Perovskite-related Structure Research Articles

Strong reddish-orange light emission from stress-activated Srn+1SnnO3n+1:Sm3+ (n = 1, 2, ∞) with perovskite-related structures

We report mechanoluminescence (ML) in Sm3+-doped Srn+1SnnO3n+1 (n = 1, 2, ∞) phosphors with perovskite-related structures, which consist of layered units depending on the number of n. The intensity of the ML was strongly dependent on the layered structure; the ML intensity for Sr3Sn2O7:Sm3+ was three orders of magnitude higher than that for SrSnO3:Sm3+. The charge transfer state (CTS) band in the photoluminescence excitation spectra was observed for Sr3Sn2O7:Sm3+ and Sr2SnO4:Sm3+, indicating that the efficient energy transfers from the host to the Sm3+ ions. The formation of CTS and the layered structure may be responsible for this ML enhancement.

Effect of structural packing on the luminescence properties in tungsten bronze compounds M2KNb5O15 (M=Ca, Sr, Ba)

Tungsten bronze compounds M2KNb5O15 (M=Ca, Sr, Ba) were successfully synthesized, and the luminescence properties were investigated. Among the three compounds, Ca2KNb5O15 showed an obviously broad band of host luminescence at 460nm with exciting at 269nm. By doping Eu3+ into the M sites, Ca2KNb5O15:Eu3+ displayed strong red emission from Eu3+ ions characteristic transitions, nearly four times higher than Sr2KNb5O15:Eu3+ and seven times higher than Ba2KNb5O15:Eu3+. Crystal packing factor (PF) was introduced to account for this luminescence difference, lower PF being correlated to higher luminescence intensity for perovskite-related structure. Both the as-prepared compounds and the literature examples were proved to fit this correlation. This can be explained through the influence of the structural packing on the environment distortion and crystal field splitting of the doping site.

Effect of calcium stoichiometry on the dielectric response of CaCu3Ti4O12 ceramics

Ca x Cu 3 Ti 4 O 12 ( x = 0.90, 0.97, 1.0, 1.1 and 1.15) polycrystalline powders with variation in calcium content were prepared via the oxalate precursor route. The structural, morphological and dielectric properties of the ceramics fabricated using these powders were studied using X-ray diffraction, scanning electron microscope along with energy dispersive X-ray analysis, transmission electron microscopy, electron spin resonance (ESR) spectroscopy and impedance analyzer. The X-ray diffraction patterns obtained for the x = 0.97, 1.0 and 1.1 powdered ceramics could be indexed to a body-centered cubic perovskite related structure associated with the space group Im3. The ESR studies confirmed the absence of oxygen vacancies in the ceramics that were prepared using the oxalate precursor route. The dielectric properties of these suggest that the calcium deficient sample ( x = 0.97) has a reduced dielectric loss while retaining the high dielectric constant which is of significant industrial relevance.

Perovskites in Solid Oxide Fuel Cells

Perovskite oxides comprise large families among the structures of oxide compounds, and several perovskite-related structures are also known. Because of their diversity in chemical composition, properties and high chemical stability, perovskite oxides are widely used for preparing solid oxide fuel cell (SOFC) components. In this work a few examples of perovskite cathode and anode materials and their necessary modifications were shortly reviewed. In particular, nickel-substituted lanthanum ferrite and iron-substituted strontium titanate as cathode materials as well as niobium-doped strontium titanate, as anode material, are described. Electrodes based on the modified perovskite oxides are very promising SOFC components.

Influence of Zr doping on the dielectric properties of CaCu3Ti4O12 ceramics

Pure and Zr-substituted CaCu3(Ti1−xZrx)4O12 (x = 0, 0.01, 0.02, 0.03) ceramics were prepared by the Pechini method. X-ray powder diffraction analysis indicated the formation of single-phase compound, and all the diffraction peaks were completely indexed by the body-centered cubic perovskite-related structure. The effects of Zr4+ ion substituting partially Ti4+ ion on the dielectric properties were investigated in frequency range between 100 Hz and 1 GHz. The low frequency (f ≤ 105 Hz) dielectric constant decreases with Zr substitution and the high frequency (f ≥ 107 Hz) dielectric constant is unchanged. Interestingly, a low-frequency relaxation was observed at room temperature through Zr substitution. The observed dielectric properties in Zr-substituted samples were discussed using the internal barrier layer capacitor model. A corresponding equivalent circuit was adopted to explain the dielectric dispersion. The characteristic frequency of low-frequency relaxation rises due to the decrease of the resistivity of grain boundary with Zr substitution, which is likely responsible for the large low-frequency response at room temperature.

