Perovskite Symmetry Research Articles

Lattice instabilities in bulk EuTiO3

The phase purity and the lattice dynamics in bulk EuTiO3 were investigated both microscopically, using X-ray and neutron diffraction, 151-Eu-M\"ossbauer spectroscopy, and 151-Eu nuclear inelastic scattering, and macroscopically using calorimetry, resonant ultrasound spectroscopy, and magnetometry. Furthermore, our investigations were corroborated by ab initio theoretical studies. The perovskite symmetry, Pm-3m, is unstable at the M- and R- points of the Brillouin zone. The lattice instabilities are lifted when the structure relaxes in one of the symmetries: I4/mcm, Imma, R-3c with relative relaxation energy around -25 meV. Intimate phase analysis confirmed phase purity of our ceramics. A prominent peak in the Eu specific density of phonon states at 11.5 meV can be modelled in all candidate symmetries. A stiffening on heating around room temperature is indicative of a phase transition similar to the one observed in SrTiO3, however, although previous studies reported the structural phase transition to tetragonal I4/mcm phase our detailed sample purity analysis and thorough structural studies using complementary techniques did not confirm a direct phase transition. Instead, in the same temperature range, Eu delocalization is observed which might explain the lattice dynamical instabilities.

Oxygen permeation through tape-cast asymmetric all-La0.6Sr0.4Co0.2Fe0.8O3−δ membranes

One of the most promising materials for oxygen separation amongst ceramic oxygen transport membranes (OTMs) is La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) due to its relatively high oxygen permeability combined with high stability. In this work, asymmetric thin-film LSCF membranes supported over a porous LSCF support were manufactured by inverse sequential tape casting. Moreover, surface activation was accomplished by depositing a porous LSCF activation layer in order to promote oxygen evolution reaction, i.e. O2− to O2 oxidation, in the permeate membrane side. In this case, the porous layer allows the surface area available for oxygen activation to be enlarged. Both the manufacturing of the asymmetric thin-film membranes and the surface activation are described in detail. A thorough study of the oxygen permeation is presented for disk-shaped 30µm thick LSCF-supported membranes considering the following operating parameters: temperature (1000–600°C), sweep flow rate (300–750mlmin−1 argon) and oxygen partial pressure in the feed (0.21–1atm). High permeation fluxes were achieved, e.g., 11.87mlmin−1cm−2 at 1000°C and 300mlmin−1 argon sweep when using pure oxygen as feed. A change in the apparent activation energy at about 850ºC was related to a reversible structural change in the perovskite symmetry (cubic↔rhombohedral), as revealed by XRD measurements. Furthermore, the application of an activation layer allowed the permeation process to be improved, especially at low temperatures, i.e. below 800°C. Specifically, an improvement of up to about 300% at 600°C is observed upon application of the activation layer. The activated membrane reached a flux of 13.3mlmin−1cm−2 at 1000°C under an O2/Ar gradient. Additional permeation tests using CO2-rich sweep gas demonstrated the good stability and performance of these LSCF asymmetric membranes at 900 and 1000ºC.

Charge compensation and magnetic properties in Sr and Cu doped La-Fe perovskites

Orthorhombic lanthanum orthoferrites La0.8Sr0.2Fe1-yCuyO3-w (y = 0 and 0.10) have been studied using X-rays and neutron powder diffraction (XRPD and NPD), magnetization measurements and 57Fe Mossbauer spectroscopy. Rietveld refinements on XRPD and NPD data show that they adopt an orthorhombic ABO3 perovskite symmetry with La/Sr and Fe/Cu atoms randomly distributed on crystal A and B sites, respectively. The magnetic structure at room temperature is antiferromagnetic, with the Fe/Cu magnetic moments aligned along the a axis. Magnetization curves versus temperature show that the compounds exhibit an overall antiferromagnetic and a weak ferromagnetic behaviour in the range 5-298 K. 57Fe Mossbauer spectroscopy measurements indicate that Fe3+ and Fe5+ ions coexist in both compounds, and the relative percentage of Fe5+ is almost the same at 77 and 170 K, rejecting a charge disproportion mechanism.

