V.A. Cherepanov

Quintuple perovskites Ln2Ba3Fe5−xCoxO15−δ (Ln = Sm, Eu): nanoscale ordering and unconventional magnetism

Quintuple perovskites Ln2Ba3Fe5−xCoxO15−δ, with Ln = Sm, Eu have been synthesized for x values varying from 0 to 2. The HRTEM and HAADF investigations show that these oxides are ordered at the nanoscale with a tetragonal “ap × ap × 5ap” lattice, corresponding to an ordered fivefold stacking of the Ba and Ln layers. These structures are in fact chemically twinned, so that only cubic or pseudo-cubic symmetry is detected by XRD investigation. The detailed magnetic study shows the existence of short range antiferromagnetic ordering, canted in nature in the temperature range from 5 to 330 K. This particular behavior is interpreted on the basis of local antiferromagnetic M–O–M (M = Fe/Co) interactions within the nanodomains that are limited by pinning of the Co/Fe spins at the boundaries.

Homogeneity range, oxygen nonstoichiometry, thermal expansion and transport properties of La2−xSrxNi1−yFeyO4+δ

A series of La2−xSrxNi1−yFeyO4+δ complex oxides adopting the K2NiF4-type structure (sp.gr. I4/mmm) was prepared via the decomposition of citrate–nitrate precursors, followed by multiple annealing treatments at 1100 °C in air. Strontium for lanthanum substitution in La2−xSrxNi1−yFeyO4+δ leads to a progressive increase of iron solubility (y) which reaches maximum values at x = 1.0–1.2. The crystal structure of single-phase samples was refined by the Reitveld method. The unit cell volume increases with y and decreases with x in La2−xSrxNi1−yFeyO4+δ. The solid solutions of La2−xSrxNi1−yFeyO4+δ were shown by TGA to be over-stoichiometric (δ > 0) at x = 0.5 and 0.6 and oxygen deficient (δ < 0) at x = 0.8 within the temperature range of 25–1050 °C in air. The thermal expansion coefficient of La2−xSrxNi1−yFeyO4+δ increases with x and y, varying within the range of (12–16) × 10−6 K−1 up to 700 °C in air. In the higher temperature range, the value of the TEC increases up to 20 × 10−6 K−1 due to a chemical expansion contribution. The total conductivity and Seebeck coefficient were measured in air from RT to 1100 °C. The maximum conductivity value equal to 289 S cm−1 was obtained for La1.2Sr0.8Ni0.9Fe0.1O4+δ at 460 °C in air. The conduction is temperature activated for all samples within the composition range under study. The temperature dependencies of the Seebeck coefficient were explained in the approximation of the small-polaron hopping mechanism. The charge carriers were electron holes localized on nickel forming Ni3+ cations in low- and high-spin states.

Tuning oxygen content and distribution by substitution at Co site in 112 YBaCo2O5+δ: impact on transport and thermal expansion properties

Polycrystalline “112” ordered oxygen deficient double perovskites YBaCo2−xMexO5+δ (Me = Fe, Cu, Ni) were synthesized by a glycerol-nitrate route with 0.0 ≤ x ≤ 0.7 for Me = Fe, 0.0 ≤ x ≤ 0.6 for Me = Cu and x = 0.1 for Me = Ni. The combined X-ray diffraction, electron microscopy and thermo-gravimetric studies show that all these oxides exhibit the ap × ap × 2ap tetragonal structure (S.G. P4/mmm); moreover, the oxygen content increases continuously with x in the iron-substituted oxide YBaCo2−xFexO5+δ, whereas the opposite is observed for the copper phase YBaCo2−xCuxO5+δ. This difference, which is due to the more electropositive character of Fe3+ compared to Cu2+, is hindered in the Ni2+ case due to its inability to accommodate the pyramidal coordination. The changes of the conductivity of these compounds versus temperature are closely related to their oxygen loss, in agreement with a defect structure model suggested earlier. Thermal expansion measurements prove the absence of phase transition in all oxides within the temperature range studied. The chemical compatibility of YBaCo1.4Fe0.6O5+δ with the electrolyte Ce0.8Sm0.2O2−δ is also demonstrated.

Phase equilibria, crystal structure at 1373 K and properties of complex oxides in the Nd–Co–Fe–O system

Phase equilibria in the Nd–Co–Fe–O system were systematically studied at 1373 K in air. The homogeneity range and crystal structure of solid solution NdCo1–xFexO3 (0.0 ≤ x ≤ 1.0) have been studied by the X-ray powder diffraction method. The structural parameters of complex oxides have been refined by the full-profile Rietveld method. It was shown that all oxides reveal practically stoichiometric oxygen composition within the entire temperature range under investigation. The values of thermal expansion coefficients for the cobaltites NdCo1–xFexO3 (x = 0.3, 0.7) have been calculated within the wide temperature range in air. Chemical stability of NdCo1–xFexO3 (x = 0.3, 0.7) in respect to the solid electrolyte materials (Ce0.8Sm0.2O2–δ and La0.88Sr0.12Ga0.82Mg0.18O3-δ) was examined. Electrical conductivity of NdCo1–xFexO3 (x = 0.3, 0.7) was measured as a function of temperature within the range 300–1373 K in air. It was shown that substitution of cobalt for iron leads to the decrease of conductivity. The isothermal-isobaric cross-section of the phase diagram for the Nd–Co–Fe–O system at 1373 K in air has been presented.