Formation, cationic site exchange and surface structure of mechanosynthesized EuCrO3 nanocrystalline particles

Nanocrystalline EuCrO3 particles (∼25 nm) have been prepared by pre-milling a 1 : 1 molar mixture of Eu2O3 and Cr2O3 for 60 h followed by sintering at 700 °C (12 h). This temperature is ∼500–600 °C lower than those at which the material, in bulk form, is conventionally prepared. Rietveld analysis of the x-ray powder diffraction pattern of the EuCrO3 nanoparticles favours a structural model involving a slight degree of cationic exchange where ∼11% of the Eu3+ and Cr3+ ions exchange their normal dodecahedral A- and octahedral B-sites, respectively, in the perovskite-related structure. This cationic site exchange, which is unusual in a perovskite structure, has been well supported by the corresponding room-temperature 151Eu Mössbauer spectrum of the nanoparticles that in addition to displaying a distribution in the principal component of the EFG tensor (Vzz) at the usual A-sites of the 151Eu nuclei, also revealed the presence of a subcomponent with ∼11% area fraction and a considerably increased |Vzz| value that was associated with Eu3+ ions at octahedral B-sites. X-ray photoelectron and Auger electron spectroscopic techniques reveal a complex surface structure where extremely thin layers of un-reacted Eu2O3 and Cr2O3 cover most of the EuCrO3 nanoparticles' surfaces together with some traces of elemental Cr. The binding energies associated with Eu3+ 3d5/2, Eu3+ 4d3/2, Cr3+ 2p3/2 and O2− 1s core-level electrons in EuCrO3 are estimated from the x-ray photoelectron data for the first time.

Large anisotropic thermoelectricity in perovskite related layered structure: SrnNbnO3n+2 (n = 4,5)

We have systematically synthesized a series of perovskite related layered structures, Strontium-Niobates expressed as SrnNbnO3n+2 (n = 4 ~ 5) and focused on the thermoelectricity in n = 4 and 5 type materials here. To explore their thermoelectricities and anisotropic properties, we have measured the thermal and charge transport properties along all crystallographic axes. The values of thermoelectric parameters were strongly anisotropic and there exists a large anisotropy even in in-plane direction of the layered structure. As a result, the best performance of thermoelectricity is commonly observed in the a-axis. The respective ZT for Sr1.8La0.2Nb2O7 and Sr5Nb5O17 at room temperature is 3.5×10−2 and 3.6×10−3.

New Layered Oxide-Fluoride Perovskites: KNaNbOF5 and KNaMO2F4 (M = Mo6+, W6+)

KNaNbOF5 and KNaMO2F4 (M = Mo6+, W6+), three new layered oxide-fluoride perovskites with the general formula ABB’X6, form from the combination of a second-order Jahn-Teller d0 transition metal and an alkali metal (Na+) on the B-site. Alternating layers of cation vacancies and K+ cations on the A-site complete the structure. The K+ cations are found in the A-site layer where the fluoride ions are located. The A-site is vacant in the adjacent A-site layer where the axial oxides are located. This unusual layered arrangement of unoccupied A-sites and under bonded oxygen has not been observed previously although many perovskite-related structures are known.

Structure and X-ray powder reference patterns for hexagonal perovskite-related phases, (Sr0.8Ca0.2)5Co4O12 and Sr6Co5O15

Two selected members of the homologous series An+2BBn′O3n+3 (A=Sr and Ca; B and B′=Co) have been investigated for their crystal structures because of their potential applications as thermoelectric materials. A combined Rietveld refinement and spin-polarized magnetic geometry optimization technique was employed for the structural studies. Both the n=3 member, (Sr0.8Ca0.2)5Co4O12, and the n=4 member, Sr6Co5O15, have distorted hexagonal perovskite-related structures that possess one-dimensional cobalt oxide chains separated by alkaline-earth cations. The linear chains consist of one unit of CoO6 trigonal prism alternating with n units of CoO6 octahedra. Crystal structures and reference powder X-ray diffraction patterns of (Sr0.8Ca0.2)5Co4O11 [P3c1, a=9.4196(2) Å, c=19.9857(6) Å, V=825.83 Å3, and Dx=5.358 g/cm3] and Sr6Co5O15 [R32, a=9.497 64(12) Å, c=12.3956(2) Å, V=968.34 Å3, and Dx=5.455 g/cm3] are reported.

