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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 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.

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.

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.

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

We describe in this work the synthesis by the Pechini method of five Mo(VI)- and W(VI)-containing complex perovskites and their structural characterisation by HREM and XRD. The compounds studied, Ba(B2/3′B1/3″)O3 (B′=In and Y; B″=W and Mo) and Sr(In2/3W1/3)O3, were obtained after firing the precursor powders for 8 h at 1200°C. Thermal analysis showed that the formation mechanism of the five perovskites is similar and implies the formation of barium carbonate and barium tungstates or molybdates of different stoichiometries as intermediate phases. Interesting enough, these similar mechanisms yield to materials of a quite different structure. Indeed, Ba(In2/3Mo1/3)O3 and Sr(In2/3W1/3)O3 were found to be disordered perovskites (unit cells: ap×ap×ap and 2 a p × 2 a p ×2a p , respectively); on the other hand, two phases coexist in the sample Ba(In2/3W1/3)O3 at the synthesis conditions: an ordered predominant phase (unit cell: 2ap×2ap×2ap) and, as a minor phase, a disordered perovskite (unit cell: ap×ap×ap). Finally, the two yttrium-containing compounds were found to be ordered perovskites (2ap×2ap×2ap).

A solid solution of Nd1−xSrxCoO3−δ (with x=0, 1/3, 2/3, and 1) has been prepared and characterized by a combination of X-ray diffraction, electron microscopy, and electron energy-loss spectroscopy (EELS). The structural characterization indicates that Nd-doped materials present an orthorhombic symmetry with a=√2xap, b=√2xap, and c=2xap (ap refers to lattice parameter of simple cubic perovskite), while SrCoO2.5 has an orthorhombic symmetry with a=√2xap, b=4xap, and c=√2xap. EELS analysis revealed that Co are in 3+ oxidation states but in different spin configurations.

The oxide series La2-x Sr x CoTiO6 (0 ≤ x ≤ 1.0) belong to the perovskite family with general formula ABO3. The evolution of the room-temperature structure as a function of the Sr content was studied using complementary techniques by applying the symmetry-adapted modes formalism (AMPLIMODES). In the compositional range presented in this article (0 ≤ x ≤ 0.5), the compounds adopt distorted perovskite structures of monoclinic (space group P21/n) or orthorhombic (space group Pnma) symmetry, both with octahedral tilting scheme (a - a - c +) (out of phase along two perovskite main directions and in phase along the third direction). The main difference between these structures is the existence of rock-salt order of B ions in the monoclinic symmetry, which is lost for x ≥ 0.30. As the Sr content increases, a better matching of the A - O and B - O distances occurs. This is produced by an elongation of the A - O distance as La3+ is replaced by the larger ion Sr2+, and the shortening of the B - O distance due to the oxidation of Co2+ to Co3+ induced by the aliovalent substitution. As a result, the cuboctahedral A-site cavity becomes less and less distorted; the A ion tends to occupy its ideal positions, increasing its coordination and giving rise to a more symmetrical structure. In the whole compositional range, the symmetry-adapted atomic displacements (modes) responsible for the out-of-phase tilting of the BO6 octahedra remain active but those associated with the in-phase tilting become negligible, anticipating for x ≥ 0.6 a transition to a new structure with tilting scheme either (a 0 a 0 c -) (space group I4/mcm) or (a - a - a 0) (space group Imma) or (a - a - a -) (space group R3c). © 2014., We thank the Spanish Ministerio de Ciencia e Innovación and Comunidad de Madrid for funding the projects MAT2010-19837-C06-01 and S-2009/PPQ-1626, respectively. Financial support from Universidad CEU San Pablo is also acknowledged. We acknowledge the Spanish Ministerio de Ciencia e Innovación and Consejo Superior de Investigaciones Científicas for financial support.

The accurate control of both stoichiometry and synthesis conditions has allowed the stabilization of new Ba–Co oxides with one-dimensional structures related to the 2H- AB O 3 hexagonal perovskite. Ba 26 Co 23 O 69 , Ba 44 Co 39 O 117 , and Ba 28 Co 25 O 75 are new members of the (Ba 8 Co 6 O 18 ) α (Ba 8 Co 8 O 24 ) β polysomatic orthorhombic series whose structural features have been characterized by electron diffraction and high-resolution electron microscopy. These polysomes are described as the ordered intergrowth between structural slabs of one-half unit cell of the o -Ba 8 Co 7 O 21 and one Ba 8 Co 8 O 24 2H-block in different ratios.

The systematic search of new intermediate phases between 2H-BaCoO 3 and Ca 3 Co 2 O 6 structures has shown, for the first time, the existence of polymorphism in monodimensional phases of the homologous series ( A 3 B 2 O 6 ) α ( A 3 B 3 O 9 ) β . A new Ba 8 Co 7 O 21 phase, different from the trigonal phase previously reported, has been synthesized. The new material is orthorhombic (SG Fd 2 d ) with unit cell parameters a o =1.14, b o =1.98, and c o =1.75 nm. Both orthorhombic and trigonal phases are built up of isolated rows of polyhedra parallel to the c -axis formed by one trigonal prism and six ocathedra sharing faces, but with a different spatial arrengement between the chains. The orthorhombic structure can be derived from a hcp stacking of Ba 8 O 24 and Ba 8 Co 2 O 18 layers.

