Orthorhombic Perovskite Phase Research Articles

Stable, easily sintered Ca–Zn-doped YCrO 3 as novel interconnect materials for co-fired yttrium-stabilized zirconia-based solid oxide fuel cells

In order to develop co-fired yttrium-stabilized zirconia (YSZ)-based solid oxide fuel cells (SOFCs), stable and easily sintered Y 0.7Ca 0.3Cr 1− x Zn x O 3−δ ( x = 0–0.15) perovskite oxides were synthesized by the microwave-aided sol–gel process and then examined as novel ceramic interconnect materials. (The characterizations focused on phase structure, sintering behavior, relative density, electrical conductivity and thermal expansion.) The XRD analysis indicates that a pure orthorhombic perovskite phase was obtained for all the samples. Cell volume decreases as x increases from 0 to 0.10. The Y 0.7Ca 0.3Cr 0.9Zn 0.1O 3– δ (YCCZ10) powder exhibited the best sintering ability, and a relative density of 96.6% could be obtained for the sample sintered at 1400 °C for 4 h in air. The electrical conductivities of the specimens increase with the Zn 2+ content at x ≤ 0.10, but then remarkably decrease at x = 0.15, which might relate to the over-range of the substitution amount of Zn (0.15) for Cr position. YCCZ10 shows a remarkable electrical conductivity of 20.9 S cm −1 at 850 °C in air, and a very suitable thermal expansion coefficient value of 10.8 × 10 −6 K −1 (YSZ: ∼10.8 × 10 −6 K −1). These investigations have indicated that YCCZ10 is a promising interconnect material for co-fired YSZ-based SOFCs.

The synthesis and characterization of the Sm0.5Sr0.5Co1−xCuxO3−δ cathode by the glycine–nitrate process

Abstract In this study, a new oxygen-deficient cathode material, Sm 0.5 Sr 0.5 Co 1− x Cu x O 3− δ (SSCCu) was developed. It is expected to enhance the efficiency of intermediate-temperature solid oxide fuel cells (IT-SOFCs). The structure, conductivity and electrochemical performance of SSCCu were examined as a function of copper content. The structure of Sm 0.5 Sr 0.5 Co 0.9 Cu 0.1 O 3− δ and Sm 0.5 Sr 0.5 Co 0.8 Cu 0.2 O 3− δ samples was a single orthorhombic perovskite phase. Second phase SrCoO 2.8 , however, formed in the Sm 0.5 Sr 0.5 Co 0.7 Cu 0.3 O 3− δ and Sm 0.5 Sr 0.5 Co 0.6 Cu 0.4 O 3− δ samples. The conductivity of the Sm 0.5 Sr 0.5 Co 0.7 Cu 0.3 O 3− δ cathode was higher than that of other samples. However, the Sm 0.5 Sr 0.5 Co 0.8 Cu 0.2 O 3− δ electrode exhibited the lowest overpotential of 25 mV at 400 mA cm −2 and the lowest area special resistance of 0.2 Ω cm 2 at 700 °C.

Characterization of RMnO3 (R=Sc, Y, Dy-Lu): High-pressure synthesized metastable perovskites and their hexagonal precursor phases

Abstract A series of R MnO 3 samples with R = Sc, Y, Dy-Lu was synthesized through a two-step route in which first the hexagonal oxides, h - R MnO 3 , were prepared under normal pressure using an EDTA method and then converted to orthorhombic perovskites, o - R MnO 3 , through a high-pressure treatment at 5 GPa and 800 °C. Since all the aforementioned members but the R = Sc member were converted to an orthorhombic perovskite phase under the 5-GPa high-pressure condition, an additional sample of InMnO 3 of the hexagonal structure was prepared and tested for the high-pressure conversion. The stability limit of the orthorhombic perovskite structure under the 5-GPa high-pressure condition was concluded to be located between InMnO 3 and LuMnO 3 in terms of the size of the A-site cation constituent, R III or In III . The decrease in the size of R III was found to increase the distortion of o - R MnO 3 with a decrease in the Mn O Mn bond angle from 144.3° for Dy to 140.3° for Lu. All the hexagonal and orthorhombic R MnO 3 phases exhibited antiferromagnetic characteristics at low temperatures.

