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The effects of the contents of iron and cobalt on the crystal structure, oxygen content, thermal expansion coefficient, and electrical–electrochemical properties of materials Eu2SrCoxFe2–xO7−δ (x =...

The potential of the perovskite system Nd1-xSrxCoO3-δ (x = 1/3 and 2/3) as cathode material for solid oxide fuel cells (SOFCs) has been investigated via detailed structural, electrical, and electrochemical characterization. The average structure of x = 1/3 is orthorhombic with a complex microstructure consisting of intergrown, adjacent, perpendicularly oriented domains. This orthorhombic symmetry remains throughout the temperature range 373-1073 K, as observed by neutron powder diffraction. A higher Sr content of x = 2/3 leads to stabilization of the cubic perovskite with a homogeneous microstructure and with a higher oxygen vacancy content and cobalt oxidation state than the orthorhombic phase at SOFC operation temperature. Both materials are p-type electronic conductors with high total conductivities of 690 and 1675 S·cm-1 at 473 K in air for x = 1/3 and 2/3, respectively. Under working conditions, both compounds exhibit similar electronic conductivities, since x = 2/3 loses more oxygen on heating than x = 1/3, associated with a greater loss of p-type charger carriers. However, composite cathodes prepared with Nd1/3Sr2/3CoO3-δ and Ce0.8Gd0.2O2-δ present lower ASR values (0.10 Ω·cm2 at 973 K in air) than composites prepared with Nd2/3Sr1/3CoO3-δ and Ce0.8Gd0.2O2-δ (0.34 Ω·cm2). The high activity for the oxygen electrochemical reaction at intermediate temperatures is likely attributable to a large disordered oxygen-vacancy concentration, resulting in a very promising SOFC cathode for real devices.

A new oxide of the Ruddlesden–Popper series in the (Eu,Sr)n+1(Co,Fe)nO3n+1 system has been isolated and structurally characterized. X-ray diffraction and electron microscopy show that polycrystalline Eu2SrCo1.5Fe0.5O7 constitutes the n = 2 member of a homologous series, the essential feature of which is the existence of two connected Co/Fe octahedral layers with a perovskite-like structure, linked by an EuO rock-salt like layer. Random distributions of 3d-metals in the perovskite blocks and interstitial and oxygen vacancies in the rock-salt layers (anti Frenkel defects) are observed by electron microscopy. Such facts are commonly associated with high electronic conduction and ionic conductivity. The electrochemical study shows that the area-specific resistance of this compound is 0.26 Ω cm2 at 700 °C in air, a value which is comparable to those of the best state-of-the-art Ruddlesden–Popper materials used as cathodes in intermediate temperature solid oxide fuel cells.

The potential of the perovskite system Nd

Silica has many industrial (i.e., glass formers) and scientific applications. The understanding and prediction of the interesting properties of such materials are dependent on the knowledge of detailed atomic structures. In this work, amorphous silica subjected to an accelerated alkali silica reaction (ASR) was recorded at different time intervals so as to follow the evolution of the structure by means of high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), and electron pair distribution function (e-PDF), combined with X-ray powder diffraction (XRPD). An increase in the size of the amorphous silica nanostructures and nanopores was observed by HRTEM, which was accompanied by the possible formation of Si–OH surface species. All of the studied samples were found to be amorphous, as observed by HRTEM, a fact that was also confirmed by XRPD and e-PDF analysis. A broad diffuse peak observed in the XRPD pattern showed a shift toward higher angles following the higher reaction times of the ASR-treated material. A comparison of the EELS spectra revealed varying spectral features in the peak edges with different reaction times due to the interaction evolution between oxygen and the silicon and OH ions. Solid-state nuclear magnetic resonance (NMR) was also used to elucidate the silica nanostructures. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

The compound NaFe2(PO4)(SO4)2 is successfully synthesized via a solid state reaction route and its crystal structure is determined using powder X-ray diffraction data. NaFe2(PO4)(SO4)2 phase is also characterized by cyclic voltammetry, galvanostatic cycling and electrochemical impedance spectroscopy. NaFe2(PO4)(SO4)2 crystallizes with the well-known NASICON-type structure. SAED and HRTEM experiments confirm the structural model, and no ordering between the PO4−3 and SO4−2 polyanions is detected. The electrochemical tests indicate that NaFe2(PO4)(SO4)2 is a 3 V sodium intercalating cathode. The electrical conductivity is relatively low (2.2 × 10−6 Scm−1 at 200 °C) and the obtained activation energy is ∼0.60eV. The GITT experiments indicate that the diffusivity values are in the range of 10−11-10−12 cm2/s within the measured sodium concentrations.

