Perovskite-type Crystal Structure Research Articles

Nanostructured codoped neodymium orthoferrite synthesized via the hydrothermal route for photocatalytic annihilation of crystal violet dye

Current research demonstrates the synergistic effects of gadolinium (Gd)/copper (Cu) codoping and nanotechnology on various physicochemical and photocatalytic features of neodymium iron oxide (NIO) perovskites. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) studies the synthesis of pure neodymium oxide (NIO-I) and its codoped counterpart (NIO-II) with a perovskite-type crystal structure via simple hydrothermal techniques. Through codoping and nanostructural synthesis, the surface characteristics, as well as the optical, electrical, and optoelectronic properties of the neodymium iron oxide perovskite, were enhanced, as confirmed by Brunner−Emmett−Teller (BET), UV/Vis, current−voltage (I−V), and photoluminescence (PL) studies. In terms of photocatalytic activity, the NIO-II perovskite outperforms its undoped NIO-I counterpart, eliminating 91.8 % of crystal violet (CV) dye in less than 50 min with a rate constant value (k) of 0.019 min−1. However, the undoped counterpart removed 63.8 % of the CV dye, with a comparatively lower rate constant of 0.013 min−1 under the same conditions. Under alkaline conditions, the NIO-II catalyst demonstrates superior efficiency in degrading CV dye; moreover, it maintains an efficiency of 96.8 %, even after undergoing five cycles of reuse. Through the use of rare earth and transition metal codoping, this study reveals a new way to fine-tune the structural and other pertinent properties of semiconductor perovskites, ensuring their practical use for the treatment of polluted water.

Reversible Solid Oxide Cells: Cycling and Long-Term Durability of Air Electrodes

Introduction Reversible solid oxide cell (r-SOC) enables both power generation and electrolysis, when the cell can be operated as a solid oxide fuel cell (SOFC) and as a solid oxide electrolysis cell (SOEC). Many studies are being conducted to improve their performance and durability (1,2).Lanthanum strontium cobalt ferrite (LSCF) is an air electrode material with a perovskite-type crystal structure which exhibits high electronic and ionic conductivity, oxygen diffusivity, and electrocatalytic activity (3). However, performance and durability of the cells with Sr-containing oxide air electrodes have to be carefully analyzed, as Sr ions tend to easily diffuse during sintering and in long-term operation, reacting with Zr ions from the solid electrolyte to form a highly resistance reaction layer such as SrZrO3 (4-6). While the performance and durability of LSCF-based air electrodes and the Sr diffusion in SOFC operation have been studied, there are still limited number of studies on the SOEC operation, and especially on the r-SOC operation. The diffusion of constituent ions such as Co and Fe may not be negligible in SOCs.Here in this study, we fabricate YSZ electrolyte-supported r-SOCs and conduct durability studies in both SOFC and SOEC modes. STEM-EDS observation of the air electrode materials is also made to investigate the durability and diffusion of various elements in the LSCF-based air electrode of r-SOCs. Experimental R-SOCs with yttria stabilized zirconia electrolytes (YSZ, 200 µm thick) were prepared. Ni-GDC co-impregnated fuel electrode was prepared, where a mixture of La0.1Sr0.9TiO3 (LST) and Gd0.1Ce0.9O2 (GDC) at a volume ratio of 50:50 was used as the porous electrode backbone, onto which Ni-containing solution of 1 µL was impregnated for Ni loading of 0.167 mg cm-2 (7). Gd0.1Ce0.9O2 (GDC) buffer layer was prepared between the electrolyte and the air electrode to prevent elemental diffusion (8). La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) was used for the air electrode. In addition, a Pt reference electrode was deposited onto the electrolyte on the fuel electrode side. The voltage measurement terminals of the electrochemical measurement setup were connected between the reference electrode and the air electrode to evaluate the air electrode potential. 50%-humidified hydrogen (100 ml min-1) was supplied to the fuel electrode, and air (150 ml min-1) was supplied to the air electrode. Negative current density means the value in the SOEC mode, while positive current density in the SOFC mode. Results and discussion As shown in Fig. 2 describing one cycle, current density was varied at 800°C for the 1000-cycle durability tests in the r-SOC operation. The range of current density was between -0.2 A cm-2 and 0.2 A cm-2. The air electrode potential at different current densities and the electrode impedance in every 100 cycles were measured. For the 1000-hour electrolysis durability test in the SOEC mode, current density of -0.2 A cm-2 was applied for 1000 hours at 800°C. The air electrode potential and the impedance in every 100 hours were measured (9). It has been found that a gradual degradation of the r-SOCs associated with an increase in air electrode potential in the SOEC mode and a decrease in the potential in the SOFC mode could be distinguished. However, such degradation tended to be stabilized with the number of cycles.STEM-EDS observations were performed to analyze the elemental distribution around the air electrode and the electrolyte in the cell. It was found that such elemental diffusion could occur during sintering and durability experiments.AcknowledgmentsA part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO) (Project No. JPNP20005). Collaborative support by Prof. H. L. Tuller, and Prof. B. Yildiz at Massachusetts Institute of Technology (MIT) for their continuous support is gratefully acknowledged. Figure 1

