Oxygen-deficient Perovskite Research Articles

Synthesis and characterization of La 4BaCu 5O 13+ δ and La 4BaCu 5− xMxO 13+ δ: M=Fe, Co, Ni, Zn

La 4BaCu 5− x M x O 13+ δ : M=Fe, Co, Ni, Zn were prepared by the solid-state route at 1000°C. Solid solution limits of x=1.0(1) [Fe], x=1.1(1) [Co], x=1.56(7) [Ni] and x=0.47(1) [Zn] were determined from XRD and EPMA results. Rietveld refinement of combined XRD/neutron powder diffraction data was carried out on undoped La 4BaCu 5O 13+ δ and x=1 for M=Fe, Co, Ni. For La 4BaCu 5O 13+ δ , which is an oxygen-deficient perovskite, the presence of square planar CuO 4 groups, disordered over the Cu(2) sites with CuO 5 square pyramids, is indicated, together with, for δ<0, either square planar CuO 4 or square pyramidal CuO 5 and octahedral CuO 6 groups disordered over the Cu(1) sites. For M=Fe, Ni, there was preferential substitution onto the one-fold octahedral site; for M=Co, substitution took place on both the one-fold octahedral and four-fold square pyramidal sites.

Magnetically tunable nonlinear current–voltage characteristics in oxygen deficient manganite perovskites

La 0.667 Ca 0.333 Mn 1− x M x O 3− δ (M=Mg, Li or Re) exhibit insulating behaviour and nonlinear current–voltage ( J – E ) relationship with voltage-limiting characteristics at temperatures below the ferromagnetic transition ( T c ). The high current region is set in at field strengths <60 V/cm. Nonlinearity exponent, α in the relation J = kE α increases inversely with temperature. In presence of an external magnetic field, the J – E curves show higher current density at lower field strengths. Microstructural studies indicate that there is no segregation of secondary phases in the grain boundary regions. There is remarkable changes in ρ ( T ) as well as J – E curves with the grain size. Annealing studies in lower p O 2 atmospheres indicate that there is significant out-diffusion of oxygen ions through the grain boundary layer (GBL) regions creating oxygen vacancies in the GBL regions. The concentration of Mn 4+ ions is lowered at the GBL due to oxygen vacancies, reducing the probability of hopping and resulting in insulating behaviour. Therefore an insulating barrier is introduced between two conducting grains and the carrier motion between the grains is inhibited. Thus below T c , where sufficient increase in resistivity is observed the conduction may be arising as a result of spin dependent tunneling across the barrier. External electric field lowers the barrier height and establishes carrier transport across the barrier. Above certain field strength, barrier height diminishes significantly and thereby allowing large number of carriers for conduction, giving rise to highly nonlinear conductivity.

In-situ diffraction study of Ba2In2O5

Abstract In-situ neutron and X-ray diffraction experiments were used to determine the crystal structure of Ba 2 In 2 O 5 as related to temperature. Ba 2 In 2 O 5 has a Brownmillerite-type crystal structure from room temperature to 900 °C, consisting of an orthorhombic unit cell with lattice parameters 6.0864(3)×16.7903(7)×5.9697(3) A and Icmm (74) space group symmetry. At 900 °C, oxygen vacancies begin to disorder. By 925 °C, barium indate crystallizes in a tetragonal, 6.0348(4)×17.0688(22) A, unit cell with I4cm (108) space group symmetry. This oxygen vacancy order–disorder transition is associated with an evolution to fast oxide-ion conduction. At 1040 °C, Ba 2 In 2 O 5 becomes a cubic oxygen-deficient perovskite, 4.2743(1) A, in the Pm3m space group. At 1080 °C, Ba 2 In 2 O 5 begins to decompose.