New oxides of the (NdSr)n+1MO3n+1 (M = Co and Mn or Fe) series are reported. Compounds of composition NdSrCo0.75Fe0.25O4.10, NdSrCo0.75Mn0.25O4.08 and Nd0.5Sr1.5Co0.75Mn0.25O3.86 are the n = 1 members of the Ruddlesden–Popper homologous series (K2NiF4 structural type) as determined by X-ray diffraction and different transmission electron microscopy techniques. Their crystal structure consists of connected (Co–Fe/Mn)O6 octahedra blocks separated by (Nd/Sr)O rock-salt like layers along the c-axis. Interstitial oxygen atoms or anion vacancies are induced depending on composition. Oxides with interstitial oxygen show good performances as intermediate temperature solid oxide fuel cell (IT-SOFC) cathode. The area-specific resistance values of electrodes made of these oxides at 973 K in air are 0.18 Ω cm2 for NdSrCo0.75Fe0.25O4.10 and NdSrCo0.75Mn0.25O4.08 (comparable to the one of the state-of-the-art materials proposed as cathodes in IT-SOFC), and 1.38 Ω cm2 for Nd0.5Sr1.5Co0.75Mn0.25O3.86.

The partial substitution of up to 5% Nd+3 by Ca+2 results in the oxide Nd1.90Ca0.10MgTiO5.94 that presents some remarkable structural features with a noticeable influence on its properties. In this oxide with a monoclinic perovskite-like structure and an octahedral tilting scheme (a-a-b+), both A- and B-ions are arranged in a rock-salt like manner, representing therefore the first example of a type of perovskite theoretically predicted. Besides this unprecedented arrangement of A- and B-ions, the oxygen vacancies created through doping with acceptor ions are trapped by association with the acceptor defects and hence the mobility of these vacancies is strongly limited. The oxygen conductivity of the substituted material is lower and the activation energy for oxygen motion is higher than those of the parent oxide, in which the concentration of anion vacancies is only due to intrinsic defects.

The partial substitution of up to 5% Nd+3 by Ca+2 results in the oxide Nd1.90Ca0.10MgTiO5.94 that presents some remarkable structural features with noticeable influence on its properties. In this oxide with monoclinic perovskite-like structure and octahedral tilting scheme (a-a-b+), both A- and B-ions are arranged in a rock-salt like manner and represents thereby the first example of a type of perovskite theoretically predicted. Besides this unprecedented arrangement of A- and B-ions, the oxygen vacancies created through doping with acceptor ions are trapped by association with the acceptor defects (𝑉𝑂••-𝐶𝑎𝑁𝑑´) and hence the mobility of these vacancies is strongly limited. The oxygen conductivity of the substituted material is lower and the activation energy for oxygen motion higher than those of the parent oxide, in which the concentration of anions vacancies is only due to intrinsic defects. pre-print

A solid solution forms for Sr3NdNb(3-x)Ti(x)O(12-δ) with approximate limits 0 ≤ x ≤ 0.06. The system crystallizes with a 12R-type hexagonal perovskite structure in the space group R3, as determined by neutron diffraction and selected area electron diffraction. The electrical properties of the end members have been investigated by impedance spectroscopy in the temperature range 550-800 °C under various gas atmospheres and as a function of oxygen and water-vapour partial pressure. Proton transport dominates under wet oxidising conditions in the temperature range 550-700 °C, as confirmed by the H(+)/D(+) isotope effect. Acceptor doping considerably enhances proton conductivity with a value of 3.3 × 10(-6) S cm(-1) for the bulk response of x = 0.06 at 700 °C in moistened air. The presence of a -¼ slope for both doped and undoped samples in the range 10(-19) ≤ pO2 ≤ 10(-8) atm at 900 °C indicates n-type transport under reducing conditions following the extrinsic model attributable to acceptor centres. The conductivity is essentially independent of pO2 at 600 °C under dry oxidising conditions, consistent with oxide-ion transport; a positive power-law dependence at higher temperature indicates extrinsic behaviour and a significant electron-hole contribution. The dielectric constant at RT of nominally stoichiometric Sr3NdNb3O12 is εr ∼ 37, with a moderately high quality factor of Q × f ∼ 16,400 GHz at fr ∼ 6.4 GHz. The temperature coefficient of resonant frequency of x = 0 is τf ∼ 12 ppm °C(-1), which lowers to -3 ppm °C(-1) for the Ti-doped phase x = 0.06.

We report the rich magnetic behaviour of Sr2CoNb1-xTixO6 (0 ≤ x ≤ 0.5) oxides as a result of their complex microstructure. Although these oxides show an average simple-cubic perovskite structure, they present a flexible microstructure due to short-range ordering between Co/Ti and Nb cations in the perovskite B-sites. The microstructure consists of double-cubic perovskite domains grown in a simple-cubic perovskite matrix. The size and number of the double-cubic perovskite domains decrease as the Ti content increases. As a result of aliovalent substitution of Nb(5+) by Ti(4+) in the parent Sr2CoNbO6 mixed-valence Co(3+)/Co(4+) oxides are obtained. A spin glass-like state has been observed at low temperatures for all the series, with freezing temperatures increasing with the Ti-content in the range 22 to 33 K. Furthermore, the x = 0.3 and x = 0.5 samples show non-interacting superparamagnetic particle-like dynamics associated with relatively high amounts of Co(4+), with "blocking temperatures" of 13 and ∼16 K, respectively. The complex magnetic behaviour of the title oxides seems to be connected with the clustering of magnetic Co(3+) and the distribution of Co(4+) as a result of the microstructure.

