Characteristics of the structure and crystallochemical criteria for the existence of compounds of the type AnBnO3n+2

The calculated (models of the ideal layered perovskite-like structures (LPS)) and experimental values of the unit cell parameters of compounds of the A_nB_nO_3n+2 type were analyzed. The interconnection between the peculiarities of the distortion of the LPS in these compounds, the number of layers of BO₆ octahedra in the perovskite-like blocks, and the limits of the morphotropic transitions were established. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 2, pp. 92–95, March–April, 2006.     

Mechanisms for the formation of layered A<Subscript>4</Subscript>B<Subscript>3</Subscript>O<Subscript>12</Subscript> compounds from coprecipitated hydroxocarbonate and hydroxide systems

We have established and analyzed the sequences of phase transitions in synthesis of layered compounds in the A_nB_n–1O_3n family ( \textA₃^\textII\textLnB₃^\textV\textO₁₂ {\text{A}}_3^{\text{II}}{\text{LnB}}_3^{\text{V}}{{\text{O}}_{{12}}} (A^II = Ba, Sr, Ln = La, Nd, B^V = Nb, Ta) and La₄Ti₃O₁₂ with n = 4) from coprecipitated hydroxocarbonate and hydroxide systems, including steps involving the formation, solid-phase reaction, or structural rearrangement of intermediates.     

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.     

Phase formation features and magnetic properties of complex oxides in the systems Sr-Co-M-O (M = Zn, Cu)

It was shown that, in the systems Sr-Co-M-O (M=Zn, Cu), the first stage of the crystallization of solid solutions Sr₃Co_{2 ? x} M _x O₆ with a tetragonal structure of the Ruddlesden-Popper type is the formation of quasi-one-dimensional oxides of the family A_3m+3n A _n ′B_3m+n O_9m+6n . The magnetic properties of new compounds, Sr₃Co_1.35Cu_0.65O₆ and Sr₁₄Co_8.4Cu_2.6O₃₃, were measured, and a relationship between the structure and the existence of magnetic ordering at low temperatures was established.     

Study of the incommensurability of (La4Ca2)Ti6O20 and (Nd4Ca2)Ti6O20, members n = 6 of the AnBnO3n+2 family: Interpretation of the lattice modulation using a four-dimensional space

(La₄Ca₂)Ti₆O₂₀ and (Nd₄Ca₂)Ti₆O₂₀, members n = 6 of the A_nB_nO_3n+2 family can be prepared in an incommensurate form, the thermal stability of which was studied by X-ray diffraction. The lattice modulation observed is a displacive type. We interpreted that by means of a four-dimensional space.     

Catalytic hydrogenation of carbon monoxide over nanostructured perovskite-like gadolinium and strontium ferrites

The catalytic properties of perovskite-like ferrites (A_{n + 1}B_nO_{3n + 1}, where n = 1, 2, 3, …, ∞; A = Gd, Sr; and B = Fe) synthesized via ceramic and sol-gel technology in the hydrogenation of carbon monoxide are studied. The interrelation between catalytic activity, selectivity to olefins and synthetic methods for complex oxide preparation, the number of perovskite layers, crystallite size, composition, and the valence state of iron is established.     

Synthesis and crystal structure of the A6B5O18 perovskite-like compounds

New members of the A_nB_n−1O_3n perovskite-like family (Ba₅KNb₅O₁₈ and Sr₆Nb₄SnO₁₈ compounds) with n = 6 have been synthesized and studied by the X-ray powder diffraction. Their crystal structures were found to belong to the Ba₆Nb₄TiO₁₈-type with a = 0.57840(7) nm, c = 4.2532(5) nm and a = 0.5661(1) nm, c = 4.186(1) nm for Ba₅KNb₅O₁₈ and Sr₆Nb₄SnO₁₈, respectively. It was shown that Ba and K (A-atoms) are completely disordered in the crystal structure of Ba₅KNb₅O₁₈ compound. But Nb and Sn atoms (B-atoms) in the crystal structure of the Sr₆Nb₄SnO₁₈ compound are quite ordered with the preferred Sn⁺⁴ and Nb⁵⁺ cations localization in the center of perovskite-like block and on the boundaries of these blocks, respectively. Temperature and frequency dependencies of the real components of electric conductivity σ^I and dielectric permeability ɛ^I; specific electric conductivity at the direct current σ_dc have been obtained by the impedance spectroscopy method for Sr₆Nb₄SnO₁₈.     

