Efectos del orden estructural en las propiedades eléctricas y magnéticas de óxidos multi-catiónicos con estructura derivada de la perovskita

Los óxidos mixtos con estructura tipo perovskita, de fórmula general ABO3, forman familias de compuestos de gran interés tanto desde el punto de vista científico como tecnológico. Esta estructura ofrece una gran variedad de posibilidades de sustitución de cationes en las posiciones A y/o B para formar sistemas complejos con dos o más tipos de cationes en ambas posiciones. Esta flexibilidad composicional está asociada a una flexibilidad estructural que permite acomodar la variedad de tamaños y estados de oxidación de los diferentes cationes. En este sentido, el orden de los cationes en posiciones A y B de óxidos con estructura derivada del tipo perovskita ha generado un gran interés en los últimos años debido a su gran impacto sobre las propiedades eléctricas y magnéticas, lo que los convierte en materiales con un gran interés funcional. Entre los diferentes modelos de orden que se pueden establecer, el orden de tipo laminar de los cationes en las posiciones A ha sido poco explorado por su mayor dificultad de estabilización. Un método eficaz para inducir este tipo de orden consiste en la co-existencia de vacantes aniónicas, que también pueden llegar a ordenarse, estableciendo un orden laminar adicional dentro de la subred aniónica con interesantes implicaciones en las propiedades de estos óxidos. El objetivo general de este trabajo ha consistido en preparar óxidos con estructura derivada de la perovskita, con orden laminar en ambas subredes, catiónica y aniónica, y estudiar los efectos del orden estructural en sus propiedades eléctricas y magnéticas..

Influence of Structural (Cation and Anion) Order in the Superconducting Properties of Ozone-Oxidized Mo0.3Cu0.7Sr2RECu2Oy (RE = Yb, Tm, Gd, Nd, and Pr)

The influence of rare earth (RE) elements on superconducting properties of the transition element (TE)-substituted TExCu1−xSr2RECu2Oy cuprates has not been sufficiently emphasized so far. In the case of molibdocuprates with the general formula Mo0.3Cu0.3Sr2RECu2Oy, all the RE element containing compounds except La, Ce, and Lu can be prepared at room pressure. The influence of the crystal structure on the superconducting properties after ozone oxidation of the present system is reported selecting three groups of RE elements attending to their different atom sizes: small (Yb and Tm), medium (Gd), and big (Nd and Pr). Advanced transmission electron microscopy, various diffraction techniques, and spectroscopic analysis have been used to demonstrate that the increase of structural disorder complemented with a decrease in the hole content play a major role in the vanishing of superconductivity within the present system

Structural and dielectric properties of ultra-fast microwave-processed La_0.3Ca_0.7Fe_0.7Cr_0.3O3-delta ceramics

Perovskite La_0.3Ca_0.7Fe_0.7Cr_0.3O_(3-delta) (LCFCr) is a mixed ionic and electronic conductor (MIEC) that can be employed as an electrode material in reversible solid oxide fuel cells (RSOFCs). In this work, an ultra-fast (15 min) one-step microwave (MW)-assisted combustion synthesis route has been developed to obtain phase pure and highly crystalline LCFCr powder. The synthesized powders exhibited a sponge-like microstructure with increased electrochemical reaction sites. Neutron thermodiffraction analysis revealed a structural transition above 500 degrees C from the room temperature (RT) orthorhombic Pnma to a rhombohedral R3c perovskite phase. The oxygen vacancy concentration was found to increase from delta = 0.272(7) at RT to delta = 0.333(5) at 900 degrees C. Furthermore, a 3-dimensional G-type antiferromagnetic structure was detected at RT. MW-sintering of pressed green ceramic pellets was carried out at 950 degrees C for 1 h, using a MW-transparent quartz fiber crucible or alternatively a SiC crucible acting as a MW-absorber. Impedance spectroscopy data on sintered ceramic pellets revealed electronic inhomogeneity as demonstrated by the occurrence of three dielectric relaxation processes associated with two grain boundary (GB)-like contributions and one bulk. The dielectric inhomogeneity encountered may be restricted to the extrinsic GB areas, which may be rather thin. More homogeneous dielectric properties of the GBs were found in the pellet that was sintered in the SiC crucible

Fluorochemicals from fluorspar via a phosphate-enabled mechanochemical process that bypasses HF

All fluorochemicals-including elemental fluorine and nucleophilic, electrophilic, and radical fluorinating reagents-are prepared from hydrogen fluoride (HF). This highly toxic and corrosive gas is produced by the reaction of acid-grade fluorspar (>97% CaF2) with sulfuric acid under harsh conditions. The use of fluorspar to produce fluorochemicals via a process that bypasses HF is highly desirable but remains an unsolved problem because of the prohibitive insolubility of CaF2. Inspired by calcium phosphate biomineralization, we herein disclose a protocol of treating acid-grade fluorspar with dipotassium hydrogen phosphate (K2HPO4) under mechanochemical conditions. The process affords a solid composed of crystalline K3(HPO4)F and K2-xCay(PO3F)a(PO4)b, which is found suitable for forging sulfur-fluorine and carbon-fluorine bonds

Multiferroism Induced by Spontaneous Structural Ordering in Antiferromagnetic Iron Perovskites

Room-temperature multiferroism in polycrystalline antiferromagnetic Fe perovskites is reported for the first time. In the perovskite-type oxides RE1.2Ba1.2Ca0.6Fe3O8 (RE = Gd, Tb), the interplay of layered ordering of Gd(Tb), Ba, and Ca atoms with the ordering of FeO4-tetrahedra (T) and FeO6-octahedra (O) results in a polar crystal structure. The layered structure consists of the stacking sequence of RE/Ca-RE/Ca-Ba-RE/Ca layers in combination with the TOOT sequence in a unit cell. A polar moment of 33.0 μC/cm2 for the Gd-oxide (23.2 μC/cm2 for the Tb one) is determined from the displacements of the cations, mainly Fe, and oxygen atoms along the b-axis. These oxides present antiferromagnetic ordering doubling the c-axis, and the magnetic structure in the Tb compound remains up to 690 K, which is one of the highest transition temperatures reported in Fe perovskites

Conversion of Li2FeSbO5 to the Fe(III)/Fe(V) phase LiFeSbO5 via topochemical lithium extraction.