Crystallographic and magnetic characterisation of the brownmillerite Sr 2Co 2O 5

Sr 2Co 2O 5 with the perovskite-related brownmillerite structure has been synthesised via quenching, with the orthorhombic unit cell parameters a=5.4639(3) Å, b=15.6486(8) Å and c=5.5667(3) Å based on refinement of neutron powder diffraction data collected at 4 K. Electron microscopy revealed L–R–L–R-intralayer ordering of chain orientations, which require a doubling of the unit cell along the c-parameter, consistent with the assignment of the space group Pcmb. However, on the length scale pertinent to NPD, no long-range order is observed and the disordered space group Imma appears more appropriate. The magnetic structure corresponds to G-type order with a moment of 3.00(4) μ B directed along [1 0 0].

Crystal chemistry and electrochemical properties of Ln(Sr,Ca)3(Fe,Co)3O10intergrowth oxide cathodes for solid oxidefuel cells

Ln(Sr,Ca)3(Fe,Co)3O10 (Ln = La, Nd, and Gd) oxides crystallizing in a perovskite-related intergrowth structure have been synthesized and explored as new cathode materials for solid oxide fuel cells (SOFC). The crystal structures of the Ln(Sr,Ca)3(Fe,Co)3O10 samples could be refined based on the tetragonal space groupI4/mmm. The substitution of Co for Fe in LnSr3(Fe,Co)3O10 (Ln = La and Nd) increases the amount of oxygen loss on heating, thermal expansion coefficient (TEC), total electrical conductivity, and cathode performance in SOFC. In particular, the Ln = Nd samples exhibit superior cathode performances in SOFC compared to those obtained with the Ln = La or Gd samples in LnSr3(Fe,Co)3O10. Further improvement in the cathode performance and a decrease in the TEC value have been realized by a substitution of less electropositive Ca for Sr in NdSr3−yCayFe1.5Co1.5O10, demonstrating that NdSr2.5Ca0.5Fe1.5Co1.5O10 (NSCFC) is the optimum composition among the Ln(Sr,Ca)3(Fe,Co)3O10 samples investigated. The improved performance of NSCFC is attributed to the decreased lattice distortion in the perovskite layer, which is beneficial to enhance the oxide-ion conductivity.

Large anisotropic thermoelectricity in perovskite related layered structure: SrnNbnO3n+2 (n=4,5)

We measured the thermal and charge transport properties of perovskite-related layered structures. Strontium-Niobates, which were expressed as SrnNbnO3n+2 (n=4: Sr1.8La0.2Nb2O7, n=5: Sr5Nb5O17), to explore their thermoelectricities and thermal anisotropies. The behaviors of the thermoelectric parameters (thermal conductivity, Seebeck coefficient, resistivity) were strongly anisotropic in all crystallographic axes (a, b, and c) and large anisotropy exists even in the in-plane direction of the layered structure. Especially, along the a-axis in which corner-sharing NbO6 octahedra aligned straightly, contrastive properties were observed between Sr1.8La0.2Nb2O7 and Sr5Nb5O17. For Sr1.8La0.2Nb2O7, a thermally activated charge conduction is pronounced in the temperature dependence of Seebeck coefficient and resistivity, on the other hand, it was a metallic nature for Sr5Nb5O17. In both compounds, ZT results in anisotropic due to the anisotropic properties of thermoelectric parameters, the best performance is commonly observed in the a-axis. The respective ZT values at room temperature are 3.5×10−2 and 3.6×10−3.