Irreversible metamagnetic transition and magnetic memory in small-bandwidth manganite Pr1−xCaxMnO3 (x = 0.0–0.5)

The present paper reports detailed structural and magnetic characterization of the low-bandwidth manganite Pr1−xCaxMnO3 (with x = 0.0–0.5) (PCMO) polycrystalline samples. With increasing Ca content, reduction of the unit cell volume and improvement in perovskite structure symmetry was observed at room temperature. Magnetic characterization shows the signature of coexisting AFM–FM ordering and spin-glass phase at the low doping range (x = 0.0–0.2) while increased hole doping (x = 0.3–0.5) leads to charge ordering, training effect and an irreversible metamagnetic phenomenon. The large irreversible metamagnetism in the CO phase of PCMO and the corresponding spin memory effect is a direct consequence of hysteretic first-order phase transition arising from the weakening of the CO state under the external magnetic field and trapping of the spins due to a strong pinning potential in the material.

Ferroelectric domain morphology, electrical and electromechanical properties of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics

ABSTRACTPb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics were fabricated by mixed-oxide route. The compositions along morphotropic phase boundary were investigated. Dielectric properties and piezoelectric coefficient were measured. The maximum relative permittivity is 33600 found in the (PIN-PT)x(PMN-PT)1-x ceramics with x = 0.1 at 167 °C. When increasing the amount of Pb(In1/2Nb1/2)O3, the piezoelectric coefficient of the ceramics decreases but the phase transition temperature increases. The selected-area electron diffraction patterns show the pseudo-cubic perovskite symmetry. Diffuse scattering is found in the diffraction pattern taken at higher order zone axis. Transmission electron microscopy study shows that the morphology of ferroelectric/ferroelastic domains is neither tetragonal nor rhombohedral configuration.

Structural Investigations of CaTiO3 and CaTi0.99Nb0.01O3 Ceramics

Crystal structure refinement of CaTiO3 and CaTi0.99Nb0.01O3 ceramics data measured by synchrotron x-ray diffraction technique revealed orthorhombic (Pbnm) perovskite symmetry for both the ceramics. TiO6 octahedra exhibited slight distortions from the ideal cubic perovskite symmetry that was attributed to the size ratios of the two types of cations occupying A- and B-sites.

Ferroelectric Properties and Electric Field Dependent Polarization in 0.86BaTiO3–0.14(Bi3/4Na1/4)(Mg1/4Ti3/4)O3 Ceramics

Ceramics of composition 0.86BaTiO3–0.14(Bi3/4Na1/4)(Mg1/4Ti3/4)O3 (0.86BT-0.14BNMT) are synthesized and verified to be of perovskite tetragonal symmetry. The ceramics also exhibit saddle-back shaped dielectric anomalies as a function of temperature. The high temperature anomaly at Tm (∼393 K) is essentially frequency independent; the low temperature anomaly at T1 (∼301 K at 1 kHz) however is strongly dispersive with frequency. The temperature dependence of the relaxation time, τ(T), can be fitted well with a Vogel-Fulcher type relation. The P-E relation below 228 K is of characteristic hysteresis type of a normal ferroelectric. As the temperature approaches to 333 K, the P-E loops became slim resembling relaxor characteristics. Three pyroelectric peaks were found that correspond to respective phase transitions of BaTiO3, which indicates the dominance of ferroelectric BT clusters in the solid solution compositions.

Perovskite Stabilization in Pb[(Zn1/3Ta2/3),Ti]O3 with BaTiO3 Substitution and Dielectric Properties

A multiple-octahedral system of Pb ( Zn 1/3 Ta 2/3) O 3- PbTiO 3 was modified by 20 mol% BaTiO3 and perovskite formation tendencies were studied. Crystal symmetries and lattice parameters were determined. Low-frequency dielectric constant spectra and microstructures of the ceramics were examined. A cubic perovskite symmetry changed to tetragonal with increasing fractions of PbTiO3. Dielectric maximum temperatures shifted linearly with the compositional change.