Specific features of phase equilibriums in Ln–Ba–Fe–O systems

Formation of five-layered Ln2–εBa3+εFe5O15–δ phases [exhibiting nanoscale ordering with layer-by-layer location of the cations in the Ln–Ba–(Ln,Ba)–(Ln,Ba)–Ba–Ln perovskite-type structure] has occurred in the Ln–Ba–Fe–O (Ln = Y, Pr, Nd, Sm, Eu, and Gd) systems at 1100°С in air. Partial substitution of iron with cobalt (Ln2–εBa3+εFe5–yCoyO15–δ, Ln = Nd, Sm, Eu) has stabilized formation of the ordered structure. The oxygen content in the complex oxides has been determined in air over a wide temperature range by means of high-temperature thermogravimetry and iodometric titration. The change in oxygen content with temperature for the phases with five-layered ordering was significantly smaller than for the disordered phases.

Crystal Structure and Physicochemical Properties of Doped Lanthanum Manganites

Substituted lanthanum-strontium manganites La0.7Sr0.3Mn0.9Me0.1O3 ± δ (Me = Ti, Cr, Fe, and Cu) are obtained by standard ceramic and glycerin-nitrate techniques. High-temperature powder X-ray diffraction is employed to study the crystal structure of La0.7Sr0.3Mn0.9Me0.1O3 ± δ oxides. It is shown that in the range 298–1023 K in air, La0.7Sr0.3Mn0.9Me0.103 ± δ manganites crystallized in an orthorhombic cell (space group R-3c). The isobaric temperature dependences of unit cell parameters are determined. Thermal expansion coefficients are calculated for La0.7Sr0.3Mn0.9Me0.103 ± δ oxides. The conductivity of La0.7Sr0.3Mn0.9Me0.103 ± δ is studied as a function of temperature in the range 500 K ≤ T ≤ 1200 K in air. It is shown that substituting 3d metal for manganese considerably lowers the conductivity of basic La0.7Sr0.3Mn0.9O3 ± δ. The chemical stability of iron-substituted manganite La0.7Sr0.3Mn0.9Fe0.1O3 ± δ is studied with respect to the electrolyte material.

Nanoscale Ordering in Oxygen Deficient Quintuple Perovskite Sm2 ϵBa3+ϵFe5O15-δ revealed by TEM