High-Temperature Sorption Process for Air Separation and Oxygen Removal

This paper describes the concept of a new, efficient high-temperature oxygen sorption process based on a perovskite-type ceramic sorbent for oxygen removal and air separation. The new sorption process takes advantage of the unique properties of certain perovskite-type ceramics that can adsorb a large quantity of oxygen, but not other gases, at high temperatures (300−800 °C). The essential principle of this new sorption process is based on the changing of oxygen nonstoichiometry of the perovskite-type ceramics with temperature and oxygen partial pressure. Two highly oxygen-deficient perovskite oxides, La0.1Sr0.9Co0.5Fe0.5O3-δ, and La0.1Sr0.9Co0.9Fe0.1O3-δ, were examined as candidate materials for the oxygen sorption process. Oxygen sorption equilibrium properties were studied by thermogravimetric analysis (TGA) at 500 and 600 °C and oxygen pressures ranging from 1.3 × 10-4 to 1 atm. The oxygen removal performance at 500 and 600 °C was also investigated in a fixed-bed adsorption column. An infinitely large ...

Spin, charge, and orbital ordering in the ferrimagnetic insulatorYBaMn2O5

The oxygen-deficient (double) perovskite YBaMn$_2$O$_5$, containing corner-linked MnO$_5$ square pyramids, is found to exhibit ferrimagnetic ordering in its ground state. In the present work we report generalized-gradient-corrected, relativistic first-principles full-potential density-functional calculations performed on YBaMn$_2$O$_5$ in the nonmagnetic, ferromagnetic and ferrimagnetic states. The charge, orbital and spin orderings are explained with site-, angular momentum- and orbital-projected density of states, charge-density plots, electronic structure and total energy studies. YBaMn$_2$O$_5$ is found to stabilize in a G-type ferrimagnetic state in accordance with experimental results. The experimentally observed insulating behavior appears only when we include ferrimagnetic ordering in our calculation. We observed significant optical anisotropy in this material originating from the combined effect of ferrimagnetic ordering and crystal field splitting. In order to gain knowledge about the presence of different valence states for Mn in YBaMn$_2$O$_5$ we have calculated $K$-edge x-ray absorption near-edge spectra for the Mn and O atoms. The presence of the different valence states for Mn is clearly established from the x-ray absorption near-edge spectra, hyperfine field parameters and the magnetic properties study. Among the experimentally proposed structures, the recently reported description based on $P$4/$nmm$ is found to represent the stable structure.

Induction of Colossal Magnetoresistance in the Double Perovskite Sr2CoMoO6

Sr2CoMoO6 perovskite has been prepared in polycrystalline form by thermal treatment, in air, of previously decomposed citrate precursors. This material has been studied by X-ray (XRD) and neutron powder diffraction (NPD), thermal analysis, and magnetic, magnetotransport, and Hall effect measurements. At room temperature, the crystal structure is tetragonal with a space group I4/m, with a = 5.565 03(5) and c = 7.948 10(8) A. The crystal contains alternating CoO6 and MoO6 octahedra, tilted by 6.8° in the basal ab plane. Magnetic measurements indicate an antiferromagnetic ordering below TN = 37 K. As prepared, the sample is an electrical insulator. The topotactic reduction of the stoichiometric sample, in H2/N2 flows, leads to oxygen-deficient double perovskites, Sr2CoMoO6-δ, δ = 0.03 and 0.14. The magnetic behavior of the reduced samples suggests the presence of ferromagnetic domains characterized by Curie temperatures of TC = 350−370 K. The conductivity dramatically increases upon H2 reduction; the number ...

Cationic ordering of small RE3+ cations (RE=Y, Ho, Er, Yb) in the cuprates La1.9−xRExSr(Ca)1.1Cu2O6−δ: similarities between the oxygen-deficient P2/RS and T* structures