Deposition and characterisation of epitaxial oxide thin films for SOFCs

This paper reviews the recent advances in the use of thin films, mostly epitaxial, for fundamental studies of materials for solid oxide fuel cell (SOFC) applications. These studies include the influence of film microstructure, crystal orientation and strain in oxide ionic conducting materials used as electrolytes, such as fluorites, and in mixed ionic and electronic conducting materials used as electrodes, typically oxides with perovskite or perovskite-related layered structures. The recent effort towards the enhancement of the electrochemical performance of SOFC materials through the deposition of artificial film heterostructures is also presented. These thin films have been engineered at a nanoscale level, such as the case of epitaxial multilayers or nanocomposite cermet materials. The recent progress in the implementation of thin films in SOFC devices is also reported.

Square Coordinated MnO2-Units in BiMn7O12

Room temperature high-resolution synchrotron X-ray diffraction (HRSXRD) measurements were carried out on polycrystalline samples of BiMn(7)O(12) and LaMn(7)O(12). Rietveld refinements of HRSXRD data describe the crystal structures of both compounds in the monoclinic space group I2/m. The maximum entropy method-based pattern fitting (MPF) method inversion of HRSXRD data was utilized to obtain the spatial electron density (ED) distribution. Obtained results show clear differences in ED distributions around Mn-O bonds between isomorphic BiMn(7)O(12) and LaMn(7)O(12). The scheme of ED distributions in BiMn(7)O(12) indicates the presence of orbital ordering in the MnO(2) planar arrangement that appears as prominent features in very distorted MnO(12) and MnO(6) polyhedrons in the perovskite related structure of [BiMn(3)](A-site)[Mn(4)](B-site)O(12).

Studies on Chemical Stability and Electrical Properties of Proton Conducting Perovskite-Like Doped BaCeO3

The chemical stability and electrical properties of three promising perovskite-related structures , , and were tested in air, humidified and , as well as in . Powder X-ray diffraction studies confirmed the formation of a cubic perovskite-like structure. The change in the lattice constant was consistent with B-site substitution in . All the investigated compounds formed barium carbonate in at elevated temperatures and were found to be chemically unstable in boiling . The data showed that these three compounds are chemically stable in humidified at ; however, at , the formation of barium carbonate was observed. The electrical conductivity in wet and/or was found to be higher than that in the -containing atmosphere, confirming proton conduction in the doped . The co-doped showed the highest total conductivity of in at with an activation energy of 0.36 eV in the temperature range of .

Studies on chemical stability in CO2 and H2O and electrical conductivity of perovskite-type Ba3In2Zr1−x Ce x O8 (x = 0, 0.5, 1)

We report the synthesis, structure, microstructure, chemical stability in H2O and CO2, and electrical transport properties of an oxide ion-conducting perovskite-related structure Ba3In2MO8 (M = Zr, Ce, Zr0.5Ce0.5). Powder X-ray diffraction confirmed the formation of a simple cubic perovskite-like structure for Ba3In2ZrO8 (a = 4.205(9) A), Ba3In2CeO8 (a = 4.234(1) A), and Ba3In2Zr0.5Ce0.5O8 (a = 4.285(8) A). The increase in lattice constant is consistent with the Shannon’s ionic radius trend. Among the three samples investigated, Ba3In2ZrO8 was found to be stable against reaction with pure CO2 at elevated temperature, while the Ce and 1:1 Zr and Ce compounds were unstable at 600 °C. Ba3In2ZrO8, Ba3In2CeO8, and Ba3In2Zr0.5Ce0.5O8 were found to be chemically unstable in H2O at about 50 °C. The bulk electrical conductivity of the samples prepared at different temperatures was found to be nearly the same; the total conductivity (bulk + grain–boundary + electrode) seems to change with sintering temperature. Both Ba3In2ZrO8 and Ba3In2CeO8, prepared at 1,400 °C, exhibited comparable electrical conductivity of about 6 × 10−3 S cm−1 at 800 °C, which is comparable to that of conventional Y2O3-doped ZrO2 electrolyte. These compounds are very promising electroltes, provided that their chemical and mechnical stabitities are improved without losing any ionic conductivity.