Crystal structure, magnetism, and thermoelectric properties of Nd1−x Sr x FeO3−δ (0.1 ≤ x ≤ 0.9)

Polycrystalline Nd1−x Sr x FeO3−δ (0.1 ≤ x ≤ 0.9) samples exhibit a single-phase perovskite-type crystal structure and G-type antiferromagnetism with small ferromagnetic order. Assuming that the Fe site is in the mixed-valence state, the spin state of Fe changes from a low-spin (LS) or intermediate spin Fe3+-dominant state at x ≤ 0.5 to a LS Fe4+ dominant state at x ≥ 0.6. This strongly suggests that the charge carriers change from holes to electrons. In fact, at temperatures less than 500 K, these samples show a p-type Seebeck coefficient for 0.1 ≤ x ≤ 0.5 and an n-type Seebeck coefficient for 0.6 ≤ x ≤ 0.9. However, for 0.6 ≤ x ≤ 0.9, Fe4+ is reduced to Fe3+ because of an oxygen deficiency in the 500–600 K temperature range. In particular, Nd0.7Sr0.3FeO2.99(1) shows the largest p-type ZT = 0.025 at 765 K, whereas compositions with high n-type ZT could not be identified.

Investigation on magnetic and magnetocaloric properties of Sr0.25Ca0.75Mn0.9Ti0.1O3 perovskite

Interest in magnetic refrigeration, which is based on the magnetocaloric effect (MCE), has greatly increased during the past two decades. As a less-polluting and more effective cooling technology than gas compression, magnetic refrigeration is one of the prominent and possible options. Perovskite Oxides played a major contribution for the development of magnetic refrigeration (MR). Sr0.25Ca0.75Mn1-xTixO3 (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) polycrystalline samples were synthesized by conventional solid-state reaction. Its cubic perovskite-type crystal structure is discovered to be of the Pm-3m space group. At T = 31.3 K, the alloy experiences antiferromagnetic transition for the composition of Sr0.25Ca0.75Mn0.9Ti0.1O3. It demonstrates that the greatest magnetocaloric reports are 8 J/kg K for a magnetic field of 7 Tesla and 3.2 J/kg K for 1 Tesla. These (ΔSM) value is comparable to the magnetization values of the ferromagnetic Heusler alloys and are very high in these kinds of antiferromagnetic perovskite systems. For the Sr0.25Ca0.75Mn0.9Ti0.1O3 material, this is the first report of substantial magnetic entropy changes brought on by a weak magnetic field.

Perovskite as a spin current generator

We theoretically show that materials with perovskite-type crystal structures provide a platform for spin current generation, taking advantage of a mechanism requiring neither the spin-orbit coupling nor a ferromagnetic moment, but is based on spin-split band structures in certain kinds of collinear antiferromagnetic states. By investigating a multiband Hubbard model for transition metal compounds by means of the Hartree-Fock approximation and the Boltzmann transport theory, we find that a pure spin current is induced by an electric field applied to a C-type antiferromagnetic metallic phase. The spin current generation originates from a cooperative effect of spatially anisotropic electron transfer integrals owing to the GdFeO$_3$-type lattice distortion, which is ubiquitous in many perovskites, and the collinear spin configuration. We discuss our finding from the symmetry point of view, in comparison with other spin current generator candidates with collinear antiferomagnetism. We also propose several ways to detect the phenomenon in candidate perovskite materials.