The partial substitution of small rare-earth RE3+ cations (RE=Y, Ho, Er, Yb) for La3+ in the cuprates La1.9Sr(Ca)1.1Cu2O6 results in ordering phenomena of the A cations in the oxygen-deficient double perovskite–rocksalt P2/RS intergrowth structure. The preference of these RE3+ cations for the cubic-like 8-fold coordination in the middle of the P2 blocks (A1 sites) is the driving mechanism of formation of the solid solutions La1.9−xRExSr(Ca)1.1Cu2O6−δ. The homogeneity range is rather wide in the case of Sr: xmax=0.5 (Yb)→0.8 (Ho), as its extent is much more limited in the case of Ca: xmax=0.15 (Yb)→0.3 (Ho), owing to a competition between RE3+ and Ca2+ for the occupancy of the A1 sites. In the Sr–Ho solid solution, Ho3+ is found both in the A1 and A2 sites (9-fold coordination): this can be understood in terms of a not so large destabilization of the 4f10 electronic structure of Ho3+ in the crystal field of the A2 sites. The thermal decomposition of La1.2Sr1.1Er0.7Cu2O5.95 at 1050°C gives rise to the formation of a T*-type cuprate whose likely composition is La1Sr0.3Er0.7CuO3.85, accompanied by a T-type cuprate and CuO. Such a result, never obtained up to now, exemplifies the strong geometrical similarities between the A–O polyhedral networks of the P2/RS and T* structures, namely: A[P2/RS]≡A[T*]+12A[T].

Synthesis, Structure, and Physical Properties of Cobalt Perovskites: Sr3CoSb2O9 and Sr2CoSbO6−δ

The perovskites Sr3CoSb2O9, Sr2CoSbO6, and Sr2CoSbO5.63 have been synthesized in polycrystalline form by solid state reactions from acetate precursors as obtained by freeze-drying. The crystal structure of insulating Sr3CoSb2O9 has been refined from X-ray data by the Rietveld method and was found to be orthorhombic (Immm) with a=5.6411(1) Å, b=5.6610(1) Å, c=7.9829(2) Å. From the molar susceptibility curves an effective magnetic moment μeff=5.39μB and a spin-glass transition at ca. 6 K are deduced. The semiconducting perovskite Sr2CoSbO6 was synthesized by applying an elevated O2 pressure (3 kbar), and Co exclusively in the oxidation state +3 was attained. The Rietveld refinement of this compound has revealed a rhombohedral cell (R3m) with a=5.5992(2) Å and c=13.6609(2) Å and partial order (77%) of the B cations. An effective magnetic moment per formula unit of μeff=3.83μB and a spin-glass transition at ca. 47 K were observed. A semiconducting and oxygen-deficient perovskite, Sr2CoSbO5.63, was obtained by annealing Sr2CoSbO6 at 950°C in Ar. The Rietveld refinement proved it to be a double-cubic perovskite (Fm3m) with a=7.9658(1) Å, and a higher order of Co and Sb at the B sites (84%) was found, due to the formation of Co2+. From the magnetic susceptibility data an effective moment of 4.85μB and a spin-glass transition at ca.17 K were deduced.

Magnetoresistance in the ferromagnetic metallic perovskite SrFe1−xCoxO3

Abstract The investigation of the magnetic and transport properties of the oxygen deficient perovskites SrFe1−xCoxO3−δ shows that these compounds exhibit both ferromagnetism and metallicity in a wide compositional range (0≤x≤0.70). Negative magnetoresistance is evidenced for the first time in these oxides, in contrast to SrCoO3−δ. These properties are explained by superexchange interactions between cobalt and iron according to the scheme Fe3+OCo4+↔Fe4+OCo3+. This model is strongly supported by 57Fe Mossbauer measurements which show the existence of two sites at room temperature, high spin localized Fe4+ and delocalized Fe3+α sites, whereas magnetic disordering suggesting spin fluctuations is observed at 5 K as soon as cobalt is introduced into the SrFeO3 structure.

Synthesis and structure of Sr2MnGaO5, a new layered manganese oxide

A new layered manganese oxide, Sr2MnGaO5, has been synthesised and Rietveld analysis of X-ray powder diffraction data has shown it to be an oxygen deficient perovskite with the brownmillerite structure: a = 5.5033(6) A, b = 16.234(2) A, c = 5.3717(6) A. Incomplete order of the oxygen vacancies is best described using the space group Icmm. The oxygen content can readily be varied to form Sr2MnGaO5 + δ (0 ≤ δ ≤ 0.5) to provide Mn oxidation states between Mn(III) and Mn(IV).