A new oxide of the Ruddlesden–Popper series has been isolated and structurally characterized in the Eu–Sr–Co–Mn–O system. X-ray diffraction and electron microscopy show that polycrystalline Eu2SrCo1.5Mn0.5O7 constitutes the n = 2 member of a homologous series, the essential feature of which is the existence of two connected Co/Mn octahedral layers, separated by Eu atoms. Electrochemical studies show that the area-specific resistance of this compound is 0.15 Ω cm2 at 700 °C in air, a performance which is comparable to that of the best state-of-the-art materials used as cathodes in intermediate temperature solid oxide fuel cells. Below 150 K the title material presents two different magnetic phenomena. The first one corresponds to the formation of ferromagnetic nanoclusters (TC ∼ 121 K) within an ordered Co2+/Mn4+ atomic configuration, whereas at lower temperature (∼21 K) a spin glass state occurs.

Graphical abstract: The new compounds RE{sub 4}O{sub 4}[AsO{sub 4}]Cl (RE = La, Pr, Nd, Sm, Eu, Gd) were synthesised by solid state reaction via a salt flux route and investigated by HRTEM, SAED, and single crystal X-ray diffraction. - Highlights: • We discovered the series of RE{sub 4}O{sub 4}[AsO{sub 4}]Cl (RE = La, Pr, Nd, Sm, Eu, Gd) compounds. • The RE{sub 4}O{sub 4}[AsO{sub 4}]Cl single crystals were grown using NaCl/KCl flux. • The RE{sub 4}O{sub 4}[AsO{sub 4}]Cl structures were solved using single crystal X-ray diffraction data. • The layered RE{sub 4}O{sub 4}[AsO{sub 4}]Cl compounds were further characterized using HRTEM and SAED. • We observed an alternation of ordered-[RE{sub 4}O{sub 4}]{sup 4+} and disordered-[ClAsO{sub 4}]{sup 4–} layers. - Abstract: The new compounds RE{sub 4}O{sub 4}[AsO{sub 4}]Cl (RE = La, Pr, Nd, Sm, Eu, Gd) were synthesised by solid state reaction via a salt flux route and investigated by HRTEM, SAED, and single crystal X-ray diffraction. The samples crystallise with a tetragonal cell, space group P4{sub 2}/mnm and Z = 2. Their crystal structure consists of an alternation of [RE{sub 4}O{sub 4}]{sup 4+} and [ClAsO{sub 4}]{sup 4–} layers. The [RE{sub 4}O{sub 4}]{sup 4+} layer contains ORE{sub 4/4} tetrahedra which share commonmore » edges. The anions AsO{sub 4}{sup 3–} and Cl{sup –} are located between these layers in disordered manner. SAED and HRTEM experiments confirmed this structural model and enabled us to propose an ordered model for the [ClAsO{sub 4}]{sup 4–} layers.« less

Aliovalent substitution of Nb(5+) by Ti(4+) in Sr2LuNbO6 is limited to 10% of Nb atoms. A full structural determination by NPD confirms this and reveals that the structure is better described as a superstructure of the simple cubic perovskite (as previously reported) with the monoclinic cell 2(1/2)ap× 2(1/2)ap× 2ap and β≈ 90° (S.G. P21/n). The substituted materials present both oxygen-vacancies induced by charge compensation and Sr-deficiency. Therefore, their formula should be given as Sr2-yLuNb1-xTixO6-δ. Electrical properties can be fully understood considering these compositional defects. The parent compound Sr2LuNbO6 presents low electrical conductivity in air, which improves by more than one order of magnitude upon Ti substitution. In any case, the title oxides show low electrical conductivity in a wide oxygen partial pressure (pO2) range (10(-25) atm ≤pO2≤ 10(-1) atm). At high pO2 the conductivity increases with pO2 due to oxygen-vacancy annihilation and hole creation, according to a general p-type semiconducting mechanism; A-site substoichiometry and Ti-substitution are the origin of this behaviour. In the low pO2 region, the conductivity increases as the oxygen partial pressure decreases. Reduction of cations, Nb(5+) or Ti(4+), supports n-type conduction by electrons and oxygen vacancy creation. For the intermediate pO2 range a low ionic conduction contribution is observed. Although the estimated ionic conductivity is not high in the substituted compounds, the strategy seems to be valid since a significant enhancement of ionic conduction is observed upon aliovalent substitution.