Structure, morphology and electrochemical properties of SrTiO3 perovskite: Photocatalytic and supercapacitor applications

In order to explore an alternative photocatalyst for environmental remediation, we report the sol-gel process for the synthesis of strontium titanate (SrTiO3) nanoparticles (STNPs). The as-synthesized STNPs were found to poses cubic perovskite-type crystal structure with an average crystallite size of 22 nm were well-characterized by X-ray diffraction (XRD). The agglomerated and cubic like morphologies were identified by using scanning electron microscopy (SEM). The elemental analysis by energy dispersive X-Ray spectroscopy (EDAX) confirmed the composition of STNPs. The synthesized nanoparticles were further characterized by Fourier transform infrared (FTIR) analysis. STNPs displayed good photocatalytic degradation activity for malachite green (MG) and rhodamine blue (Rh–B) dye pollutants under UV light irradiation in the time duration of 120 min. Furthermore, cyclic voltammetric studies revealed that the STNP electrode, because of its increased surface area, produced maximal specific capacitance of 208.47 Fg−1 at 1 mA/cm2 with a high cycle stability of 1500 cycles. The superior supercapacitance behaviour of SrTiO3 electrode materials strengthens its stance to be developed as a highly promising material for energy related applications. In addition, STNPs can serve as better materials for the environmental remediation of pollutant dyes.

Structural, magnetic and cryogenic magneto-caloric properties in RE2FeCrO6 (RE = Er and Tm) compounds

In this work, polycrystalline Er2FeCrO6 (EFCO) and Tm2FeCrO6 (TFCO) oxides were fabricated via conventional sol-gel method, and studied with respect of crystal structure together with cryogenic magnetic and magneto-caloric properties. Both oxides are confirmed to exhibit B-site disordered hexagonal perovskite-type crystal structure. Two magnetic transitions around 11.7 and 5.7 K for EFCO, whereas only one transition around 10.5 K for TFCO, have been observed. Both oxides exhibit considerable cryogenic reversible magneto-caloric effects. The values of magnetic entropy change peak and relative cooling power (refrigerant capacity) with 0–5 T magnetic field change reach 11.95 J/kg-K and 215.8 (169.8) J/kg for EFCO, and 4.78 J/kg-K and 123.6 (97.8) J/kg for TFCO, respectively, indicating the present Er2FeCrO6 oxide is also considerable for cryogenic magnetic cooling application.

Lanthanum Doped Strontium Titanate Nanomaterial for Photocatalytic and Supercapacitor Applications

We report the synthesis of lanthanum doped strontium titanate (Sr1-xLaxTiO3,x=0.1) by sol-gel method. The physical properties of the as-synthesized sample were systematically studied through X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDAX). Diffraction peaks in XRD supported the cubic formation of perovskite-type crystal structure. The image analysis of nanomaterial by SEM and TEM techniques disclosed aggregates of nanoparticles with grain size about 20 nm. The study by UV-DRS exposed the band energy gaps (Eg) of 3.4 eV for strontium titanate nanoparticles, respectively. The degradation studies for three days were carried out for three dyes. Malachite green and rhodamine blue, strontium titanate nanoparticles showed utmost photocatalytic activity for rhodamine blue under UV light irradiation (from 0 to 80 min) as compared to malachite green. Properties of electrochemistry were looked into by cyclic voltammetry and galvanostatic charge/discharge in 1M KCl electrolyte. The Sr0.9La0.1TiO3 electrode displayed maximal specific capacitance of 306.74 F g-1 at current 1mA from galvanostatic charge-discharge curve. The rare earth doped perovskite Sr0.9La0.1TiO3 nanomaterial exhibited increased surface area with superior supercapacitance property.