Strontium tantalates with a perovskite structure : Their conductivity and high-temperature interaction with water

The interaction of perovskite-like solid solutions Sr6 – 2xTa2 + 2xO11 + 3x(x= 0–0.28) with water is studied, along with dependences of the solutions' conductivity on their composition and the atmosphere's temperature and humidity. The Sr6 – 2xTa2 + 2xO11 + 3xphases with high concentrations of structural oxygen vacancies are high-temperature mixed oxygen–hydrogen ionic conductors whose conduction is sensitive to the presence of water vapor up to 900°C. According to a thermogravimetric study, the amount of water incorporated into the complex-oxide matrix is proportional to the concentration of structural oxygen vacancies. The process of water incorporation is considered in terms of crystalline and chemical properties of the structure. The oxygen-deficient perovskites containing coordination-unsaturated metalatoms can reconstruct their coordination polyhedron by adding water molecules, with subsequent partial dissociation of water to hydroxyl groups. The proposed mechanism explains different states of water in the oxide and a two-stage nature of its removal: water molecules coordinating the metal atom and those surrounding OH–leave the core in the first and second stages, respectively.

Electrical properties of the oxygen-deficient perovskites, Ca2Mn2 − xNbxOγ: 0 ≤ x ≤ 1.2, with Mn valence varying from +2.0 to +4.0

The electrical conductivity of a range of ceramics of general formula Ca2Mn2 − xNbxOγ, with x and γ as variables, was measured using impedance spectroscopy. All materials were semiconducting but conductivity decreased with both increasing x and decreasing γ, both of which involved a gradual decrease in oxidation state of Mn from +4 in CaMnO3 to +2 in Ca2Mn0.8Nb1.2O5.8. Possible factors that control the conductivity are discussed; electrons in eg orbitals of Mn appear not to be involved directly; instead, conduction appears to depend either on the degree of t2g–Opπ–t2g orbital overlap or on σ d–p overlap, with donation of electrons from oxygen into the bonds involving eg orbitals. In both cases, the conductivity is influenced mainly by Mn–O–Mn distances and unit cell dimensions. For compositions of intermediate oxygen content γ, it was difficult to obtain materials that were electrically homogeneous due to oxygen content gradients; thus, samples prepared by partial reduction in H2 were more heavily reduced at the grain surfaces than in the grain interiors. Since the conductivity was very sensitive to γ, impedance spectroscopy provided a valuable method to investigate oxygen inhomogeneity. Two types of sample inhomogeneity were characterized: those with reduced surfaces and oxidized cores and vice versa, those with oxidized surfaces and reduced cores.

Stoichiometry and properties of new calcium–ferrite-based phases

Phase formation studies in the system Ca 2Fe 2− x Nb x O γ :0⩽ x⩽2.0 indicated the presence of two new phases, a cubic solid solution with a variable compositional extent x from 0.45⩽ x⩽0.65 and an orthorhombic phase for x=1.0. Cubic solid solution has variable oxygen content with an oxygen-deficient perovskite structure. The oxidation state of Fe was investigated by Mössbauer spectroscopy; as synthesized, for x=0.6 it contains a mixture of Fe 4+ and Fe 3+ in the ratio 13/87. Its oxygen content can be modified over the range 5.55< γ<5.89 by heat treatments under a range of conditions. For most γ values, the crystal structure was that of a cubic perovskite; at high oxygen contents, γ>5.7, transformation to an orthorhombic structure was observed. Electrical properties range from semiconducting at high γ to insulating at low γ. The orthorhombic phase Ca 2FeNbO 6 has the GdFeO 3 structure. Atomic positions and occupancies were determinated by Rietveld refinements. Mössbauer analysis revealed only the presence of Fe 3+ in this sample. Although Ca 2FeNbO 6 was heated under a range of conditions, its oxygen content could not be modified. The conductivity of Ca 2FeNbO 6 corresponds to that of an insulator material. Magnetic susceptibility data indicated an antiferromagnetic behavior.

