High-pressure synthesis, structural and complex magnetic properties of the ordered double perovskite Pb2NiReO6

The ordered double perovskite Pb2NiReO6 has been prepared at 6 GPa and temperatures ranging from 1273 to 1373 K. Its crystal structure determined by X-ray powder diffraction and selected area electron diffraction shows monoclinic symmetry with centrosymmetric space group I2/m (a = 5.6021(1) Å, b = 5.6235(1) Å, c = 7.9286(1) Å and β = 90.284°(1)). High angle annular dark field microscopy studies reveal the existence of compositional microdomains. The compound displays a re-entrant spin-glass transition from a ferrimagnetic ordering below TN 37 K between the Re+5 and Ni+3 (high spin configuration) magnetic sublattices to a spin-glass configuration. Magnetic field dependent magnetization measurements revealed wasp-waisted hysteresis loops at 5 K. These shaped features originate from the antiferromagnetic/ferromagnetic (AFM/FM) competing interactions

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

Chirp-dependent dual light emission in Na_(0.95)Er_(0.05)Nb_(0.9)Ti_(0.1)O_(3) perovskite

Polar Na_(0.95)Er_(0.05)Nb_(0.9)Ti_(0.1)O_(3) perovskite has been synthesized as a nanostructured material by microwave assisted hydrothermal method. The characterization indicates that erbium is a constituent of the crystal structure and is preferentially located in sodium positions. The compound combines the nonlinear optical properties of the host (NaNb_(0.9)Ti_(0.1)O_(2.95) and the fluorescent properties of the Er^(3+)-dopant. Under excitation by a single femtosecond (< 10 fs) laser in the near-infrared region, simultaneous dual emission signals of second harmonic generation (SHG) and up-converting fluorescence (UCF) are observed and the nonlinear dependencies of the SHG and UCF intensities on the excitation intensity are measured. In addition, both emissions are shown to be sensitive to the chirp of the exciting pulses, and for UCF, it can be explained by means of a simple theoretical model based on the density matrix equations. These nano-structured particles with chirp-dependent dual behavior can be very advantageous when used in biological systems, since they can provide complementary information in different spectral ranges and tissues and are susceptible of coherent control, which can be both useful in optical microscopy and bioimaging

Thermal route for the synthesis of maghemite/hematite core/shell nanowires

Nowadays, iron oxide-based nanostructures are key materials in many technological areas. Their physical and chemical properties can be tailored by tuning the morphology. In particular, the possibility of increasing the specific surface area has turned iron oxide nanowires (NWs) into promising functional materials in many applications. Among the different possible iron oxide NWs that can be fabricated, maghemite/hematite iron oxide core/shell structures have particular importance since they combine the magnetism of the inner maghemite core with the interesting properties of hematite in different technological fields ranging from green energy to biomedical applications. However, the study of these iron oxide structures is normally difficult due to the structural and chemical similarities between both iron oxide polymorphs. In this work, we propose a route for the synthesis of maghemite/hematite NWs based on the thermal oxidation of previously electrodeposited iron NWs. A detailed spectroscopic analysis based on Raman, Mossbauer, and X-ray absorption shows that the ratio of both oxides can be controlled during fabrication. Transmission electron microscopy has been used to check the core/shell structure of the NWs. The biocompatibility and capability of internalization of these NWs have also been proven to show the potential of these NWs in biomedical applications

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

Estudio microestructural de sulfuros metálicos MS-R2S3 (M: Mg, Ca; R: Ce, Nd, Yb)