Structural aspects of the ionic conductivity of La 1− xSr xCoO 3− δ

The crystal structure of La 1− x Sr x CoO 3− δ in the composition range 0≤ x≤0.8 was investigated by means of powder XRD. A more detailed study was performed on the x=0.7 composition by neutron diffraction, HRTEM, SAED, image analysis and PEELS. The XRD results indicate a long-range cubic perovskite symmetry (spacegroup Pm3̄m) for x>0.5 and a rhombohedrally distorted perovskite structure (spacegroup R3̄c) for x≤0.5. Neutron diffraction of La 0.3Sr 0.7CoO 3− δ , annealed at 650°C in air showed ideal cubic symmetry. HRTEM and SAED studies, however, revealed microdomains with an a c× a c×2 a c superstructure in addition to regions without such a superstructure. The superstructure could be explained by assuming the removal of all the oxygen ions from every other (001) AO 4/4 plane. PEELS measurements indicated a lower oxidation state for Co in the superstructure regions than in the non-superstructure regions. The importance of oxygen-vacancy ordering on the magnitude of ionic conductivity is discussed.

Formation and migration of cation defects in the perovskite oxide LaMnO3

Atomistic simulation techniques have been employed to investigate the energetics of cation formation and migration in cubic, rhombohedral and orthorhombic LaMnO 3 . The calculations suggest that for rhombohedral and orthorhombic lanthanum manganite, oxidative nonstoichiometry leads to the formation of cation vacancies on both La and Mn sites, though tending towards more La vacancies. The activation energy for lanthanum vacancy migration was found to increase with departure from cubic perovskite symmetry in the order: cubic cubic directions. Calculated migration energies for this path also increased with distortion from the cubic form. The effect of composition on cation migration energies was also examined.

Structure and symmetry of CaFeTi2O6 perovskite

A new calcium iron (II) titanate ordered perovskite (CaFeTi2O6) was recently synthesized from an equimolar mixture of CaTiO3 and FeTiO3 at 12-15 GPa and 1200-1400 °C. In the present paper, we discuss the structure and symmetry studies of this new compound CaFeTi2O6 using CBED and HREM techniques. The CaFeTi2O6 powder sample was crushed to small fragments with an agate mortar and pestle under purified methanol. A drop of the resulting suspension was placed on a copper grid coated with holey-carbon film. CBED and HREM studies were performed on a Philips-CM20 ST transmission electron microscope equipped with a double tilt, liquid-nitrogen-cooled specimen holder under moderate vacuum conditions over the range from 10-6-10-7 Torr. CBED makes it possible to examine the diffraction symmetry of key orientations of the crystal and therefore determine the point-group symmetry of the crystal. This information, along with the dynamic extinction information on systematic absences, can be used to determine the crystallographic space group uniquely.

Mantle layering from ScS reverberations: 2. The transition zone

This is the second paper in a four‐part sequence that examines the nature of mantle layering using the ScSn phases and internal reflections observed within the reverberative interval of SH‐polarized seismograms. Mantle reflectivity profiles for 18 seismic corridors sampling portions of Indonesia, Australia, and the western and central Pacific are constructed from low‐frequency (25‐mHz) ScS reverberations; i.e, ScSn‐type phases reflected once from internal mantle discontinuities. Modeling the reflectivity profiles using synthetic seismograms yields precise, path‐specific estimates of the travel times to the 410‐km and 660‐km discontinuities and their reflection coefficients. The data are consistent with an olivine‐dominated model of the transition zone, marked by two major phase transitions having average apparent depths of 414±2 km and 660±2 km and impedance contrasts of 0.046±0.010 and 0.072±0.010, respectively. The interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km (peak to peak) and is negatively correlated, consistent with an endothermic transition at 660 km whose Clapeyron slope magnitude is comparable to that of the 410‐km (exothermic) transition. In the case of the 660‐km discontinuity, we have been able to supplement these long‐wavelength (1500–5000 km) observations with an estimate of intrapath topography, inferred from the frequency dependence of the 660‐km reflection coefficient, which gives peak‐to‐peak variations less than 40 km on scale lengths of 500–1500 km. For the 660‐km discontinuity, the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction. However, we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660‐km discontinuity which may involve small compositional variations. Plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition, loss of reflected energy due to local curvature (or roughness) of the reflector, and/or extreme topography occurring on a small compositional component to the discontinuity (≤40% of the total impedance contrast). By stripping the 410‐km and 660‐km peaks from the composite path reflectivity profile, we identify three minor reflectors at mean depths of 520, 710, and 900 km. The shallowest may mark the β‐phase → γ‐spinel phase transition. The 710‐km discontinuity can be attributed to the ilmenite (gamet) → perovskite phase transition, although changes in perovskite symmetry can potentially provide explanations for both the 710‐km and 900‐km features.