Synthesis and Study of Hg-Based 1222-Type Superconducting Cuprates (Hg, M)Sr2(Ln, Ce)2Cu2Oz

A novel series of “1222-type” copper oxides described by the formula (Hg, M)Sr2(Ln, Ce)2Cu2Oz (M=Ti, V, Cr, Mo, W, Re; and Ln=Sm, Eu, Gd, Dy, Ho, Er) have been synthesized under high pressure. The structure was determined by combination of X-ray powder diffraction, electron diffraction and high-resolution electron microscopy measurements. The materials are isostructural with (Pb, Cu)-1222, crystallizing in a tetragonal unit cell, space group I4/mmm. The compounds adopt the 1222-type structure with an intergrowth of three types of layers: oxygen-deficient perovskite (CuO5 pyramidal layers), double fluorite-type layers, and double rock-salt layers. A correlation between lattice parameters and effective ionic radii of the (Ln, Ce) site is determined for (Hg0.75W0.25)Sr2(Ln1.5Ce0.5)Cu2Oz, Ln=Sm–Er. Except for Cr-containing compound, bulk superconductivity is observed in all the synthesized compounds. The value of Tc is related to both the effective ionic radius of the (Ln, Ce) site and the valence of the M-element. Among these compounds, the sample with nominal composition of (Hg0.75W0.25)Sr2(Gd1.5Ce0.5)Cu2Oz recorded the highest Tc up to 75 K.

Fast oxygen ion conductors—from doped to ordered systems

This contribution briefly introduces the traditional acceptor doping of oxides, charge compensated by oxygen vacancies to bring about the oxygen ion conduction utilised in solid oxide electrolytes. It is next shown how a simple defect–chemical approach to defect–defect interactions is a useful first-approximation tool in the interpretation of data and design of improved materials, e.g. by optimising dopants. Moreover, it is shown how the trapping of vacancies at acceptor defects can be avoided altogether if the cation sublattice can be made to order completely. This is realised in a few known systems and we illustrate the principle using the oxygen deficient complex perovskite Sr4(Sr2Nb2)O11 as an example, in which the Sr and Nb on the B-site sublattice can be perfectly ordered into a NaCl-structure-type arrangement, leaving all oxygen ion sites equivalent, without trapping sites. The design of new materials should thus aim at cation order, and the possibility of achieving this is discussed also for pyrochlore type oxides. A Kröger–Vink type notation for an inherently deficient sublattice without defects in the other sublattices is proposed.

Magnetism of oxygen deficient perovskite La 8− xSr xCu 8O 20

We have measured muon spin relaxation (μSR) of La 8− x Sr x Cu 8O 20, an oxygen deficient perovskite, across two magnetic phase transitions at T= T c1 and T c2. Static magnetic order was observed below T∼140 K, where weak ferromagnetism appears in magnetization measurements ( T c1). No significant change appeared in the μSR static field width at the lower transition temperature T c2, where an anomaly is seen in the Hall coefficient. The static magnetism thus originates from localized spins presumably in the one-dimensional Cu–O chains.

Metal-insulator transition induced by a spin-state transition inTbBaCo2O5+δ (δ=0.5)

Temperature variation of the crystal structure has been investigated for ${\mathrm{TbBaCo}}_{2}{\mathrm{O}}_{5+\ensuremath{\delta}} (\ensuremath{\delta}=0.5),$ which shows a metal-insulator (MI) transition at ${T}_{\mathrm{MI}}\ensuremath{\approx}340 \mathrm{K}.$ The compound has an oxygen-deficient perovskite structure, with alternating ${\mathrm{CoO}}_{6}$ octahedral and ${\mathrm{CoO}}_{5}$ pyramidal chains along the crystallographic $b$ axis. We have found that both the pyramids and octahedra are significantly distorted in the insulating phase below ${T}_{\mathrm{MI}},$ suggesting a ${d}_{{3x}^{2}\ensuremath{-}{r}^{2}}{/d}_{{3y}^{2}\ensuremath{-}{r}^{2}}\ensuremath{-}\mathrm{type}$ orbital ordering.