The n integer members of the A n +2 B n B ′O 3 n +3 homologous series keeping B = B ′=Co the same have been characterized by selected area electron diffraction and high resolution electron microscopy. Its commensurate modulated structure is made up of chains, separated by A atoms, running along the c direction containing a succession of one Co atom ( B ′) in prismatic trigonal sites and n Co atoms octahedrally coordinated sharing faces. All of them can be described as ordered intergrowths of two structures of the end members of the family, 2H–BaCoO 3 ( n =∞) and Ca 3 Co 2 O 6 ( n =1). The modulation observed in all terms can be described as a function of the difference between the c -axis of the Co sublattice with respect to the 2H structure. Members of the series can be obtained by modifying the distance between layers keeping B =Co the same, as shown in (Sr 1− x Ba x ) 6 Co 5 O 15 ( n =4), Sr 5 Co 4 O 12 ( n =3), and (Sr 0.5 Ca 0.5 ) 4 Co 3 O 9 ( n =2).

The new phases (Ba 0.5 Sr 0.5 ) 7 Co 6 O 18 , Ba 8 Co 7 O 21 , and Ba 9 Co 8 O 12 have been shown by electron diffraction and high-resolution electron microscopy to be three new members ( n =5, n =6, and n =7) of the 2H hexagonal perovskite-related A n +2 B ′ B n O 3 n +3 homologous series. Their modulated structures are made up of chains, running along the c direction and separated by Ba(Sr) ions, containing a succession of one Co atom in prismatic trigonal sites and n Co atoms in octahedral coordination sharing faces. The strategy to synthesize different terms of this family is discussed in terms of the distance between A O 3 layers.

An electron diffraction and high-resolution electron microscopy study shows that oxygen vacancies in BaCoO 2.94 lead to a threefold superstructure along the (11 2 0) 2H direction, the …hhh… hexagonal sequence characteristic of the 2H type being maintained. ording to compositional and structural data, the most probable structural model should be formed by three A O 3 layers intergrowing with one A O 2.67 layer along the c -axis. In such oxygen-deficient layers, one of each six oxygen atoms in every oxygen row is removed, leading to the observed superstructure.

International audience; The new ternary vanadate Na3MnV2O7.5 has been synthesized by solid state reaction route. Its crystal structure was determined from both powder X-ray diffraction data and high-resolution transmission electron microscopy (HRTEM) analysis. Na3MnV2O7.5 (P2/c, a = 7.4004(4) Å, b = 5.6065(3) Å, c = 17.0876(11) Å, β = 90.0114(2) °, and Z = 4) crystallizes with a new type of oxygen-deficient perovskite ABO3-δ (δ = 1/2) with the sodium atoms occupying the A site and the vanadium and manganese atoms perfectly ordered in the B site. In Na3MnV2O7.5 the manganese and the vanadium atoms are octahedral and tetrahedral coordinated, respectively. The MnO6 and VO4 polyhedra share edges and form a perovskite 3D-framework with high disorder of the oxygen atoms. HRTEM confirmed this structural model. Na3MnV2O7.5 was characterized by magnetic susceptibility and specific heat measurements. It is antiferromagnetic with no magnetic ordering down to 2 K.

International audience; A new ternary manganese vanadate, NaMnVO(4), was synthesized by solid state reaction route, and its crystal structure and magnetic properties were characterized by X-ray diffraction, magnetic susceptibility and specific heat measurements, and by density functional calculations. NaMnVO(4) crystallizes in the maricite-type structure with space group Pnma, a = 9.563(1) A, b = 6.882(1) A, c = 5.316(1) A, and Z = 4. NaMnVO(4) contains MnO(4) chains made up of edge-sharing MnO(6) octahedra, and these chains are interlinked by VO(4) tetrahedra. The magnetic susceptibility has a broad maximum at T(max) = 24 K and follows the Curie-Weiss behavior above 70 K with θ = -62 K. NaMnVO(4) undergoes a three-dimensional antiferromagnetic ordering at T(N) = 11.8 K. The spin exchanges of NaMnVO(4) are dominated by the intrachain antiferromagnetic exchange, and the interchain spin exchanges are spin-frustrated. The most probable magnetic structure of the ordered magnetic state below T(N) was predicted on the basis of the extracted spin exchanges.

International audience; The crystal and magnetic structures and electrical properties of two new Mn-rich 6H′-type hexagonal perovskites in the BaMn1−xFexO3−δ system have been investigated. Structural characterization performed by X-ray, electron, and neutron diffraction and high resolution electron microscopy indicates that both BaMn0.85Fe0.15O2.87 and BaMn0.6Fe0.4O2.72 crystallize in the 6H′ hexagonal polytype (P6m2 space group). The structure is formed by tetramers and dimers of face-sharing octahedra that are linked by corners. The anion deficiency is located at random through the hexagonal layers and increases with the Fe-content. In both phases, the central position of the tetramers is fully occupied by Mn, the remaining Mn and Fe cations being randomly distributed over different polyhedra. The Mssbauer spectroscopy data show Fe to be present only as FeIII in octahedral and tetrahedral coordination. The magnetic structure is formed by ferromagnetic sheets with the magnetic moments aligned along the x-axis and stacked antiferromagnetically perpendicular to the c-axis. The electrical properties have been characterized by impedance spectroscopy and reveal both compounds behave as leaky insulators at room temperature with bulk permittivity values

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 =...