We report a synthesis method to stabilize TiO2 rutile nanoparticles (around 10 nm) and keep the particle size when reduced down to TiO1.84. TiO2−δ nanoparticles exhibit room-temperature ferromagnetism that becomes stronger for TiO1.84. The reduction mechanism to stabilize Magneli phases excludes a relevant influence of oxygen vacancies in the modification of the magnetic properties. The arrangement of Ti3+ could give rise to hopping of the single 3d electron inducing local ferromagnetic-like behavior.

Substitution of La3+ by Sr2+ in the perovskite La2CoTiO6 yields materials of the La2−xSrxCoTiO6 series. The dominant charge-compensating mechanism is oxidation of Co2+ if they are prepared at air. The as prepared oxides can be reduced inducing a large amount of oxygen vacancies while keeping the perovskite structure. The electrical behaviour of the La2−xSrxCoTiO6 series is dominated by p-type electronic conduction in a wide pO2 range through non-adiabatic hopping of small-polarons. The electrical conductivity increases with x, except for the x = 0.5 material which shows an unexpectedly low conductivity due to microstructural and short-range ordering effects. The highest conductivity material, La1.60Sr0.40CoTiO6, is selected to study the electrochemical properties of the series. This compound is chemically compatible with YSZ up to 1373 K, in both oxidizing and reducing atmospheres. The preliminary evaluation of the electrode performance reveals that La1.60Sr0.40CoTiO6-based electrodes exhibit polarization resistances of typically 0.8 Ω cm2 at 1073 K in oxygen, which are close to the values obtained for LSM-based cathodes. Thus, the electrochemical behaviour of this oxide as cathode is particularly encouraging since the electrode microstructure is not optimized; it is expected that an improved microstructure will perform at least similarly to the state-of-art in SOFCs materials.

A new hexagonal polytype in the BaMn1-xFexO3-δ system has been stabilized. Powdered Ba0.8Sr0.2MnIV0.4FeIII0.6O2.70 crystallizes in the 4H hexagonal polytype (space group P63/mmc) according to X-ray diffraction. HAADF images and chemical maps with atomic resolution have been obtained by combining Cs-corrected electron microscopy and EELS spectroscopy. The structure is formed by dimers of face-sharing octahedra linked by corners. EELS data show a random distribution of the transition metals ions identified by Fe and Mn-L2,3 chemical maps. A systematic difference in contrast observed in the O–K signal mapping suggests that anion deficiency is randomly located along the hexagonal layers in agreement with ND data. The magnetic structure consists of ferromagnetic sheets with the magnetic moments aligned along the x-axis and coupled antiferromagnetically along the c-axis.

Structural properties of rare earth niobates (RE3NbO7, RE = Y, Er, Yb, Lu) have been studied using selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). SAED patterns show diffuse scattering features along the [111] direction of the pure fluorite (Fm–3m) structure due to a short-range ordering of anion vacancies in the fluorite structure or the change of coordination number around some cations. The latter is related to the presence of small domains of pyrochlore phase (Fd-3m), which are observed in the HRTEM images.

A solid solution of 6H-type BaTi1-yCoyO3-δ samples where 0.1 ≤ y ≤ 0.4 and δ ≤ 0.05 has been prepared in air and characterized by a combination of X-ray, neutron, and electron diffraction, X-ray absorption spectroscopy, hydrogen-reduction thermogravimetric analysis, high resolution electron microscopy, magnetic susceptibility measurements, and impedance spectroscopy. For y = 0.1, Co acts as an acceptor dopant and is incorporated as CoIII ions with charge balanced achieved by the formation of O(2) oxygen vacancies in the h-BaO3 layers that separate pairs of face-sharing octahedra. For samples with y > 0.1, Co is present as both CoIII and CoIV ions and in all cases δ ∼ 0.03. This indicates a change from primarily aliovalent (CoIII) to isovalent (CoIV) doping with increasing Co-content with a general formula BaTiIV1-y(CoIII0.06CoIVy-0.06)O2.97 for 0.2 ≤ y ≤ 0.4. All samples are semiconducting at room temperature with relative permittivity in the range ∼20−50. The bulk conductivity increases with y and displa...

The new oxide-bromide-oxoarsenates (V) La 4 O 4 [AsO 4 ]Br and Pr 4 O 4 [AsO 4 ]Br were synthesised successfully by a solid state reaction route using a salt flux. Both compounds crystallise with a tetragonal cell. The twinned Pr 4 O 4 [AsO 4 ]Br structure was determined from single crystal X-ray diffraction data ( P 4 2 / m , a = 834.91(9), c = 1321.6(2) pm, V = 0.9213 nm 3 and Z = 4). It consists of alternating [Pr 4 O 4 ] 4+ and [BrAsO 4 ] 4− layers. The [Pr 4 O 4 ] +4 layer contains OPr 4/4 tetrahedra which share common edges. The AsO 4 3− and Br − anions are located between these layers and form infinite chains along a at z = 0 and along b at z = 1/2. SAED and HRTEM experiments confirmed this well ordered structural model. X-ray powder diffraction data reveal isotypism of La 4 O 4 [AsO 4 ]Br: a = 843.07(1), c = 1355.28 pm, V = 0.9633 nm 3 .