A single crystal X-ray and HRTEM study of new series of compounds DyCux (x=4.5, 4 and 3.5)

A series of monoclinic compounds DyCu_x (x=4.5,4 and 3.5) is described. It is constructed from structural blocks AB₅ (cubic AuBe₅ structure type) and AB₂ (cubic MgCu₂ structure type) by stacking nAB₅+AB₂ and giving the compositions A₂B₇,AB₄,A₄B₁₇,A₅B₂₂,A₆B₂₇,…,AB₅. The resulting monoclinic superstructures can be derived from the cubic AuBe₅ type by introducing planar defects parallel to {hhh} that lead to a nearly orthogonal ≈(n+2)×(n+2)×(n+2−0.5) supercells. The present series is analogous to the monoclinic-hexagonal-trigonal-orthorhombic series A_n+1B_5n+2 obtained by the stacking (n-1)AB₅+A₂B₇ where AB₅ is of the hexagonal CaCu₅ structure type and A₂B₇ is of the monoclinic Zr₂Ni₇ structure type.     

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.     

Unusual valences and fast electron exchange in MoFe2O4

The distribution of d electrons over the cations in MoFe₂O₄, which is represented by the formal valence assignment, is shown to be complicated by the equilibrium reactionsFe²⁺_B+Fe³⁺_A+Mo³⁺Fe³⁺_B+Fe²⁺_A+Mo⁴⁺We have used thermal treatment to confirm that the Mo are primarily on octahedral sites; Fe_A[Mo_BFe_B]O₄. K-shell absorption and Mössbauer data at T = 423 K > T_c demonstrate that the iron has an average valence near 2.5+ with fast electron transfer (τ_h < 10⁻⁸ sec) on both octahedral and tetrahedral sites. Paramagnetic susceptibility data give a Curie constant C_M = 7.95 ± 0.2 emu/mole and a Weiss constant θ_p = −445 K; magnetometer measurements confirm a compensation point near 160 K. Transport data give a surprisingly high electronic conductivity, but also give an activated mobility similar to that found in AlFe₂O₄ and CrFe₂O₄ where mixed Fe^3+/2+ valences on both A and B sites have been demonstrated. However, a positive Seebeck coefficient and a preexponential factor one order of magnitude higher in MoFe₂O₄ point to involvement of a fraction of the Mo atoms in electronic transport, which would be consistent with the observation of a τ_h < 10⁻⁸ sec on the A sites of a spinel. An energy diagram consistent with these data and other information about the relative redox potentials of these ions in oxides are proposed for this system.     

Formation of H3O+ from alcohols and ethers induced by intense laser fields

The processes of H₃O⁺ production from alcohols (ethanol, 2‐propanol, 1‐propanol, 2‐butanol) and ethers (diethyl ether and ethyl methyl ether), and their deuterium‐substituted species, by intense laser fields (800 nm, 100 fs, ～1 × 10¹⁴ W/cm) were investigated through time‐of‐flight (TOF) mass spectrometry. H₃O⁺ formation was observed for all these compounds except for ethyl methyl ether. From the analysis of TOF signals of H_(3?n)D_nO⁺ (n = 0, 1, 2, and 3) that have expanding tails with increasing flight time, it has been confirmed that the reaction proceeds through metastable dissociation from the intermediate species C₂H_(5?m)D_mO⁺(m = 0–5). The common shape of the H_(3?n)D_nO⁺ signal profiles contains two major distributions in the time constant, i.e., fast and slow components of <50 ns and ～500 ns, respectively. The H_(3?n)D_nO⁺ branching ratio is interpreted to be the result of complete scrambling of four hydrogen atoms at the C? C site in C₂H₄‐OH⁺, and partial exchange (18–38%) of a hydrogen atom in the OH group with four other hydrogen atoms within 1 ns prior to H_(3?n)D_nO⁺ production. Ab initio calculations for the isomers and transition states of C₂H₅O⁺ were also performed, and the observed H_(3?n)D_nO⁺ production mechanism has been discussed. In addition, a stable isomer having a complex structure and two isomerization pathways were discovered to contribute to the H₃O⁺ formation process. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis of Supramolecular A8Bn Miktoarm Star Copolymers by Host-Guest Complexation

