ABSTRACT
In this work, we have studied structural and magnetic properties of LaFeO3 as a function of the particle size d, from bulk (d >> 1 µm) to nanoscale (d ≈ 30 nm). A large number of twins were observed for large particles that disappear for small particle sizes. This could be related to the softening of the FeO6 distortion as particle size decreases. It was observed that the bulk sample showed spin canting that disappeared for d ~ 125 nm and can be associated with the smoothening of the orthorhombic distortion. On the other hand, for d < 60 nm, the surface/volume ratio became high and, despite the high crystallinity of the nanoparticle, a notable exchange effect bias appeared, originated by two magnetic interactions: spin glass and antiferromagnetism. This exchange bias interaction was originated by the formation of a "magnetic core-shell": the broken bonds at the surface atoms give place to a spin glass behavior, whereas the inner atoms maintain the antiferromagnetic G-type order. The LaFeO3 bulk material was synthesized by the ceramic method, whereas the LaFeO3 nanoparticles were synthesized by the sol-gel method; the particle size was varied by annealing the samples at different temperatures. The physical properties of the materials have been investigated by XRD, HRTEM, TGA, and AC and DC magnetometry.
ABSTRACT
Kagome antiferromagnetic lattices are of high interest because the geometric frustration is expected to give rise to highly degenerated ground states that may host exotic properties such as quantum spin liquid (QSL). Ca10Cr7O28 has been reported to display all the features expected for a QSL. At present, most of the literature reports on samples synthesized with starting materials ratio CaO/Cr2O3 3:1, which leads to a material with small amounts of CaCrO4 and CaO as secondary phases; this impurity excess affects not only the magnetic properties but also the structural ones. In this work, samples with starting material ratios CaO/Cr2O3 3:1, 2.9:1, 2.85:1, and 2.8:1 have been synthesized and studied by X-ray diffraction with Rietveld refinements, selected area electron diffraction measurements, high-resolution transmission electron microscopy (HRTEM), low-temperature magnetometry, and magnetic calorimetry. This result shows that a highly pure Ca10Cr7O28 phase is obtained for a CaO/Cr2O3 ratio of 2.85:1 instead of the 3:1 usually reported; the incorrect stoichiometric ratio leads to a larger distortion of the corner-sharing triangular arrangement of magnetic ions Cr+5 with S = 1/2 in the Kagome lattice. In addition, our study reveals that there exists another frustration pathway which is an asymmetric zigzag spin ladder along the directions [211], [12-1], and [1-1-1], in which the Cr-Cr distances are shorter than in the Kagome layers.
ABSTRACT
Nanocomposite films consisting of gold nanoparticles embedded in an yttria-stabilized zirconia matrix (Au-YSZ) have been synthesized with different gold loadings by reactive magnetron sputtering followed by ex situ annealing in air or laser interference patterning (LIP) treatment. It is shown that the electrical conductivity of the nanocomposite films can be modified to a large extent by changing the gold loading, by thermal annealing, or by LIP. The structural and microstructural analyses evidenced the segregation of metallic gold in crystalline form for all synthesis conditions and treatments applied. Thermal annealing above 400 °C is observed to trigger the growth of pre-existing nanoparticles in the volume of the films. Moreover, pronounced segregation of gold to the film surface is observed for Au/(Au + Zr + Y) ratios above 0.40, which may prevent the use of thermal annealing to functionalize gold-rich Au-YSZ coatings. In contrast, significant modifications of the microstructure were detected within the interference spot (spot size close to 2 × 2 mm) of LIP treatments only for the regions corresponding to constructive interference. As a consequence, besides its already demonstrated ability to modify the friction behavior of Au-YSZ films, the LIP treatment enables local tailoring of their electrical resistivity. The combination of these characteristics can be of great interest for sliding electrical contacts.
ABSTRACT
The cationic framework structure of a whole new family of compounds with the general formula Bi(2(n + 2))Mo(n)O(6(n + 1)) (n = 3, 4, 5 and 6) has been elucidated by transmission electron microscopy (TEM) methods. High-resolution transmission electron microscopy (HRTEM) has been used to postulate heavy-atom models based on the known structure of the n = 3 phase, Bi(10)Mo(3)O(24). These models were tested by HRTEM image simulation, electron diffraction and powder X-ray diffraction simulation methods which agreed with the experimental results. The four known phases of this family correspond to n = 3, 4, 5 and 6 members and all show fluorite superstructures. They consist of a common delta-Bi(2)O(3) fluorite-type framework, inside of which are distributed ribbons of {MoO(4)} tetrahedra which are infinite along b, one tetrahedron thick along c, and of variable widths of 3, 4, 5 or 6 {MoO(4)} tetrahedra along a depending on the family member (n value). These {MoO(4)} tetrahedra are isolated, i.e. without sharing any corner as in the [Bi(12)O(14)] columnar structural-type phase Bi[Bi(12)O(14)][MoO(4)](4)[VO(4)]. The structure of all these family members can be described as crystallographic shear derivatives from Aurivillius-type phases such as Bi(2)MoO(6), the n = infinity end member. All these compounds are good oxygen-ion conductors.