Esta tesis consiste en la preparación y caracterización estructural-microestructural mediante microscopía electrónica de transmisión (TEM), técnicas asociadas y difracción de rayos X, de nuevos sulfuros basados en tierras raras. Se ha realizado el estudio de sus posibles aplicaciones como pigmentos inorgánicos ecológicos en plásticos y pinturas. Los sistemas estudiados consisten en soluciones sólidas coloreadas, con diferente comportamiento estructural. La solución sólida (Ce1-yNdy)10S14+xO1-x presenta estructura tipo b-Pr10S14O. Los sistemas M1-xR(2/3)x�(1/3)xS (M=Ca, Mg; R=Yb) (�: vacantes catiónicas) presentan estructura tipo NaCl. Cuando M: Ca se observa intensidad difractada difusa en los diagramas de difracción de electrones, asociada a la presencia incipiente de clusters de una superestructura cúbica que se observa a altas concentraciones de Yb3+. Para M: Mg, se ha caracterizado mediante TEM y Espectroscopia EELS la transición desde el tipo NaCl al tipo espinela, a través de orden a corto alcance, provocado por clusters tridimensionales y con segregación de defectos extensos. El sistema M(3/2)yR2-y�0.25 - (1/2)yS3 (M = Ca; R = Nd) presenta estructura tipo ?-Th3P4 al introducir Ca2+, suponiendo la estabilización de dicho tipo estructural a 1173 K, mientras que en ausencia de Ca2+ se observa la fase a-Nd2S3. Además se han preparado nuevas fases en el sistema Mg-Yb-S, cuyas estructuras han sido determinadas mediante TEM y técnicas asociadas.. Su estructura puede derivarse a partir del tipo NaCl por la aplicación repetida y periódica de la operación cristalográfica de maclado químico a nivel de celda unidad. Las estequiometrías observadas son MgYb2S4 y Mg2Yb6S11 (grupo espacial Cmcm); M3Yb8S15 y MgYb4S7 (grupo espacial C2/m). El color de las muestras obtenidas se ha cuantificado mediante las coordenadas colorimétricas L*, a* y b*. El sistema Ce-Nd-S-O presenta color rojo, cuyo tono varía con su composición. Los sistemas Ca-Nd-S y Ca-Yb-S presentan color verde, mientras que el sistema Mg-Yb-S presenta color amarillo de alta calidad

Structured diffuse scattering, local crystal chemistry and metal ion ordering in the (1 - x )MgS • x 3Yb 2 S 3 , 0≤ x ≤~0.45, 'defect' NaCl system

Rare earth sulfide solid solutions of 'defect' NaCl average structure type exhibit excellent colour characteristics and thus represent good prospects for use in the paints and plastics industry. As the colour characteristics of such systems are closely related to the local distribution of rare earth ions, it is important to know as much as possible about the latter. In this contribution, the reciprocal space shape of a highly structured, diffuse intensity distribution observed in a (1 -x)MgS xYb2/3S, x = 0.30, sample is used to show that the smallest polyhedral building blocks of the average NaCl structure type should each have, as far as possible, the same composition as the overall macroscopic composition. Local crystal chemical considerations suggest that both the Yb3+ ions and the vacancies would prefer to be opposite Mg2+ ions

Unravelling the complex nanostructure of La0.5−xLi0.5−xSr2xTiO3 Li ionic conductors

The origin of the intricate nanostructure of LaLiSrTiO (0.0625 ≤ x ≤ 0.25) perovskite-type Li ion conductors has been investigated. Reciprocal space electron diffraction analysis and aberration-corrected STEM by combining annular bright field (ABF) and high angle annular dark field (HAADF) imaging methods have been used to elucidate the complex local atomic arrangements which cannot be adequately described by average crystal structure models. Two different local crystal structures endotaxially-related at the nanoscale without compositional phase separation associated, constituting the crystals. Self-organization of the two different ordered regions arises as a consequence of the competition between two distortive forces in the crystal lattice: octahedral tilting and second-order Jahn-Teller distortion of TiO octahedra. Changes in the distribution of A species suggest different Li ion conduction pathways for the two structures and this scenario has difficult long-range Li mobility. The detailed study performed may be helpful in understanding the local structural changes affecting Li and their relation to the conductivity in LLTO-derived ionic conductors.Authors acknowledge the financial support through projects MAT2013-46452-C4 and CM-S2013/MIT-2753 and the ICTS-Centro Nacional de Microscopia Electrónica for instrumental facilities