Preparation and dielectric properties of (Ca0.61,Nd0.26)TiO3 nanoparticles by a sol–gel method

Ultrafine (Ca0.61,Nd0.26)TiO3 powder was synthesized by a sol–gel method. The X-ray diffraction result of the powder showed the presence of an orthorhombic GdFeO3-type perovskite phase with minute amounts of CaCO3. As the heat-treatment temperature of the xerogel was increased to 800 °C, CaCO3 disappeared completely. The average particle size calculated from the transmission electron microscopy micrograph of the powder was 5–10 nm. Compared with conventional solid-state method (1400 °C for 4 h), the ultrafine (Ca0.61,Nd0.26)TiO3 powder can be sintered compactly at relatively low temperature (1200 °C for 2 h). Microwave dielectric characteristics of (Ca0.61,Nd0.26)TiO3 ceramics fabricated from nanoparticles prepared via the sol–gel process were improved. At 1200 °C, very dense ceramics with uniform grains and good dielectric properties of dielectric constant (e) = 90.2, Q × f value (the product of quality factor Q and frequency f) = 25200 GHz, and a temperature coefficient of resonant frequency (τf) = +243ppm °C−1 were achieved.

Structural transformation and magnetic competition inYb(Mn1−xFex)O3

Structural and magnetic properties of the $\mathrm{Yb}({\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}){\mathrm{O}}_{3}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}1)$ system have been systematically investigated. Initial samples were prepared via a sol-gel method. A pure hexagonal phase was only obtained for samples with $x\ensuremath{\leqslant}0.5$. With high-pressure annealing, a pure orthorhombic perovskite phase was achieved for all the compositions. The $^{57}\mathrm{Fe}$ M\"ossbauer spectrum for $x=0.5$ shows that only ${\mathrm{Fe}}^{3+}$ ions exist in the system; there was no evidence of chemical inhomogeneities. With increasing $x$, the N\'eel temperature ${T}_{N}$ increases for both hexagonal and orthorhombic phases. The orthorhombic $\mathrm{Yb}({\mathrm{Mn}}_{0.5}{\mathrm{Fe}}_{0.5}){\mathrm{O}}_{3}$ shows an interesting weak ferromagnetic state in the temperature range of $239--298\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the ferromagnetism disappearing abruptly on cooling below ${T}_{t}=239\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The transition at ${T}_{t}$ appears to be a reorientation of the spin axis of a type-$G$ antiferromagnetic order from the orthorhombic $a$ axis to the $b$ axis in the (010) plane.

Dielectric and Piezoelectric Properties of NaNbO3–LiNbO3–SrTiO3 Ceramics

To develop Pb-free piezoelectric ceramics, the dielectric and piezoelectric properties of NaNbO3–LiNbO3–SrTiO3 perovskite ceramics were investigated as functions of composition. As for ceramics prepared in this study, the orthorhombic perovskite phase was formed at lower temperatures and was transformed to a tetragonal form and then to a cubic form with an increase in temperature. The tetragonal-cubic transition temperature (Curie temperature, Tc) gradually shifted toward lower temperatures with an increase in SrTiO3 content. Among the ceramics obtained, the 0.86NaNbO3–0.09LiNbO3–0.05SrTiO3 composition showed relatively good piezoelectric properties, namely, kp = 0.20, d33 = 53 pC/N, and Qm = 500, for high-frequency applications for the Pb-free piezoelectric ceramics reproted.

Effects of structural characteristics on microwave dielectric properties of (1 − x)Ca 0.85Nd 0.1TiO 3– xLnAlO 3 (Ln = Sm, Er and Dy) ceramics

Dependence of microwave dielectric properties on the structural characteristics of (1 − x)Ca 0.85Nd 0.1TiO 3– xLnAlO 3 (Ln = Sm, Dy and Er) ceramics were investigated as a function of LnAlO 3 content (0.05 ≤ x ≤ 0.25). For the specimens with SmAlO 3, a single phase with orthorhombic perovskite was obtained through the entire composition, however, Dy 2Ti 2O 7 and Er 2Ti 2O 7 were detected as a secondary phase along with the orthorhombic perovskite phase for the specimens with DyAlO 3 ( x = 0.25) and ErAlO 3 (0.10 ≤ x ≤ 0.25), respectively. With an increase of LnAlO 3 content, the dielectric constant decreased due to the smaller ionic polarizability of LnAlO 3 than Ca 0.85Nd 0.1TiO 3. The temperature coefficient of the resonant frequency (TCF) decreased with LnAlO 3 content resulted from an increase of oxygen octahedral distortion.