We report a new synthetic method to construct supramolecular A₈B_n (n=1, 2, 4) miktoarm star copolymers by host-guest complexation between a resorcinarene-based coordination capsule possessing eight polystyrene chains and 4,4-diacetoxybiphenyl guest molecules that retain one, two or four polymethyl acrylate chains. The formation of the supramolecular A₈B_n (n=1, 2, 4) miktoarm star copolymers was confirmed by dynamic light scattering (DLS), size-exclusion chromatography (SEC), and diffusion-ordered NMR spectroscopy (DOSY). Differential scanning calorimetry (DSC) measurements revealed that the miktoarm copolymers were phase-separated in the bulk. The micro-Brownian motion of the A₈B₄ structure was markedly enhanced in the bulk due to a weak segregation interaction between the immiscible arms.     

A new layered perovskite,KSrNb2O6F,by powder neutron diffraction

The structure of a new layered oxyfluoride, viz. potassium strontium diniobium hexaoxide fluoride, KSrNb₂O₆F, was refined from powder neutron diffraction data in the orthorhombic space group Immm. The oxyfluoride compound is an n = 2 member of the Dion–Jacobson‐type family of general formula A[A′_n−1B_nX_3n+1], which consists of double layered perovskite slabs, [SrNb₂O₆F]⁻, between which K⁺ ions are located. Within the perovskite slabs, the NbO₅F octahedra are significantly distorted and tilted about the a axis. A bond‐valence‐sum calculation gives evidence for O/F ordering in KSrNb₂O₆F, with the F⁻ ions located in the central sites of the corner‐sharing NbO₅F octahedra along the b axis. All atoms lie on special positions, namely Nb on m, Sr on mmm, K on m2m, F on mm2, and O on sites of symmetry m and m2m.     

NaCa4Nb5O17: a layered perovskite AnBnO3n+2 compound

Sodium tetracalcium pentaniobium heptadecaoxide, NaCa₄­Nb₅O₁₇, corresponds to the n = 5 term of the homologous A_nB_nO_3n+2 family of structures composed of ABX₃ perovskite layers. The structure consists of perovskite‐type blocks of n = 5 layers of NbO₆ octahedra, separated by an interblock region. Successive blocks along the b axis are displaced by c with respect to each other. The deformation of the NbO₆ octahedra is a minimum at the middle of each block, and increases along the direction of the b axis to a maximum at each end of the block. Ca and Na share the same intrablock sites, coordinated by 12 O atoms, whereas only Ca atoms are found in the interblock cavities, at sites with different coordination geometries.     

Vertical proton affinities of CH2O and CH2OH+ in their ground singlet,excited triplet and ionized doublet states

Vertical proton affinities were calculated with closed and open shell direct SCF-MO methods for the ground, excited triplet and ionized doublet states of CH₂O and CH₂OH⁺.The computed gas phase basicity of CH₂O follows the order: CH₂O(¹ A ₁) > CH₂O*(³ A ₁ or ³ A ₂) > CH₂O⁺(² B ₂ or ² B ₁).     

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.     