A solid oxide fuel cell based on Sr- and Mg-doped LaGaO 3 electrolyte: the role of a rare-earth oxide buffer

In this paper we review a systematic study on the properties of the superior oxide-ion conductor Sr- and Mg-doped LaGaO 3 (LSGM) and its performance in a single fuel cell. The conductivity of the oxygen-deficient perovskite phase was shown to be purely ionic over a wide range of oxygen partial pressures 10 −22≤ P O 2 ≤1 atm. The highest values of the oxide-ion conductivity, σ o=0.17, 0.08 and 0.03 S/cm, were found for La 0.8Sr 0.2Ga 0.83Mg 0.17O 2.815 at 800, 700 and 600°C, respectively; they remained stable over a weeklong test. The reactivity of Ni and LSGM suggested use of a thin interlayer at the anode–electrolyte interface to prevent formation of lanthanum nickelates; Ce 0.8Sm 0.2O 1.9 (SDC) was selected for the interlayer. The peak power density of the interlayered cell is 100 mW/cm 2 higher than that of the standard cell without the interlayer. This improvement is due to a significant reduction of the anode overpotential; the overpotential of the cathode La 0.6Sr 0.4CoO 3− δ (LSCo) remained unchanged. Comparison of the peak power density in this study and with that of a previous study, also with a 500-μm-thick electrolyte, indicates a factor of two improvement, i.e. from 270 mW/cm 2 to 550 mW/cm 2 at 800°C. The excellent cell performance showed that an LSGM-based SOFC operating at temperatures T op≤800°C is a realistic goal.

Atomic Scale Characterization Of Interfaces And Defects In Non-Stoichiometricmulticomponent Oxides

Abstract The ability to obtain chemical information with (near) atomic resolution has recently become possible by a combined approach of Z-contrast imaging with electron energy-loss spectroscopy (EELS) in scanning transmission electron microscopes. This method is particularly interesting for the characterization of structure-property relationships in novel multicomponent oxides, which possess added functionality due to their high nonstoichiometry. In this paper we demonstrate the capabilities of this method in analyzing the microstructural mechanisms of accommodation of non-stoichiometry, using two example systems: (Ba,Sr)TiO3thin films for DRAM applications, grown by MOCVD with different amounts of excess titanium, and an oxygen-deficient perovskite ceramic, SrCoOx. The experiments were performed in a JEOL JEM 201 OF field emission transmission electron microscope, operating at 200 kV, equipped with an annular dark-field detector, scanning unit and a post-column imaging filter (Gatan GIF 200). This microscope has been demonstrated to achieve probe sizes of under 1.5 Å.

A New Ordered Oxygen-Deficient Manganite Perovskite: LaBaMn2O5.5. Crystal and Magnetic Structure

A new ordered oxygen-deficient perovskite, LaBaMn2O5.5, has been synthesized, using the topotactic deoxygenation of the ordered LaBaMn2O6 perovskite. The crystal structure of this manganite was solved in the space group Ammm (a = 3.86 Å, b = 8.19 Å, c = 15.47 Å) from neutron powder diffraction data. The [Mn2O5.5]∞ framework is build up of row of MnO6 octahedra and MnO5 pyramids running along a⃗. This structure is intermediate between that of the ordered LaBaMn2O6 perovskite and that of LaBaMn2O5: it consists of ordered barium and lanthanum layers stacked alternatively along c⃗, oxygen vacancies being located at the level of the La layers. The magnetic structure is original: manganese spins form ferromagnetic spin-ladders along the b⃗ axis that are antiferromagnetically coupled along the a⃗ and c⃗ axis. The HREM investigation shows the existence of 90° oriented domains and twinning phenomena.