ChemInform Abstract: BARIUM TITANATE PEROVSKITE SINTERED WITH LITHIUM FLUORIDE

AbstractDas Sinterverhalten von BaTiO3 mit l und 2 Gew.‐% LiF wird für 2 Stöchiometrien mit Ti/Ba = 1 und 0.975 untersucht.

Barium Titanate Perovskite Sintered with Lithium Fluoride

The sintering of barium titanate with, respectively, 1 and 2 wt% LiF for two stoichiometries, Ti/Ba=1 and 0.975, was studied using two calcined powders. One was pure barium titanate; the other contained BaTi03 plus BaC03 and TiO2 that did not react when burning. The sintering chronology—intermediate phases, appearance, and disappearance of a liquid phase that has been pointed out for the first time—is directly dependent on the used calcined powder, on the LiF amount, and on the firing schedule. In the same way, the obtained perovskite symmetry varies during sintering from tetragonal to cubic and then to a second tetragonal form, whereas most of the Li and F disappear from the ceramic with two different kinetics.

Ferroelectric and Structural Properties of the Pb(Sc1/2Nb1/2)1‐x,Tix,O3 System

Extensive solid solution was observed in the system Pb(Sc1/2/,Nb1/2,)1‐x,Tix,O3. In the range 0 ≤ x ≤ 0.425 a rhombohedral ferroelectric phase was stable at 25° C. In the range 0.45 ≤ x ≤ 1.00 a tetragonal ferroelectric phase was stable at this temperature. The phase diagram of the system below 500° C strongly resembles that of PbZrO3−PbTiO3. The compound Pb(Sc1/2Nb1/2)O3 exhibited rhombohedral perovskite cell symmetry below the ferroelectric ↔ paraelectric transition temperature, and the angle a was acute. The radial coupling coefficient was 0.46 for the composition Sc1/2Nb1/2)0.575Ti0.4250O3. At 25°C this composition consisted primarily of the rhombohedral phase with a small amount of the tetragonal phase present. The ferroelectric ↔ paraelectric transition occurred over a temperature range in the rhombohedral phase field. The spontaneous polarization was finite at temperatures considerably above the temperature of the permittivity maximum for a given rhombohedral solid solution.

Evidence for New Phase occurring in Portland Cements

THE X-ray powder diffraction patterns normally obtained from Portland cements contain only lines which can with confidence be said to be due to the presence of the four major phases, alite (modified C3S), C2S, C3A*, a ferrite phase in the solid solution series C6A2F–C4AF–C2F and minor constituents such as CaO, MgO and CaSO4. Recently, however, I have obtained from a few cements diffraction patterns in which a number of unusual lines were present. The new lines, from their position and intensity, appear to be produced by material having a structure closely similar to perovskite, that is, orthorhombic, pseudo-cubic, α′= 3.8 A. This material cannot be true perovskite (CaO.TiO2) because the TiO2 content of the cements is too small (Table 1). The structure of C3A bears some resemblance to that of perovskite, being cubic with a cell-side four times that of the pseudo-cell of perovskite, and careful consideration of the intensities of reflexions suggests that these cements may contain little or no C3A and that it has been replaced by a similar phase with the lower symmetry of perovskite.