Dielectric and Piezoelectric Properties of KNbO3–NaNbO3–LiNbO3–SrTiO3 Ceramics

For the purpose of the developing Pb-free piezoelectric ceramics, the dielectric and piezoelectric properties of two series of KNbO3–NaNbO3–LiNbO3–SrTiO3 (KN–NN–LN–ST) perovskite ceramics, namely 0.47KN–0.47NN–(0.06-x)LN–xST (0≤x≤0.06) and 0.45KN–0.45NN–(0.10-y)LN–yST (0.02≤y≤0.10) were investigated as functions of composition. In both series, the orthorhombic perovskite phase formed at lower temperatures, and was transformed to tetragonal form and then cubic form with increasing temperature. The orthorhombic–tetragonal transition temperature (Tot) and tetragonal–cubic transition temperature (Curie temperature, Tc) gradually shifted toward lower temperatures with increasing ST content. For the 0.47KN–0.47NN–(0.06-x)LN–xST series, the Tot=100 °C and Tc=500 °C for x=0, and the Tot=-20 °C and Tc=220 °C for x=0.06. For the 0.45KN–0.45NN–(0.10-y)LN–yST series, Tot=-30 °C and Tc=450 °C for y=0.02, and Tot=-60 °C and Tc=330 °C for y=0.05. Among the ceramics obtained, the 0.45KN–0.45NN–0.06LN–0.04ST composition showed relatively good piezoelectric properties, kp=0.33 and d33=120 pC/N, which are comparable to those of Pb-free piezoelectric ceramics previously reported.

Structural evolution of La–Cr–O thin films: Part I. Microstructure and phase development

The structural evolution of La–Cr–O thin films and the formation mechanisms of the LaCrO3 perovskite phase have been studied. X-ray amorphous La–Cr–O protective coatings were deposited by magnetron sputtering on metallic interconnect materials. During the annealing of the material in air a two-step phase transition from La–Cr–O to a monoclinic LaCrO4 monazite and further to an orthorhombic LaCrO3 perovskite phase was observed. The formation of a fine nanoporous structure is a result of the significant increase in density of the final LaCrO3 perovskite in comparison with monazite LaCrO4 phase. While the porous structure was not sought after for this application, these distinctive nanostructures may have numerous applications in catalysis, separation membranes or for other SOFC components.

Molecular dynamics of MgSiO 3 perovskite melting

The melting curve of MgSiO3 perovskite is simulated using molecular dynamics simulations method at high pressure. It is shown that the simulated equation of state of MgSiO3 perovskite is very successful in reproducing accurately the experimental data. The pressure dependence of the simulated melting temperature of MgSiO3 perovskite reproduces the stability of the orthorhombic perovskite phase up to high pressure of 130GPa at ambient temperature, consistent with the theoretical data of the other calculations. It is shown that its transformation to the cubic phase and melting at high pressure and high temperature are in agreement with recent experiments.

Electrical properties and ethanol-sensing characteristics of perovskite La 1 − xPb xFeO 3

The nanocrystalline La 1 − x Pb x FeO 3 ( x = 0.0–0.50) powders with orthorhombic perovskite phase were prepared by a sol–gel method. The Pb-doping restrained the growth of the crystallite size. The electrical properties and ethanol-sensing characteristics of the La 1 − x Pb x FeO 3 system were investigated. The results demonstrated that the resistance and conduction type (p- or n-type) of the perovskite oxides changed with the increasing composition x. Among the oxides with compositions of x = 0.30–0.50, La 0.68Pb 0.32FeO 3 ( x = 0.32) showed the best ethanol gas-sensing characteristics.