ABSTRACT
Oxygen engineering techniques performed under adequate controlled atmosphere show that the CaMnO(3)-CaMnO(2) topotactic reduction-oxidation process proceeds via oxygen diffusion while the cationic sublattice remains almost unaltered. Extra superlattice reflections in selected area electron diffraction patterns indicate doubling of the CaMnO(2) rock-salt cell along the cubic directions of a distorted rhombohedral cell originated by ordering of Ca(2+) and Mn(2+) ions distributed in nanoclusters into a NaCl-type matrix, as evidenced by dark field electron microscope images. The local nature of the information provided by the transmission electron microscopy techniques used to characterize the rock-salt type Ca(1-x)Mn(x)O(2) solid solution clearly hints at the existence of subtle extra ordering in other upper oxides of the Ca-Mn-O system. The combination of local characterization techniques like electron microscopy with more average ones like powder X-ray and neutron diffraction allows a very complete characterization of the system.
ABSTRACT
Transmission electron microscopy observations on a new complex oxybromide with nominal composition Bi(4)Fe(1/3)W(2/3)O(8)Br, heated at high temperature, reveal the transformation of its basic structure yielding two types of crystals. The first crystal type shows ordered and disordered extended defects leading to a new family of intergrowths between one Sillén block and n Aurivillius blocks and occasionally between one Aurivillius block and n Sillén blocks. The second type presents a compositionally modulated structure, determined by electron diffraction, with an average composition Bi(4)Fe(1/2)W(1/2)O(8 - delta)Br and unit-cell parameters a = (1/gamma) 3.8, b = 3.8, c = 14.5 A (gamma = 0.10-0.15) in the superspace group Immm[(1 - gamma)00] no. 71.1.
ABSTRACT
In this work we report some new well-defined carbon nanostructures produced by direct chlorination of metallocenes (ferrocene and cobaltocene) and NbC, at temperatures from 100 to 900 degrees C. Thus, amorphous carbon nanotubes with variable dimensions depending on reaction temperature were produced from ferrocene. When cobaltocene is the carbon precursor the main product are solid amorphous nanospheres. The high refractory metal carbide NbC as carbon source favours the growth of nanospherical cabbage-like particles with a higher degree of graphene sheets order. Besides, NbC crystallites encapsulated in an amorphous carbon shell were also found at lower temperatures (T< or =700 degrees C).
ABSTRACT
The investigation of the tin-rich part of the ternary system Co/Ni/Sn yielded the phase Co(1)(-)(x)()Ni(x)()Sn(2) with the range of composition 0.23(3) < x < 0.59(3). When using a large excess of tin, Co(1)(-)(x)()Ni(x)()Sn(2) crystallizes at 500 degrees C in a structure isotypic to that of tetragonal PdSn(2) whereas a modification with the orthorhombic CoGe(2) structure type always forms from a stoichiometric mixture of the elemental components or from a tin melt at temperatures above 550 degrees C. The structures of Co(0.625)Ni(0.375)Sn(2) were determined by single-crystal X-ray diffraction methods (PdSn(2)-type space group I4(1)/acd, a = 6.2360(5) Å, c = 23.588(2) Å, Z = 16; CoGe(2)-type space group Aba2, a = 6.2439(4) Å, b = 6.2493(4) Å, c = 11.778(1) Å, Z = 8). Both structures have the same building unit consisting of three consecutive planar nets 4(4), 3(2)434, 4(4) formed by tin atoms. The condensation of the building blocks in the c direction gives rise to different stacking sequences. In the PdSn(2)-type, a ABCD sequence is realized and the c axis is doubled compared to the ideal CoGe(2)-type with AB stacking. Electron diffraction and high-resolution electron microscopy studies revealed the existence of other, complex, stacking variants as well as stacking faults in the Co(1)(-)(x)()Ni(x)()Sn(2) system.