Crystal Growth of A3A′MO6 Oxides

AbstractSingle crystals of two new oxides, Sr 3 LiNbO 6 and Sr 3 LiTaO 6 , and the analogous antimonates, Ba 3 NaSbO 6 , Sr 3 NaSbO 6 and Sr 3 LiSbO 6 , were grown out of hydroxide melts contained in silver/alumina crucibles. The crystal structures of all five oxides are isotypic with that of the 2H-hexagonal perovskite related A 3 A′MO 6 family of oxides and consist of one-dimensional chains of face-shared MO 6 octahedra and A′O 6 trigonal prisms; infinite chains of distorted AO 8 square antiprisms separate these chains from one another.Graphical AbstractTwo new oxides, Sr 3 LiMO 6 (M = Nb, Ta) and analogous Sr 3 NaSbO 6 , Sr 3 LiSbO 6 and Ba 3 NaSbO 6 , isostructural with the 2-H hexagonal perovskite related structure type, were synthesized from hydroxide fluxes. [IMAGE]

Characterization and optimization of Ln 1.7Sr 0.3CuO 4 (Ln = La, Nd)-based cathodes for intermediate temperature solid oxide fuel cells

The Ln 1.7Sr 0.3CuO 4 (Ln = Nd, La), i.e., NSCu and LSCu materials with perovskite-related structure were synthesized and evaluated as new cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). The crystal structure, thermal expansion, electrical conductivity and electrochemical performance of Ln 1.7Sr 0.3CuO 4 cathodes have been investigated by X-ray diffraction, dilatometry, DC four-probe method, AC impedance and cyclic voltammetry (CV) techniques. The conductivity of NSCu and LSCu reached 106 S cm −1 and 125 S cm −1 at 800 °C. NSCu cathode exhibited much lower area specific resistances (ASR) than LSCu, e.g., the ASR values for NSCu cathode on Sm-doped ceria (SDC) electrolyte in air were 0.07 Ω cm 2, 0.09 Ω cm 2 and 0.24 Ω cm 2 at 800 °C, 750 °C and 700 °C, while the corresponding values for LSCu were 0.11 Ω cm 2, 0.21 Ω cm 2 and 0.44 Ω cm 2, respectively. To further optimize the electrochemical performance of the cathodes, a certain amount of Sm-doped ceria (SDC) electrolyte was introduced. The polarization resistance of NSCu–SDC30 and LSCu–SDC30 was 0.12 Ω cm 2 and 0.18 Ω cm 2 at 700 °C in air, nearly half of pure NSCu and LSCu, respectively.

Substrate-Dependent Growth of HoMnO3 Films

Manganites with small rare earths (RE = Ho, Er, Tm, Yb, Lu, or Y) and hexagonal structures belong to a class of materials known as hexagonal ferroelectrics, in contrast to those with large rare earths and orthorhombic, perovskite-related structures showing different physical properties. In this work, we study the effects of growth conditions on the stabilization of HoMnO3 films with both hexagonal and orthorhombic structures. Based on our experimental results, a HoMnO3 film with a hexagonal structure can be stabilized on Si(111) at 850 °C and above. Pure orthorhombic phases grow on SrTiO3(100), SrTiO3(110), LaAlO3(100), and LaAlO3(110) in a temperature range from 750 to 850 °C. Multiphase samples can be grown on MgO(100) at all temperatures.

CO 2 reforming of methane over La 2NiO 4/α-Al 2O 3 prepared by microwave assisted self-combustion method

La 2NiO 4/α-Al 2O 3 catalyst was prepared by a microwaves assisted self-combustion method using glycine or urea as fuel and metal nitrates as oxidizers. In order to understand the effect of the preparation method and the fuel type used on the catalytic properties, a sample was prepared by an incipient wetness nitrate impregnation method (without fuel). All obtained samples were used as catalysts precursors for the CO 2 reforming of methane to syngas between 500 and 800 °C. The catalysts were characterized by X-ray diffraction (XRD), specific surface area measurements (BET), temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), before and after reactivity tests. XRD analyses indicate the formation of La 3Ni 2O 6.92, La 2NiO 4 and LaAl x Ni x−1 O 3 crystalline structures independently of the preparation method. All samples prepared with fuel showed high CH 4 and CO 2 conversions with low carbon deposition. No change was observed in the crystalline structure of the perovskite-related oxides. On the other hand, the sample prepared by nitrate impregnation showed high activity but destruction of the perovskite-related structure with significant carbon formation, mainly multi-walled carbon nanotubes.