Chemical doping of 15 atom % Mn by Ti in 2H-BaMnO3 leads to two new polytypes, 4H- and 9R-BaMn0.85Ti0.15O3−δ, depending on the oxygen content. The 4H polytype (a = 5.71355(4) and c = 9.41230(8) A, space group P63/mmc) was prepared in an inert atmosphere and forms face sharing dimers linked by corners based on a stacking sequence (hc)2 where h and c refer to hexagonal and cubic BaO3 layers, respectively. Selected area electron diffraction patterns and high resolution electron microscopy reveal the coexistence of two types of crystals; the majority corresponds to an ordered (hc)2-4H polytype whereas a minority fraction shows a disordered microstructure. Such differences may be associated with nonhomogeneous cationic distribution within the crystals. Oxidation in air at 1523 K for 48 days leads to conversion to the 9R polytype (a = 5.68007(1) and c = 20.98208(6) A, space group R3m) with a (hhc)3 stacking sequence; Mn4+ ions are exclusively located on the central B-site (M1) of the face sharing trimers linke...

Titanosilicate ETS-10 crystals were prepared by hydrothermal synthesis varying Ti source (TiCl 3 and commercial TiO 2 -anatase), time in autoclave and seeding with previously prepared ETS-10 crystals. The crystalline powders were characterized by X-ray diffraction, N 2 adsorption, thermogravimetric analysis, and scanning and transmission electron microscopies. Control of the particle size of ETS-10 crystals ranging from 0.32 μm × 0.41 μm to 16.4 μm × 32.5 μm was successfully achieved varying the seeding and synthesis conditions. In particular, it was found that the use of TiO 2 -anatase alone or together with TiCl 3 promotes heterogeneous primary nucleation. Transmission electron microscopy demonstrated that the largest crystals obtained here were twinned.

Selected area electron diffraction and high-resolution electron microscopy have been used to fully characterize ITQ-13 and ITQ-34. ITQ-13 is made up by a stacking sequence of AAAA... layers along the a-axis, while the stacking sequence is ABAB... for ITQ-34. These polytypes can intergrow to crystallize as new materials with different concentrations of each polytype as observed by X-ray powder diffraction. High-resolution images show that in all compositional range both materials intergrow in a disordered sequence along the a direction of the ITQ-13.

The influence of the partial substitution of Sr by Ba in the SrCoO3−y on its structure and properties has been investigated. SrBaCo2O5 is stable at room temperature as a cubic phase with a high oxygen vacancy concentration. This structure is stable up to 800 K. Above this temperature, the phase decomposes into a monodimensional rhombohedral (SrBa)7Co6O18 phase. At 1200 K, a reversible rhombohedral → cubic transformation is observed, the rhombohedral phase remaining stable until room temperature. SrBaCo2O5 shows antiferromagnetic behavior with TN ≈ 525 K. As determined by neutron diffraction data, the magnetic Co spins align ferromagnetically on the (111) planes and these planes are stacked antiferromagnetically in the normal direction (type II antiferromagnet). The temperature dependence of the electrical conductivity indicates SrBaCo2O5 to be a semiconducting p-type material with an activation energy of ∼0.3 eV.

The crystal structures of 6H-type BaMn0.15Ti0.85O3, BaMn1/4Ti3/4O2.95, and BaMn1/2Ti1/2O2.84 and 12R-type BaMn2/3Ti1/3O3 have been established by a combination of X-ray, neutron, and electron diffraction, and high-resolution electron microscopy. The 6H-type structure (space group P63/mmc) can be described by a stacking sequence (hcc)2 along the c-axis with any anion deficiency located exclusively in the h-BaO3 layers. Ti atoms display a strong preference for the corner-shared octahedral site, whereas both Mn and Ti are distributed over the octahedral sites in the face sharing dimers. The 12R-type structure (space group R3m) can be described by a stacking sequence (hhcc)3. Ti atoms again display a strong preference for the isolated corner-sharing octahedral site, whereas Mn atoms occupy the central site of the face-sharing trimers. The electrical properties have been characterized by impedance spectroscopy and reveal the fully oxidized compounds to be electrical insulators with relative permittivity value...