Fe3+ and Al solubilities in MgSiO3 perovskite: implication of the Fe3+AlO3 substitution in MgSiO3 perovskite at the lower mantle condition

Perovskite in the starting composition of Mg3Fe3+AlSi3O12 was investigated at 24 and 51 GPa and 2100 K using a laser‐heated diamond anvil cell (LHDAC) and an analytical transmission electron microscopy (ATEM) to understand the solubilities of Fe3+ and Al into MgSiO3‐dominated perovskite. A single phase of orthorhombic perovskite was formed at 51 GPa and 2100 K and the Fe3+AlO3 bearing perovskite showed no structural change during decompression. The samples synthesized at 24 GPa and 2100 K showed minor amounts of magnesioferrite and stishovite phases in spaces along the perovskite grain boundaries. These results indicate that the solubility of ferric iron incorporated as the Fe3+AlO3 component in MgSiO3‐based perovskite is higher and slightly lower than 25 mole % at 51 and 24 GPa, respectively.

A New Elaboration Route by Sol-Gel Process for Cerium Doped SrHfO3Films and Powders

We present for the first time the elaboration via sol gel route of cerium (1 mol%) doped SrHfO3 powders and films. The sol is elaborated using hafnium and strontium ethoxides as precursors and cerium nitrate as dopant. The structure of powders and films are characterized by convergent methods: Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, Raman spectroscopy and optical measurements conducted by the prism coupling method. The powder crystallises from amorphous to pure SrHfO3 orthorhombic perovskite phase after a 800°C heat treatment. Nevertheless HfO2 monoclinic phase coexists with orthorhombic perovskite phase after a 1000°C heat treatment. The film is amorphous for annealing temperatures lower than 700°C and presents good waveguiding performances. The film heat-treated at 700°C exhibits a refractive index of 1.810 ± 0.001 (λ = 543.5 nm) for a thickness around 375 nm. The attenuation coefficient obtained on the 400°C heat-treated film is α = 4.0 ± 0.5 dB/cm (λ = 632.8 nm). The film starts to crystallize at 750°C into the SrHfO3 orthorhombic phase but HfO2 monoclinic phase is also detected after a heat treatment at 1000°C. The potentiality of sol gel Ce3+:SrHfO3 powders and films for scintillation applications are investigated.

Key structural and defect chemical aspects of Cd–In–Sn–O transparent conducting oxides

The equilibrium phase diagram (1175 °C in air), solubility ranges and room temperature physical properties (electrical, optical) were established for the various n-type transparent conductors in the CdO–In 2O 3–SnO 2 system. Extended solubility ranges were found to exist at constant Cd/Sn ratio for the bixbyite, spinel and orthorhombic perovskite phases. This is attributed to the isovalent and nearly size-matched nature of the co-substitution. Still larger solubility ranges were found for metastably-processed thin films. Co-doped bixyite and spinel phases were found to be inherently off-stoichiometric with respect to cation composition, leading to self-doped n-type character, in bulk and films. A correlation was found between optimized conductivity in the various phases and the density of octahedral Cd, In and/or Sn species in their crystal structures.

Synthesis of cadmium titanate powders by a sol-gel-hydrothermal method

Cadmium titanate (CdTiO3) powders have been synthesized by a sol-gel-hydrothermal method at 200°C. The as-synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Raman spectroscopy, and thermo-gravimetric and differential thermal analysis (TG-DTA). The results indicate that the as-obtained powders are ilmenite phase CdTiO3 with rhombohedral structure. This phase changes to orthorhombic perovskite phase upon annealing at about 1050°C.

TEM studies on La2−2xCa1+2xMn2O7 (x=0.5 and 0.25)

The nominal compositions and structures of the compounds La2-2xCa1+2xMn2O7 (x=0.5 and 0.25) prepared by solid-state reaction have been studied by transmission electron microscopy and high-resolution electron microscopy. Besides the observation of the major phase of a two-layer structure in the nominal composition La1.0Ca2.0Mn2O7, the embedded n = 3, n = 4 Ruddlesden-Popper phases and La rich phase in the two-layer structure were observed as defects. No layered structure was observed in the nominal composition La1.5Ca1.5Mn2O7. Instead, a major phase of orthorhombically distorted perovskite, a minor phase of an ideal cubic perovskite, and a small fraction of calcium-oxide were observed in the sample. The coexistence of the orthorhombic phase and ideal cubic perovskite phase, as well as the same orthorhombic phase with perpendicular directions, was observed in the nominal composition La1.5Ca1.5Mn2O7. (C) 2002 Elsevier Science B.V. All rights reserved.