Stability of Optical Responses from Lead-free Perovskite Films

Materials with perovskite-type crystal structure attract much attention due to their unique optical properties, such as high quantum yield of photoluminescence and high carrier mobility. However, their further utilization as an active media in photonic and optoelectronic devices is limited by the lack of stability of their optical responses together with the presence of lead cations in the chemical composition. Here, a detailed study of the optical properties and their stability under ambient conditions of perovskite films with the chemical composition (CH3NH3)xAyIz, where A = Pb, Bi, Sb, was performed. The changes in optical responses of the samples were monitored for 3 months. It was shown that the stability of perovskite films was improved by surface passivation with a thin layer of polymethylmethacrylate onto their surface. The developed procedures of film formation and following surface passivation are of interest in the design of active optical materials based on perovskite films with the increased optical performance.

On the homogeneity of high entropy oxides: An investigation at the atomic scale

Homogeneous distribution of the constituent elements at the atomic scale in high entropy oxides (HEOs) was demonstrated for the first time by atom probe tomography (APT). Three different single-phase HEOs having rocksalt, fluorite, and perovskite type crystal structures were synthesized using nebulized spray pyrolysis process. The as-synthesized powders were compacted into pellets under high pressure of 6 GPa. Both as-synthesized and compacted powders were investigated by X-ray diffraction and transmission electron microscopy. APT results showed that the constituent cations in all three HEO systems were homogenously distributed with no significant segregation or concentration fluctuation even at the atomic scale.

Pressure Impact on the Stability and Distortion of the Crystal Structure of CeScO3.

The effects of high pressure on the crystal structure of orthorhombic (Pnma) perovskite-type cerium scandate were studied in situ under high pressure by means of synchrotron X-ray powder diffraction, using a diamond-anvil cell. We found that the perovskite-type crystal structure remains stable up to 40 GPa, the highest pressure reached in the experiments. The evolution of unit-cell parameters with pressure indicated an anisotropic compression. The room-temperature pressure-volume equation of state (EOS) obtained from the experiments indicated the EOS parameters V0 = 262.5(3) Å3, B0 = 165(7) GPa, and B0' = 6.3(5). From the evolution of microscopic structural parameters like bond distances and coordination polyhedra of cerium and scandium, the macroscopic behavior of CeScO3 under compression was explained and reasoned for its large pressure stability. The reported results are discussed in comparison with high-pressure results from other perovskites.

Synthesis, Phase Transition of La0.69Sr0.31MnO3 Nanoperovskites Toward Its Versatile Structural, Electrical and Mechanical Properties Employing Ultrasonic Measurements

The present investigation correlates the propagation of ultrasonic waves with various micro structural features of nano La0.69Sr0.31MnO3 (LSMO) perovskites with different grain sizes. A solid state reaction followed by the ball milling technique was used to synthesize the nano LSMO perovskite samples with various grain sizes. The occurrence of ferro-paramagnetic transition temperature (Tc) was explored through the observed anomalous behaviour in ultrasonic velocities, attenuations and elastic moduli. As the particle size gets reduced, lower Tc and broader transitions are observed due to the distribution of grain boundaries. The diffusion of the FM–PM phase transition along with decrease in Tc is correlated due to the effect of particle size. Furthermore structural, vibrational and electrical properties of nanosized LSMO perovskite samples with different grain sizes were studied by XRD, FTIR, BET surface area measurement, HRSEM, TEM and four probe conductivity studies. The phase of the synthesized nano LSMO perovskite samples are perfectly indexed to pure rhombohedral perovskite type crystal structure. The conductivity study proves the semiconductor nature of the nano LSMO perovskite samples. In addition to this, the peak broadening in ultrasonic studies at phase transition is in line with the observation made from the XRD pattern for the prepared nano LSMO perovskite samples.

Orbital- and spin-order sensitive nonlocal screening in Mn 2p X-ray photoemission of La1−xSrxMnO3

The Mn 2p X-ray photoemission spectra (XPS) of LaMnO3 (LMO) and hole-doped La0.7Sr0.3MnO3 (LSMO) are investigated using a dp model simulating Mn 3d and O 2p electrons under the perovskite-type crystal structure. The observed 2p XPS features, especially the low-binding-energy structure (LBES) of the main line, are reproduced well using an impurity Anderson model optimized from the dp model within the dynamical mean-field approximation. The LBES in both compounds is due to the nonlocal screening (NLS) between the neighboring Mn ions in the final state, but the screening character is quite different: The NLS in LSMO directly reflects the character of the ferromagnetic metal, while that in undoped LMO the C-type orbital order between and orbitals in the ab-plane. We emphasize the directive nature of the NLS in the orbital order system, which can be a sensitive probe to the order pattern.

