ABSTRACT
Perovskite-type solid-state electrolytes, Li3x La2/3-x TiO3 (LLTO), are considered among the most promising candidates for the development of all-solid-state batteries based on lithium metal. Their high bulk ionic conductivity can be modulated by substituting part of the atoms hosted in the A- or B-site of the LLTO structure. In this work, we investigate the crystal structure and the long-range charge migration processes characterizing a family of perovskites with the general formula La1/2+1/2x Li1/2-1/2x Ti1-x Al x O3 (0 ≤ x ≤ 0.6), in which the charge balance and the nominal A-site vacancies (n A = 0) are preserved. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) investigations reveal the presence of a very complex nanostructure constituted by a mixture of two different ordered nanoregions of tetragonal P4/mmm and rhombohedral R3Ì c symmetries. Broadband electrical spectroscopy studies confirm the presence of different crystalline domains and demonstrate that the structural fluctuations of the BO6 octahedra require to be intra- and intercell coupled, to enable the long-range diffusion of the lithium cation, in a similar way to the segmental mode that takes place in polymer-ion conductors. These hypotheses are corroborated by density functional theory (DFT) calculations and molecular dynamic simulations.
ABSTRACT
The origin of the intricate nanostructure of La0.5-xLi0.5-xSr2xTiO3 (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 TiO6 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.
ABSTRACT
Glycerol surplus from biodiesel synthesis still represents a major problem in the biofuel production chain. Meanwhile, those in the acrylic acid market are looking for new processes that are able to offer viable alternatives to propylene-based production. Therefore, acrylic acid synthesis from glycerol could be an effective solution to both issues. Among the viable routes, one-pot synthesis theoretically represents the most efficient process, but it is also highly challenging from the catalyst design standpoint. A new class of complex W--Mo--V mixed-oxide catalysts, which are strongly related to the hexagonal tungsten bronze structure, able to directly convert glycerol into acrylic acid with yields of up to 51 % are reported.
Subject(s)
Acrylates/chemistry , Glycerol/chemistry , Oxides/chemistry , Catalysis , Time FactorsABSTRACT
An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten bronze-type Sr(2)NaNb(5)O(15) (SNN) compound has been performed in the 300-100 K temperature range, by combining X-ray, neutron diffraction, and transmission electron microscopy with dielectric measurements, in order to clarify the structural effects responsible for the observed low temperature dielectric properties. Interestingly, a relevant second anomaly in the dielectric constant, in addition to the ferroelectric (FE) to paraelectric (PE) transition at T(C) = 518 K is found at T ≈ 240 K, revealing a relaxor-like behavior of the material at low temperature. This phenomenon has been previously observed in FE perovskite-type phases and referred to as the re-entrant phenomenon. However, FE polarization tends to vanish below this low temperature dielectric anomaly and this fact is not expected for a classical relaxor-ferroelectric phase. Although there is no structural transition from RT to 100 K, there is a change in the elastic properties of the material in the considered temperature range and the intense anomaly at ~240 K could be associated to a smeared-out phase transition to a frustrated FE/ferroelastic (FEL) low temperature state in correlation with subtle structural effects.
ABSTRACT
A transmission electron microscopy (TEM) study of the complex NaNb(1-x)Ta(x)O(3) (0.4 < or = x < or = 0.6) perovskites, combining high-resolution TEM and high-angle annular dark-field scanning TEM, has revealed the formation of extended areas on the crystals where niobium and tantalum order into layers in a 1:1 ratio. NaNb(1-x)Ta(x)O(3) oxides are stoichiometric, and there is neither charge difference nor significant ionic size discrepancy between Nb(V) and Ta(V) cations. As d(0) octahedrally coordinated cations, they show a propensity to second-order Jahn-Teller distortion. This distortion, however, manifests itself to different extents for the two cations and is considered the driving force for the layered ordered distribution observed. The niobium-tantalum segregation we have found can also be interpreted as a naturally occurring nanometer-scale phase separation. Albeit occurring in wide regions of the crystals and not in the entire grains, it shows a clear trend toward a long-range ordered disposition. This is reminiscent of the more general behavior of a recently documented class of perovskites that suffer spontaneous nanoscale phase separation to form a superlattice.
ABSTRACT
The thermal stability of nanostructured Ce(0.5)Zr(0.5)O(2) powders prepared by the Pechini method was studied on the nanometric scale by X-ray diffraction (XRD), energy-dispersive spectrometry (EDS), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), and Raman techniques. Obtained results demonstrate that amorphous powders coming from the thermal decomposition of the precursor transform into the stable crystalline state through one highly disordered and metastable intermediate. This is a new example of successive reactions controlled by Ostwald's rule in inorganic systems. At low calcination temperatures, the combination of Raman spectroscopy, high-resolution electron microscopy, and EDS nanoanalysis showed the formation from the precursor powder of a disordered pseudocubic phase. At 900 degrees C, metastable T' and stable T and C phases were detected in XRD patterns. As increasing temperature, crystallites growth and proportions of stable T and C phases increased at the expense of the T' phase, which completely disappeared at 1300 degrees C. In analyzed samples, the Raman technique and (crystal by crystal) EDS nanoanalyses were used to detect local phase inhomogeneity. Compositions and relative percentages of phases were investigated by XRD Rietveld analysis and discussed in terms of phase diagrams previously reported.
ABSTRACT
Mixed metal oxides with tetragonal tungsten bronze (TTB) structure, showing high activity and selectivity for the gas phase partial oxidation of olefins, have been prepared by hydrothermal synthesis from Keggin-type heteropolyacids.