Site Disorder Induced Hexagonal−Orthorhombic Transition in Y3+1-xGd3+xMnO3

We show that the transition in AMnO3 from the orthorhombic perovskite phase to the hexagonal phase is promoted by inducing site disorder on the A-site. The gap between the orthorhombic and the hexagonal phase is widened for site disordered, mixed yttrium−gadolinium manganite samples. At the cost of the orthorhombic phase, a two phase region emerges. The phase separation exhibits very unusual thermodynamical behavior. We also show that high-pressure synthesis favors the orthorhombic phase. YMnO3 is formed in the orthorhombic phase at 15 kbar.

Mineral assemblages of basalt in the lower mantle

High‐pressure X‐ray diffraction and analytical transmission electron microscopy of laser‐heated diamond cell samples show temperature‐dependent phase assemblages in mid‐ocean ridge basalt (MORB) at lower mantle pressures. The assemblage observed below ∼2250 K consists of only stishovite plus an orthorhombic perovskite phase (space group Pbnm or Cmcm) having a composition close to that of MORB but slightly depleted in SiO2. With increasing temperature, the assemblage transforms to orthorhombic and cubic perovskites plus stishovite and then at ∼3250 K to orthorhombic and cubic perovskites plus stishovite and the CaFe2O4 phase. All of these assemblages could be present in slabs that have penetrated deep into the mantle and can therefore contribute to the seismological heterogeneity of the lower mantle.

Fabrication of La 0.2Sr 0.8Co 0.8Fe 0.2O 3− δ mesoporous membranes on porous supports from polymeric precursors

Since there are many difficulties in the preparation of supported oxygen-selective perovskite dense membranes on porous supports due to the problem of compatibility between the deposited membrane and the support material, one strategy to alleviate the problem is to fabricate mesoporous perovskite membranes on porous supports before deposition of a dense top layer with similar or adapted composition. In this study, the supported mesoporous membranes of La 0.2Sr 0.8Co 0.8Fe 0.2O 3− δ (LSCF) without defect were prepared on porous surface-modified alumina supports using Pechini-type polymeric precursors by the dip-coating method. The LSCF polymeric precursor and the supported LSCF membrane were characterized by means of thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and liquid–liquid displacement porometry (LLDP). It is found that the membranes prepared are of orthorhombic perovskite phase, and the twice dip-coated membrane determined by SEM and LLDP is crack-free and about 0.7 μm in thickness, with an average pore diameter of 5 nm.

Garnet-perovskite transformation under conditions of the Earth's lower mantle: an analytical transmission electron microscopy study

Natural pyrope garnets with three different chemical compositions have been transformed at 30–60 GPa in the laser-heated diamond-anvil cell (DAC). Recovered samples were examined by analytical transmission electron microscopy (ATEM). For all three pyropes, the dominant post-garnet phase was Mg-rich orthorhombic silicate perovskite. The Al content of Mg-perovskite increased significantly with increasing pressure and temperature, and its chemical composition became close to that of the starting material. Mg-perovskite with Al 2O 3 content less than 25 mol% was quenched as a single phase of orthorhombic perovskite at ambient conditions, whereas Mg-perovskite with Al 2O 3 content of 25–28 mol% transformed into the alternating lamellae of orthorhombic perovskite and lithium niobate phases. With further increasing the Al 2O 3 content, Mg-perovskite converted into a single phase of the lithium niobate structure with polysynthetic twinning on {101̄2̄} at ambient conditions. The high Al content may also induce the stabilization of Fe 3+ in the perovskite structure accompanied by metallic iron. Two Al-rich phases, corundum and a new Al-rich phase (the NAL phase) were recognized with Mg-perovskite mostly at lower pressures. The NAL phase is close to M 3Al 4Si 1.5O 12 (M=Mg, Fe, Ca, Na, K), i.e., the middle of A 2O 3 and AB 2O 4 type compounds, and is accompanied by stishovite. The electron diffraction patterns are consistent with the space group P6 3/m or P6 3 with a=8.85 (2) Å, c=2.78 (2) Å, V=188 (1) Å 3. The existence of large cations such as Ca, K, and Na seems to stabilize the NAL phase relative to the corundum phase, but excess Ca and Na contents are likely to induce the formations of Ca-rich cubic perovskite and the calcium ferrite phase, respectively.