Thin perovskite-type ferromagnetic film (La,Sr)CoO3

Properties of materials having a perovskite-type oxide crystal structure ABO3 can easily be modified by means of partial substitution of cations located at A or B sites. Nonstoichiometric lanthanum-strontium cobalt oxides (La,Sr)CoO3 also referred to as LSCO exhibit electrical conductivity and ferromagnetic properties. Perovskite-type compounds LaCoO3 and SrCoO3 can form solid solutions in every ratios to each other. Notation (La,Sr)CoO3, or La(1-x)SrxCoO3 means that some lanthanum ions are substituted with the strontium ones at A sites of crystal cell. This paper describes a sol-gel manufacturing process of LSCO thin films, presents measurements of their resistivity vs. composition, shows thermal coefficient of selected composition resistivity as well as its magnetoresistive properties.

Texture design for microwave dielectric (Ca0.7Nd0.3)0.87TiO3 ceramics through reactive-templated grain growth

Plate-like Ca3Ti2O7 (CT) and Nd2Ti2O7 (NT) particles were synthesized in molten salts and used as reactive templates for the preparation of highly textured (Ca0.7Nd0.3)0.87TiO3 bulk ceramics (CNT) with preferred pseudocubic 〈100〉 and 〈110〉 orientations, respectively. During flux growth CT and NT particles developed facets parallel to the pseudocubic {100} and {110} planes, respectively, in a perovskite unit cell, since those planes correspond to the interlayers of the layered perovskite-type crystal structures. Complementary reactants for the CNT stoichiometry were wet-mixed with the reactive templates and the slurries were tape-cast. Then stacked tapes were heat-treated for dense single-phase CNT ceramics with a distorted and A-site deficient regular perovskite-type structure. The CNT ceramics prepared with CT and NT reactive templates exhibited strong pseudocubic 100- and 110-family x-ray diffraction peaks, respectively, with other peaks drastically suppressed when non-perovskite sources were used as complementary reactants. The textured ceramics possess unique microstructures; as either parallel or obliquely stacked block structures with a pseudocubic {100} plane faceted. The pseudocubic {100}-and {110}-textured CNT ceramics exhibited ∼10 and ∼20% higher products of the dielectric quality factor and frequency, Q · f, respectively, than conventional ceramic sintered at the same temperature. When Q · f is compared based on the same grain size, the {100}-textured CNT exhibited 27% higher values than non-textured while relative permittivity and temperature coefficient of resonant frequency were of similar values. Simple geometrical relationships between electric field and penetrated pseudocubic {hk0}-type grain boundaries must lead to the reduced scattering and dielectric loss.

Multiscale simulation of domain switching behavior in polycrystalline ferroelectric materials

This paper presents a multiscale nonlinear finite element simulation aimed at revealing the relationship between the macroscopic hysteresis response and the microscopic crystal morphology of polycrystalline ferroelectric materials. We utilized an incremental form of constitutive law to consider changes in material properties caused by domain switching, and employed the asymptotic homogenization theory based on the perturbation method for scale-bridging of the macro- and microstructures. The proposed multiscale simulation was applied to two different ferroelectric materials, viz., barium titanate and lead titanate, both of which have a perovskite-type tetragonal crystal structure. The correlation between the macroscopic response under an external electric load and the microstructural change in crystal orientations caused by 90° and 180° domain switching was investigated. We also discussed the influence of the initial crystal orientation distribution and the difference between the two ferroelectric materials.