The introduction of two sorts of cations with different valence and size, such as Ba2+ or Sr2+ and Ln3+, in the A-sites of transition metal perovskite oxides has generated numerous remarkable properties such as high Tc superconductivity, oxygen storage in cobalt based oxides for the realization of solid oxide fuel cell (SOFC) cathodes , CMR in manganates . The investigation of the system Sm-Ba-Fe-O in air has allowed an oxygen deficient perovskite Sm2-eBa3+eFe5O15-δ (δ=0.75, e=0.125) to be synthesized. In contrast to the XRPD pattern which gives a cubic symmetry (ap= 3.934A), the ED patterns of this phase (Fig.1a) show superstructure spots corresponding to c=5ap”. HRTEM study (Fig1b) revealed that this phase is nanoscale ordered with a quintuple tetragonal cell, “ap ′ ap ′ 5ap”. Bearing in mind that one cubic cell corresponds to the formula Sm0.375Ba0.625FeO2.85, these tetragonal nanostructures can be formulated as Sm2-eBa3+eFe5O15-δ (δ˜0.75, e˜0.125). They consist of 5 SmO/BaO layers stacked alternately with 5 FeO2 layers along c. The HAADF-STEM image (Fig.1c) of the Sm2-eBa3+eFe5O15-δ structure along the [100] is clearly established from the contrast segregation that the Ba2+ and Sm3+ cations are ordered in (001) layers along the c-axis. One observes rows of bright dots perpendicular to c, which corresponds to three sorts of Sm or Ba cationic layers, judging from their intensity: pure Sm, mixed Ba/Sm and pure Ba layers. Thus the HAADF-STEM image can be interpreted by the following periodic stacking sequence of the A cationic layers along the c axis: “Sm-Ba-Sm/Ba-Sm/Ba-Ba-Sm”. It appears from the ABF-STEM images along [100] (Fig.1d) and [110] orientations (Fig.1e) of a single Sm2-eBa3+eFe5O15-δ domain, that the oxygen positions in all the layers are close to the ideal octahedral positions. However, a closer inspection of the images reveals that the oxygen columns in the equatorial positions close to the Sm layer deviate from their ideal octahedral position, and lie closer to the Sm3+ cations yielding a “zigzag” contrast along [100] and [110]. The “Sm-Ba-Sm/Ba-Sm/Ba-Ba-Sm” chemical ordering is also confirmed by elemental EELS mapping (Fig.2a,b). The spatially resolved EELS data show that the O-K edge spectra corresponding to the “FeO2” planes (labeled A,B,C) exhibit different intensity ratios of the two pre-peaks to the O-K edge, prepeak1/ prepeak2 at approximately 529/531 eV, depending on the nature of the surrounding “Sm,Ba” layers (Fig.2c). The O-K fine structure in the Sm plane is very similar to that of plane A, whereas those of the Ba and Ba/Sm planes are similar to B and C planes respectively. A first observation is that pre-peak 1 at ˜529 eV is less intense for oxygen anions close to Sm cations (SmO layers as well as A layers). According to the literature, the height of this pre-peak is generally rather independent of the rare earth element and should be around the same height as pre-peak 2. Pre-peak 2, related to Fe3d eg – O2p hybridized states seems invariant in the structure, apart from the c plane where it is slightly subdued, accompanied by an increase of pre-peak 1 below 530eV. Pre-peak 1 can be attributed to a charge transfer from the eg to the t2g band of Fe (the eg band is usually empty for Fe3+). This increase of pre-peak 1 related to Fe3d t2g – O2p hybridized states is also visible in the Ba/Sm mixed layers, and can be linked to the presence of oxygen vacancies in those planes. This peak is stronger in the C plane suggesting the presence of more vacancies in this plane.The spatially resolved EELS spectra of the Fe-L2,3 edge are plotted in Fig. 2c. The Fe L3 and L2 “white lines” arise from transitions of 2p3/2 3d3/23d5/2 (L3) and 2p1/2 3d3/2 (L2) and are known to be sensitive to valency and coordination. Our data shows that the A and B FeO2 planes exhibit very similar Fe-L2,3 edges, with an L3 peak maximum at 709.5 eV, and a pre-peak to L3, even if faint at 708 eV. The energy position of the L3 maximum, together with the shape and positions of the L3 and L2 are then indicative of Fe3+ in an octahedral coordination. All the acquired Fe L3 edges are significantly broadened with respect to the plotted references for 6-fold, 5-fold and 4-fold coordinated Fe3+. This broadening can be explained by a change in coordination of the Fe atoms. Bearing in mind that the measured oxygen stoichiometry is 14.25, instead of 15, this suggests that the iron coordination is mainly 6, i.e. octahedral, but may also be mixed with the presence of some FeO5 pyramids in those layers. The nanoscale ordering of this perovskite explains its peculiar magnetic properties on the basis of antiferromagnetic interactions with spin blockade at the boundary between the nanodomains. The variation of electrical conductivity and oxygen content of this oxide versus temperature suggest potential SOFC applications. Keywords: quintuple perovskite; HAADF-STEM; ABF-STEM; EELS maping; ordering

Coherent intergrowth of simple cubic and quintuple tetragonal perovskites in the system Nd{sub 2−ε}Ba{sub 3+ε}(Fe{sub ,}Co){sub 5}O{sub 15−δ}

Abstract Investigation of the Nd2−eBa3+e(Fe,Co)5O15−δ system, combining X-ray diffraction and electron microscopy, has allowed a tetragonal quintuple ordered perovskite “ap×ap×5ap” phasoid inter-grown within a single cubic perovskite matrix to be evidenced for e=0. This nanoscale chemically twinned perovskite is compared with other members, Ln=Sm, Eu, Pr. The unusual long range ordering of the layers develops strains due to size mismatch between Ba2+ and Ln3+ cations. Importantly, two factors allow the strains to be decreased: (i) special intergrowths of double (LnBaFe2O6−δ) and triple (LnBa2Fe3O9−δ) perovskite ribbons/layers oriented at 90°, (ii) nanoscale chemical twinning. The spin locking effect of the nano-domain boundaries upon the magnetic properties of these perovskites is discussed.

Nanoscale ordering in oxygen deficient quintuple perovskite Sm_{2-\epsilon}Ba_{3+\epsilon}Fe_{5}O_{15-\delta} : implication for magnetism and oxygen stoichiometry

The investigation of the system Sm–Ba–Fe-O in air has allowed an oxygen deficient perovskite Sm2-eBa3+eFe5O15-δ (δ = 0.75, e = 0.125) to be synthesized. In contrast to the XRPD pattern which gives a cubic symmetry (ap = 3.934 A), the combined HREM/EELS study shows that this phase is nanoscale ordered with a quintuple tetragonal cell, “ap × ap × 5ap”. The nanodomains exhibit a unique stacking sequence of the A-site cationic layers along the crystallographic c-axis, namely “Sm–Ba–Ba/Sm–Ba/Sm–Ba–Sm”, and are chemically twinned in the three crystallographic directions. The nanoscale ordering of this perovskite explains its peculiar magnetic properties on the basis of antiferromagnetic interactions with spin blockade at the boundary between the nanodomains. The variation of electrical conductivity and oxygen content of this oxide versus temperature suggest potential SOFC applications. They may be related to the particular distribution of oxygen vacancies in the lattice and to the 3d5L configuration of iron.