Reaction between Na2FeSbO5 and LiNO3 at 300 °C yields the metastable phase Li2FeSbO5 which is isostructural with the sodium “parent” phase (space group Pbna, a = 15.138(1) Å, b = 5.1440(3) Å, c = 10.0936(6) Å) consisting of an alternating stack of Li2Fe2O5 and Li2Sb2O5 sheets containing tetrahedral coordinated Fe3+ and octahedrally coordinated Sb5+, respectively. Further reaction between Li2FeSbO5 with NO2BF4 in acetonitrile at room temperature yields LiFeSbO5, which adopts an orthorhombic structure (space group Pbn21, a = 14.2943(4) Å, b = 5.2771(1) Å, c = 9.5610(3) Å) in which the LiFeO5 layers have shifted on lithium extraction, resulting in an octahedral coordination for the iron cations. 57Fe Mössbauer data indicate that the nominal Fe4+ cations present in LiFeSbO5 have disproportionated into a 1:1 combination of Fe3+ and Fe5+ centers which are ordered within the LiFeSbO5 structural framework. It is widely observed that Fe4+ centers tend to be unstable in delithiated Li–Fe–X–O phases currently proposed as lithium-ion battery cathode materials, so the apparent stability of highly oxidized Fe5+ centers in LiFeSbO5 is notable, suggesting cathode materials based on oxidizing Fe3+ could be possible. However, in this instance, the structural change which occurs on delithiation of Li2FeSbO5 prevents electrochemical cycling of this material

Structural Ordering Supremacy on the Oxygen Reduction Reaction of Layered Iron-Perovskites

Depto. de Química InorgánicaFac. de Ciencias QuímicasTRUEpu

Crystal and magnetic structures of the Ir(V) JeffIr = 0 double perovskite LaSrNiIrO6

LaSrNiIrO6 adopts a B-site cation-ordered double perovskite structure with a strong a–a–c​+ ​cooperative tilting distortion (space group P21/n a ​= ​5.5931(1) Å, b ​= ​5.5676(1) Å, c ​= ​7.8850(1) Å, β ​= ​90.01(1) o). Magnetization and neutron diffraction data indicate LaSrNiIrO6 adopts a ‘type II’ antiferromagnetic structure (TN ​= ​70 ​K) in which the Ni spins are arranged in an antiferromagnetic manner with their next-nearest-neighbors, with no ordered moment observed for Ir. DFT calculations, including spin-orbit coupling effects, confirm S ​= ​1 Ni2+ and Jeff ​= ​0 Ir5+ local configurations and indicate strong Ni(3dx2−y2)1 –O(2p)–Ir(5dx2−y2)0–O(2p)–Ni(3dx2−y2)1 σ-type super-super-exchange is the dominant magnetic coupling interaction in the system

Complex modulation of the crystal structure of a layered perovskite. A promising solid-oxide-fuelcell cathode

Layered-type perovskites have been the focus of research in the last few years due to their outstanding properties as solid oxide fuel cell oxygen electrodes. The synthesis of a new (GdBa)0.8Ca0.4Co0.6Fe1.4O6-d layered perovskite as a single phase as well as the crystal structure determination and catalytic activity for oxygen reduction is reported. Highly advanced methods of transmission electron microscopy have been used for structure determination at atomic resolution. The oxide shows a complex modulation of the crystal structure associated with layered-type ordering of the Gd, Ba and Ca atoms in combination with formation of different coordination polyhedra of the Fe and Co atoms. Location of the anion vacancies within particular highly ion-conducting layers of the crystal structure has been achieved by phase image reconstruction. This material exhibits excellent electrochemical behaviour and thermal stability for use as an air electrode in IT-SOFCs.Depto. de Química InorgánicaFac. de Ciencias QuímicasTRUEpu

3D to 2D Magnetic Ordering of Fe3+ Oxides Induced by Their Layered Perovskite Structure

The antiferromagnetic behavior of Fe3+ oxides of composition RE1.2Ba1.2Ca0.6Fe3O8, RE2.2Ba3.2Ca2.6Fe8O21, and REBa2Ca2Fe5O13 (RE = Gd, Tb) is highly influenced by the type of oxygen polyhedron around the Fe3+ cations and their ordering, which is coupled with the layered RE/Ba/Ca arrangement within the perovskite-related structure. Determination of the magnetic structures reveals different magnetic moments associated with Fe3+ spins in the different oxygen polyhedra (octahedron, tetrahedron, and square pyramid). The structural aspects impact on the strength of the Fe-O-Fe superexchange interactions and, therefore, on the Neel temperature ( ́ TN) of the compounds. The oxides present an interesting transition from three-dimensional (3D) to two-dimensional (2D) magnetic behavior above TN. The 2D magnetic interactions are stronger within the FeO6 octahedra layers than in the FeO4 tetrahedra layers