XAFS study on nanostructured (La;Sr)CoO and (La;Sr)(Co;Fe)O IT-SOFC cathodes

In the last years, extensive research has been devoted to develop novel materials and structures with high electrochemical performance for intermediate-temperatures solid-oxide fuel cells (IT-SOFCs) electrodes. In recent works, we have investigated the structural and electrochemical properties of nanostructured La0.6Sr0.4CoO3(LSCO) and La0.6Sr0.4(Co;Fe)O3(LSCFO) cathodes, finding that they exhibit excellent electrocatalytic properties for the oxygen reduction reaction [1,2]. These materials were prepared by a pore-wetting technique using polycarbonate porous membranes as templates. Two average pore sizes were used: 200 nm and 800 nm. Our scanning electronic microscopy (SEM) study showed that the lower pore size yielded nanorods, while nanotubes were obtained with the bigger pore size. All the samples were calcined at 10000C in order to produce materials with the desired perovskite-type crystal structure. In this work, we analyze the oxidation states of Co and Fe and the local atomic order of LSCO and LSCFO nanotubes and nanowires for various compositions by X-ray absorption spectroscopies. For this purpose we performed XANES and EXAFS studies on both Co and Fe K edges. These measurements were carried out at the D08B-XAFS2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS). XANES spectroscopy showed that Co and Fe only change slightly their oxidation state upon Fe addition. Surprisingly, XANES results indicated that the content of oxygen vacancies is low, even though it is well-known that these materials are mixed ionic-electronic conductors. EXAFS results were consistent with those expected according to the rhombohedral crystal structure determined in previous X-ray powder diffraction investigations.

Homogenization modeling of domain switching in ferroelectric materials

We presented a multiscale nonlinear finite element simulation to analyze domain switching behaviors in ferroelectric materials. We utilized an incremental form of fundamental constitutive law to consider changes in the material properties caused by domain switching. A multiscale nonlinear problem was formulated by employing the asymptotic homogenization theory based on the perturbation method and implemented using finite element analysis. The developed simulation was applied to barium titanate with a Perovskite-type tetragonal crystal structure. The 90° and 180° domain switching behaviors of a single crystal were computed for verification. The nonlinear behaviors of a bulk polycrystal with virtual microstructure were analyzed as a case study. The variation of the crystal orientation distribution in the polycrystalline microstructure was analyzed to reveal its influence on macroscopic hysteresis and butterfly curves.

Influence of type of precursors on the sol-gel synthesis of the LaCoO3 nanoparticles

Abstract Perovskite-type LaCoO3 was prepared by sol-gel method using nitrates / chlorides as precursors and citric acid as chelating agent. Chemical composition was obtained by means of EDX method. The structures of sintered samples were investigated by scanning electron microscopy (SEM), IR and XRD analysis. The results of X-ray diffraction indicated that the LaCoO3nanopowders obtained using nitrate as precursors had a rhombohedral perovskite-type crystal structure (S. G: R-3c), while that obtained using chloride as precursors had a mixture of LaCoO3, LaOCl and Co3O4. The all lanthanum cobaltites exhibit catalytic activity on the decomposition of hydrogen peroxide, ascribed to their higher surface and Co3+ concentration

High-Pressure Synthesis of Novel Hydrides and Intermetallic Compound in Al-X Systems (X=Sr, V, Hf)

Al–based hydrides have been extensively investigated in order to their application for hydrogen storage. To explore new hydrides, the samples in Al–X–H systems (X = Sr, V, Hf) were synthesized at 873–1173 K for 2–8 h under 5 GPa with internal hydrogen source. Unidentified phases in XRD analysis were observed in samples with nominal composition of Al–50mol%SrH2, AlH3–60mol%VH2 and Al–50mol%HfH2. Judging from SEM–EDX analysis, the chemical composition of these phases were Al/Sr=1/1, Al/V=2/3 and Al/Hf=1/2. In Al–Sr system, hydrogen content was determined to be about 2.78mass% by fusion analysis. Corresponding chemical formula of the new hydride was estimated to be AlSrH3 with a perovskite–type crystal structure. Hydrogen desorption of 0.55 mass%H in the Al-60V sample was observed above about 400K by TG–TDS. The new compound, Al2V3H1.8~2.2 had a tetragonal structure. In Al–Hf system, the new compound had almost no hydrogen. Then, the chemical fomula of newly founf compound was estimated to be AlHf2 with a tetragonal structure.