A combination of X-ray, neutron and electron diffraction, and high-resolution electron microscopy have been used to establish the 5H structural type of a new hexagonal-type perovskite BaMn0.2Co0.8O2.80. The structure can be described as a (cc′chh) 5H hexagonal polytype with ordered oxygen vacancies where the cubic c′ layer corresponds to a composition of [BaO2] as opposed to [BaO3]. The resulting layer structure consists of [MnCo2O12] blocks of three sharing faces octahedra linked by corners to two unconnected [CoO4] tetrahedra. Electron Energy Switch order Loss Spectroscopy shows Mn to be present only as Mn(+IV) and therefore Co is present as mixed +III/+IV. Mn(+IV) and Co(+III) ions are distributed over the face sharing octahedral sites whereas Co(+IV) ions are located on the tetrahedral sites. The magnetic behavior is more complex than is observed for BaCoO2.80 (a ferromagnet with Tc = 47 K) and can be described by a Stoner–Wohlfarth model of random-anisotropic, noninteracting monodomain ferromagnetic clusters. The ferromagnetic clusters occur below 35 K and are assigned to groups of Co ions in octahedral and/or tetrahedral sites; however, incorporation of Mn ions in the octahedral sites disrupts the transition into long-range three-dimensional ferromagnetic order. Impedance Spectroscopy data reveals semiconducting grain conductivity at room temperature (1 × 10−2 S cm−1); however, subambient data reveal an unusual temperature dependence with a smooth changeover from a thermally activated process (0.07 eV) in the range 40–300 K to a low-temperature state below 40 K with a near-zero activation energy. The data cannot be described by conventional Arrhenius or variable-range hopping conduction models and the conduction mechanism(s) remain unresolved. Several possible suggestions for the conductivity behavior are made, including Anderson localization, anisotropic conduction associated with the 5H crystal structure or some complex correlated mechanism between the magnetic and electronic transport properties. The electrical microstructure of BaMn0.2Co0.8O2.8 ceramics consist of semiconducting grains and constrictive grain boundaries and therefore exhibit internal barrier layer capacitor (IBLC) behavior, with a high and temperature-stable apparent permittivity of 10 000 (at 10 kHz) above 100 K.

The structural characterization of NaNbWO(6), prepared by the ceramic route, has been performed. Electron diffraction has shown the presence of two related phases in a 1:1 ratio, whose lattice parameters correspond to those of the well-known tetragonal tungsten bronzes (TTB) and those of a monoclinically distorted phase. In addition to basic unit cells, the morphology of the two phases has been found to be similar, but they present a slight difference in the W/Nb ratio. (1)H and (23)Na magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectra of NaNbWO(6) and its proton-exchanged derivatives have been interpreted on the basis of the ideal TTB structure. The average structure and the morphology remain unchanged in Na(1-x)H(x)NbWO(6) derivatives. (1)H and (23)Na MAS-NMR spectroscopies have been used to monitor changes produced during exchange processes. It has been shown that the exchange of Na ions is mainly produced, but not exclusively, at tetragonal channels. However, a large amount of Na ions at the pentagonal channels do not exchange with protons, suggesting that these ions are needed to stabilize the TTB-like structure. A tentative distribution of sodium ions in the most-exchanged oxide, deduced from NMR results, approximately (Na(0.46))(p)(Na(0.08))(s)H(0.46)NbWO(6), has been proposed. NMR spectra of Na(1-x)H(x)NbWO(6) indicate that two different OH groups are formed upon exchanging. The study of samples hydrated with D(2)O allowed us to conclude that deuterons of adsorbed water exchange with protons of the two OH groups. The proton-deuteron exchange is slow at room temperature but is strongly enhanced at 90 degrees C. This observation relates to the proton conductivity displayed by exchanged products under a humid atmosphere.

We have developed a new fast electron diffractometer working with high dynamic range and linearity for crystal structure determinations. Electron diffraction (ED) patterns can be scanned serially in front of a Faraday cage detector; the total measurement time for several hundred ED reflections can be tens of seconds having high statistical accuracy for all measured intensities (1–2%). This new tool can be installed to any type of TEM without any column modification and is linked to a specially developed electron beam precession “Spinning Star” system. Precession of the electron beam (Vincent–Midgley technique) reduces dynamical effects allowing also use of accurate intensities for crystal structure analysis. We describe the technical characteristics of this new tool together with the first experimental results. Accurate measurement of electron diffraction intensities by electron diffractometer opens new possibilities not only for revealing unknown structures, but also for electrostatic potential determination and chemical bonding investigation. As an example, we present detailed atomic bonding information of CaF2 as revealed for the first time by precise electron diffractometry.

Two complex perovskite-related structures were solved by ab initio from precession electron diffraction intensities. Structure models were firstly derived from HREM images and than have been confirmed independently using two and three-dimensional sets of precession intensities. Patterson techniques prove to be effective for ab initio structure resolution, specially in case of projections with no overlapping atoms. Quality of precession intensity data may be suitable enough to resolve unknown heavy oxide structures.

