ba₄Mn₃O₁₀ has been prepared and characterized by X-ray diffraction (290 K), neutron diffraction (290, 80, 5 K) and magnetometry (5≤T(K)≤1000). At 290 K the compound is paramagnetic and isostructural with Ba₄Ti₂PtO₁₀. Mn₃O₁₂ trimers, built up from MnO₆ octahedra, are linked through common vertices to form corrugated sheets perpendicular to the y-axis of the orthorhombic unit cell (Space group Cmca, a=5.6850(1), b=13.1284(1), c=12.7327(1) Å); Ba atoms occupy the space between the layers. On cooling, the magnetic susceptibility shows a broad maximum at ∼130 K, and a sharp transition at 40 K. Neutron diffraction has shown that long-range antiferromagnetic order is present at 5 K but not at 80 K, although magnetometry at 5 K has revealed a remanent magnetization (0.002 μ_B per Mn) which is below the detection limit of the neutron experiment.     

Study of Pr1−xMn1+xO3 perovskites

The structural and magnetic properties of the Pr_1?xMn_1+xO₃ perovskites were studied. The increase of x (i.e., $\text{Pr}\text{Mn}\text{< 1}$) leads to the decrease of the orthorhombic deformation and of the Néel temperature and, simultaneously, to an increase of the ferromagnetic contribution. The latter effect is explained from the suggested distribution of the cations (Pr³⁺_1?xMn²⁺_x)_A(Mn³⁺_1?xMn⁴⁺_x)O^2?₃ by the double exchange of Mn³⁺Mn⁴⁺ pairs at the B—sublattice.     

Structural order and disorder in Co-based layered cuprates CoSr2(Y,Ce)sCu2O5+2s (s=1-3)

Crystal structures of a homologous series of Co-based layered cuprates, CoSr₂(Y,Ce)_sCu₂O_5+2s (s=1-3), have been investigated by high-resolution electron microscopy (HREM) and electron diffraction (ED) techniques. For all the three phases ED patterns showed double periodicity along a direction parallel to the CoO layers, indicating a regular alternation of two types of CoO₄-tetrahedra chains within the layers. Also seen was ordering of the chains along the layer-stacking direction for the s=1 phase (Co-1212); ED patterns simulated based on the proposed superstructure model well reproduced the observed patterns. For the s=2 (Co-1222) and s=3 (Co-1232) phases in which an additional fluorite-type layer-block is inserted between two CuO₂ planes, HREM and ED analysis revealed complete disorder of the CoO₄ chains along the layer-stacking direction. This implies that the interlayer ordering is mainly controlled by the distance between the neighboring CoO layers.     

Mixed valence tungsten oxides with a tunnel structure,P4W4nO12n+8: A nonintegral member P4W10O38 (n = 2.5)

The mixed-valence oxide P₄W₁₀O₃₈, which can be considered as the nonintegral member n = 2.5 of the series P₄W_4nO_12n+8, crystallizes in the monoclinic system with unit-cell dimensions a = 6.5656(25), b = 5.2850(15), c = 20.573(15) Å, β = 96.18(4)°, and space group P2₁. The crystal structure was solved by conventional Patterson and Fourier techniques using 2339 counter-measured reflections that obeyed the condition I > 3σ(I) and refined to an R factor of 0.074 (R_w = 0.077). Basically, the framework of the structure built up from ReO₃-type slabs connected through PO₄ tetrahedra looks like that of P₄W₈O₃₂ previously described. Unlike P₄W₈O₃₂, two successive ReO₃-type slabs have a different width corresponding to two and three WO₆ octahedra so that the structure can be considered as an intergrowth of the integral members n = 2 and n = 3 of the series P₄W_4nO_12n+8.     

Structural Characterization of YMexMn1−xO3 (Me=Cu, Ni, Co) Perovskites

The structural properties of the YMe_xMn_1−xO₃ (Me=Cu, Ni, Co) pseudobinary oxides have been studied by X-ray diffraction and electrical measurements. The powders were prepared by solid state reaction between the corresponding oxides. The incorporation in solid solution of small divalent cations, Cu²⁺, Ni²⁺, and Co²⁺, substituting for Mn in the hexagonal YMnO₃ compound, leads to a phase transition in which a perovskite-type structure is formed. The amount of substituting cation necessary for such a transition depends on the cation nature and, to a small extent, on the ionic radius. The phase transition depends strongly on the progressive substitution of the Jahn-Teller Mn³⁺ cation and therefore of the cooperative Jahn-Teller interaction weakness. The steric influence plays a secondary role, as is shown by the very small variation of the tolerance factor, t, as a function of the cation content. The solid solutions with perovskite-type structure show semiconducting behavior. The conductivity mechanism is of a thermally activated small polaron hopping.     

Layered perovskite-related ruthenium oxychlorides: crystal structure of two new compounds Ba5Ru2Cl2O9 and Ba6Ru3Cl2O12

Single crystals of the title compounds were prepared using a BaCl₂ flux and investigated by X-ray diffraction methods using MoKα radiation and a charge coupled device (CCD) detector. The crystal structures of these two new compounds were solved and refined in the hexagonal symmetry with space group P6₃/mmc, a=5.851(1) Å, c=25.009(5) Å, ρ_cal=4.94 g cm⁻³, Z=2 to a final R₁=0.069 for 20 parameters with 312 reflections for Ba₅Ru₂Cl₂O₉ and space group , a=5.815(1) Å, c=14.915(3) Å, ρ_cal=5.28 g cm⁻³, Z=1 to a final R₁=0.039 for 24 parameters with 300 reflections for Ba₆Ru₃Cl₂O₁₂. The structure of Ba₅Ru₂Cl₂O₉ is formed by the periodic stacking along [001] of three hexagonal close-packed BaO₃ layers separated by a double layer of composition Ba₂Cl₂. The BaO₃ stacking creates binuclear face-sharing octahedra units Ru₂O₉ containing Ru(V). The structure of Ba₆Ru₃Cl₂O₁₂ is built up by the periodic stacking along [001] of four hexagonal close-packed BaO₃ layers separated by a double layer of composition Ba₂Cl₂. The ruthenium ions with a mean oxidation degree +4.67 occupy the octahedral interstices formed by the four layers hexagonal perovskite slab and then constitute isolated trinuclear Ru₃O₁₂ units. These two new oxychlorides belong to the family of compounds formulated as [Ba₂Cl₂][Ba_n+1Ru_nO_3n+3], where n represents the thickness of the octahedral string in hexagonal perovskite slabs.