Synthesis and structural studies of cation-substituted Aurivillius phases ASrBi2Nb2TiO12

We report here a novel method to prepare high-quality samples of the three layered oxides ASrBi₂Nb₂TiO₁₂ACa, Sr or Ba using the pre-formed intermediates ABi₂Nb₂O₉ and SrTiO₃. The room temperature structures were refined using synchrotron X-ray and Neutron powder diffraction data in the orthorhombic space group B2cb. This symmetry arises as a consequence of cooperative tilting of the BO₆ octahedra in the [ASrNb₂TiO₁₀]²⁻ perovskite-like slabs and a polar displacement of the cations. The structure is characterized by extensive cation disorder but lacks appreciable oxygen vacancies.     

Synthesis of perovskite-related layered AnBnO3n+2 = ABOX type niobates and titanates and study of their structural,electric and magnetic properties

ABO_X niobates and titanates belonging to the homologous series A_nB_nO_3n+2 are a special group of perovskite-related layered materials. These oxides comprise the highest-T_c ferroelectrics such as CaNbO_3.50 and LaTiO_3.50, as well as thermodynamically stable bulk compounds involving well-ordered stacking sequences of layers with different thickness such as SrNbO_3.45. An extensive overview on many ABO_X compositions of the A_nB_nO_3n+2 family and its properties is presented. The crystal structure type is given by n and can be tuned by adjusting the oxygen content X. The charge carrier concentration of the electrical conducting oxides can be varied by appropriate substitutions at the A or B site. To investigate the properties of these systems, more than 150 different compositions were prepared. Most of them were grown by floating zone melting, of which many were fabricated as single crystals with precise control of the oxygen content X. For these crystalline compounds, the synthesis, structural, electric and magnetic features are discussed. Attempts to prepare series members beyond the known structure types n=4, 4.33, 4.5, 5 and 6 were not successful. For some of the known structures types n, however, pronounced non-stoichiometric homogeneity ranges with respect to the oxygen content X and cation ratio A/B were found. Thus, these systems offer many possibilities to vary the compositional, structural, chemical and physical properties. Further, measurements of the resistivity as a function of temperature T are reported for crystals of the n=4 type Sr_0.8La_0.2NbO_3.50, n=4.5 type Sr_0.96Ba_0.04NbO_3.45 and n=5 types Sr_1−YLa_YNbO_3.41 (Y=0, 0.035, 0.1), Sr_0.95NbO_3.37, CaNbO_3.41 and LaTiO_3.41. These measurements, which were performed in the temperature range 4 K≤T≤290 K and along the a-, b- and c-axis, revealed a highly anisotropic conductivity and intricate behavior. In parts of the temperature range, these materials are quasi-1D metals which display temperature-driven metal-semiconductor transitions at lower temperatures. The niobates and titanates investigated represent a new group of quasi-1D metals which are in compositional, structural and electronical proximity to non-conducting layered (anti)ferroelectrics. Furthermore, measurements of the magnetic susceptibility as a function of temperature are reported for many compounds. As a typical property at elevated temperatures, it was observed that the magnetic susceptibility rises with increasing temperature.     

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.     

Intercalation behavior of n-alkylamines into an A-site defective layered perovskite H2W2O7

Intercalation behavior of n-alkylamines into a protonated form of an A-site defective layered perovskite H₂W₂O₇ has been investigated. Results from XRD indicate these materials are layered with the corresponding interlayer spacing governed by the n-alkylamine chain length, and a reversible intercalation and deintercalation property is observed among these intercalation compounds. The IR spectra of the intercalation compounds with n-alkylamines clearly show that n-alkyl chains possess an all-trans conformation, and H₂W₂O₇ accommodate n-alkylamines (C_nH_2n+1NH₂: n=3, 4, 7, 8, 12, 16) to form intercalation compounds via an acid-base mechanism. A linear relationship between the interlayer distance and the number of carbon atoms in n-alkyl chains is observed to show a bilayer arrangement of the n-alkyl chains with a tilt angle of ∼71.6°. Elemental analysis studies reveal that the amounts of intercalated n-alkylamines are about 2.0 mol per [W₂O₇]. Despite the surface geometry of H₂W₂O₇ is almost identical to those of layered perovskites H₂[A_n−1B_nO_3n+1], the amounts of intercalated n-alkylamines of them are different. A reasonable explanation is given through our research.