A layered perovskite related compound with the Sr4CoTaO8 composition was isolated. This material constitutes the n = 1 member of the recently reported [Srn+1(Co0.5Ta0.5)nO3n+1] Ruddlesden–Popper series. XRPD, SAED and HREM reveal that this compound crystallizes as a K2NiF4-type with space group I4/mmm. The structure is formed by one Sr2CoTaO6 perovskite block that intergrows with one SrO rock salt layer; no ordering between the Co and Ta cations was established. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Two new oxides of the Ruddlesden-Popper series have been isolated and structurally characterized in the Sr-Co-Ta-O system. X-ray and electron diffraction and high-resolution electron microscopy show that polycrystalline Sr(3)CoTaO(7) constitutes the n=2 member of a new Sr(n+1)(CoTa)(n)O(3n+1) homologous series, the essential feature of which is the existence of two connected Co/Ta octahedral layers, separated by Sr atoms. Sr(2)CoTaO(6), the n=infinity member of the series, shows a particular short-range ordering of Co and Ta at the octahedral sites leading, as shown by high-resolution electron microscopy, to the disordered intergrowth of simple and double perovskite type domains. Strategies to stabilize new oxides of this series are discussed.

Polycrystalline Sr3CaMn2CoO9 and (Sr0.5Ca0.5)15Mn7Co4O33 have been synthesized and characterised by X-ray and electron diffraction, high resolution electron microscopy and magnetic measurements. These oxides constitute the α = 3, β = 1 and α = 4, β = 1 terms of the homologous series (A3B2O6)α(A3B3O9)β, respectively. Isolated rows of polyhedra sharing faces along the c-axis are made up of units of two and one face-sharing octahedra (occupied by Mn) linked by one trigonal prism (occupied by Co) in an ordered way. The chains are separated by columns of Sr/Ca atoms. Magnetic measurements suggest antiferromagnetic correlations in the two new commensurate monodimensional oxides. The influence of the size of alkaline-earth atoms on the long-range magnetic interactions is discussed.

The cobaltite Ba2CoO4 presents a monoclinic symmetry (space group P21/n) and lattice parameters a = 5.891 31(7), b = 7.59 74(3), c = 10.3638(2) A, and β = 91.90(1)°. As determined from X-ray and neutron diffraction data, its structure can be described as formed by isolated CoO4 tetrahedra defining zigzag rows running along the a axis, separated by Ba atoms in both the b and the c directions. The cobalt atoms are in a high spin Co4+ ground state, and their magnetic structure evidences the existence of antiferromagnetic and ferromagnetic alignments of the magnetic moments alternating along each of the aforementioned rows. Transport properties are related to p-type carriers.

Apatite-type lanthanum silicates have been successfully prepared at room temperature by dry milling hexagonal A-La2O3 and either amorphous or low cristobalite SiO2. Milling a stochiometric mixture of these chemicals in a planetary ball mill with a moderate rotating disc speed (350 rpm), allows the formation of the target phase after only 3 h although longer milling times are needed to eliminate all SiO2 and La2O3 traces. Thus, the mechanically activated chemical reaction proceeds faster when using amorphous silica instead of low cristobalite as silicon source and pure phases are obtained after only 9 and 18 h, respectively. As obtained powder phases are not amorphous and show an XRD pattern as well as IR and Raman bands characteristic of the lanthanum silicate. The domain size of the as-prepared phases varies gradually with the temperature of post-milling thermal treatment with activation energies of about 26(8) and 52(10) kJ mol−1 K−1 for the apatites obtained from amorphous silica and low-cristobalite, respectively. These values suggest crystallite growth to be favored when using amorphous silica as reactant.

This paper reports the mechanochemical synthesis and the structural and microstructural characterization of three titanates, A 2 Ti2O7 ( A = Y , Gd and Dy), with a pyrochlore-like structure. Starting from stoichiometric mixtures of elemental oxides TiO2 and A 2 O3, single-phase samples of highly disordered pyrochlores were obtained after milling. Differential thermal analysis of the as prepared powders showed in every case the presence of a single exothermic event at temperatures close to 800 °C. The evolution of the structure and microstructure of these highly-disordered pyrochlores with temperature was studied by combining XRD and IR and Raman spectroscopies. On heating, both the cation and anion arrays in Y2Ti2O7 and Dy2Ti2O7, order by two independent processes. The exothermic events observed in DTA have their origin in the ordering of the anion sublattice, whereas cation ordering progress smoothly with temperature. Gadolinium titanate, Gd2Ti2O7, behaves differently to the other two compounds studied concerning the cation sublattice: no evidence of cation disorder is observed, even in the just-milled sample.

Polycrystalline Eu2Mo3O12 has been synthesized and characterized by X-ray and electron diffraction, high resolution electron microscopy and magnetic measurements. This oxide shows a complex scheelite superstructure whose essential feature is the existence of isolated Mo tetrahedra. Each Mo atom has four oxygen neighbors at distances ranging from 1.75 to 1.91 A, whereas each Eu atom has eight oxygen neighbors at 2.43 A. The temperature dependence of the inverse magnetic susceptibility shows that Eu2Mo3O12 behaves as a Curie–Weiss paramagnet at high temperature (160–300 K), whereas in the range 100–20 K it shows Van Vleck paramagnetism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Electron diffraction and high resolution electron microscopy have been used to characterize two new one-dimensional superstructures in the A Rh O system (A = Ca, Sr) related to the 2H-ABO3-type. They are formed by the intergrowth of n A3A′BO6 blocks, showing the Sr4RhO6-type, with A12A′2B8O30 blocks, constituted by two A3O9 and two A3A′O6 layers alternating in the stacking sequence 1:1, leading to the A27A′7B13O60 ( n = 5 ) and A30A′8B14O66 ( n = 6 ) compositions. A crystallographic shear mechanism is proposed to describe the structural relationship between Sr4RhO6 (A3A′BO6-type) and the new superstructures.

Polycrystalline Sr9Mn5Co2O21 has been synthesized and characterized by X-ray and electron diffraction, high-resolution electron microscopy, and magnetic measurements. This oxide constitutes the α =...

Mesoporous MCM-41 materials seem adequate for their use as bioactive materials since they exhibit high specific surfaces constituted by siloxane and silanol groups, and present a high porosity, which are important conditions for bioactivity. Even though, no signs of bioactivity are shown after 2 months of in vitro assays, revealing the need of some other element such as cations that can act as defect generators, promoting the apatite layer nucleation. The addition of a small amount of glass to a MCM-41 material induce the formation of an apatite-like layer starting from the glass surface, covering after 7 days of assay the whole surface of the biphasic material MCM-41-glass.

Ba4Mn3O10 and Ba6Mn5O15 constitute the n=3 and n=5 terms of a new layered Ban+1MnnO3n+1 family, consisting of n 2H-ABO3 hexagonal perovskite-like blocks separated by a BaO layer. The structural essential feature is the presence of zig-zag layers perpendicular to [010] separated by Ba atoms and folded along c-axis. The layers are built up from (Mn3O12) units constituted by three (n=3) and five (n=5) octahedra sharing faces connected via their apices by sharing two of their three terminal corners at each end of the (Mn3O12) group. These layered phases are structurally related to the 9R and 15R Ba–Mn hexagonal polytypes since they can be derived by the insertion of BaO layers into the structural framework of the corresponding hexagonal polytype.

Novel rare-earth-containing manganites, Ba 4 RE Mn 3 O 12 ( RE =Ce, Pr), with 12 R structure, have been prepared by solid-state reaction. Although the phases are formed at 950°C, to obtain single-phase samples high temperatures (up to 1300°C) and long synthesis periods are needed. Their structure is built up from chains of BO 6 face-sharing and corner-sharing octahedra running along the c -axis giving a quasi-one-dimensional oxide. Every polyhedral column consists of (Mn 3 O 12 ) units of three face-sharing octahedra, both ends connected by the three terminal oxygen atoms to three different ( RE O 6 ) octahedra. Mixed occupation of the three octahedral positions in the structure, (Mn(1), Mn(2) and Re), was not found. Vacancies are not observed, neither in the cationic sublattice nor in the oxygen one. Thus, as in all the other 1-D manganites, the oxidation state of manganese ions seems to be four, as the rare-earth valence is. High-resolution electron microscopy suggests the eventual existence of ordered polytypes for different compositions, which could be stabilized by adjusting the thermodynamic conditions.

We describe in this work the synthesis and crystal structure of five rare earth and Mo(VI) or W(VI) containing complex perovskites. The compounds studied are Ba(Dy 2/3 Mo 1/3 )O 3 , Ba(Dy 2/3 W 1/3 )O 3 , Ba(Gd 2/3 Mo 1/3 )O 3 , Ba(Gd 2/3 W 1/3 )O 3 and Ba(Sm 2/3 W 1/3 )O 3 and were prepared starting from solutions, by the polymeric precursors method. Structural characterization by HREM, SAED and powder XRD revealed the five compounds to be ordered cubic perovskites, SG Fm -3 m (225), with a cell parameter double of that of a simple perovskite cell and increasing as the size of the trivalent lanthanide ion increases (Dy

Polycrystalline Sr4Mn2CoO9 has been synthesized and characterized by X-ray and electron diffraction, high-resolution electron microscopy, and magnetic measurements. This oxide constitutes the α = 3, β = 1 term of the homologous series (A3B2O6)α(A3B3O9)β. The essential feature of this structure is the presence of isolated rows of polyhedra sharing faces along the c-axis and formed by the alternation of one trigonal prism occupied by a Co cation and two Mn cations in octahedral coordination; the chains are separated by Sr atoms. The magnetic properties seem to indicate strong antiferromagnetic correlations along the chains, but no three-dimensional (3D) magnetic long range order is observed. The global magnetic behavior indicates a partially ordered system, close to a magnetic spin glass.