E. Garcia-Gonzalez

Structural Effects Behind the Low Temperature Nonconventional Relaxor Behavior of the Sr2NaNb5O15 Bronze

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.

Multielement crystalline and pseudocrystalline oxides as efficient catalysts for the direct transformation of glycerol into acrylic acid

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.

Cation miscibility in CeO2–ZrO2 oxides with fluorite structure. A combined TEM, SAED and XRD Rietveld analysis

Structural and microstructural features of Ce1−xZrxO2 (0 < x < 1) samples, prepared by solid state reaction at 1350 and 1500 °C, were analyzed by means of powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. In samples prepared at 1350 °C the solubility was very low, but in samples prepared at 1500 °C, it increased considerably. In all of the analyzed samples, the fluorite structure was detected; however, Rietveld analysis of the XRD patterns showed that for samples with a high cerium content the solid solution adopts cubic symmetry (S.G. Fmm) and suffers a structural transformation as the zirconium content increases, firstly to tetragonal (S.G. P42/nmc), and later to monoclinic (S.G. P21/c). The relationship between the Ce/Zr ratio and the microstructural properties has been investigated by means of selected area electron diffraction (SAED) and high resolution electron microscopy (HREM), combined with EDS microanalysis at the nanoscale level. Although no intracrystalline compositional heterogeneities were detected, samples with intermediate compositions consisted of two phases with compositions close to Ce0.2Zr0.8O2 and Ce0.7Zr0.3O2. Finally, solubility ranges deduced from Rietveld analysis are discussed in terms of phase diagrams previously reported.

Transmission Electron Microscopy Evidence of Spontaneous B-Cation Layered Distribution in NaNb1−xTaxO3

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.

Phase coexistence in NaNb(1−x)TaxO3 materials with enhanced dielectric properties

NaNb(1−x)TaxO3 (0.1 ≤ x ≤ 0.9) materials have been synthesized and investigated by combining diffraction methods and high resolution electron microscopy with dielectric measurements. The stabilization of lead-free ferroelectric materials has been established from a threshold dopant composition of 40%, probing the doping process in the B-cationic sublattice of the perovskite structure to be a suitable approach for tuning the dielectric properties of the antiferroelectric NaNbO3 oxide. Rietveld analysis of the room temperature powder X-ray diffraction data for ferroelectric compositions has provided unambiguous evidence for the coexistence of two different orthorhombic phases with lattice parameters a ≈ c ≈ √2ac, b ≈ 2ac in the 0.4 ≤ x ≤ 0.6 composition range, a non-centrosymmetric ferroelectric phase (P21ma) and a centrosymmetric paraelectric phase (Pcmn). The change in the order of the phase transitions from first order-like to second order seems to be related to the coexistence of ferroelectric and paraelectric phases, with the NaNb(1−x)TaxO3 (0.4 ≤ x ≤ 0.6) materials showing an enhanced dielectric response.

Oxygen content and microstructure in Bi4V2O11 − δ

Selected area electron diffraction and high resolution electron microscopy have been used to study the room temperature-stable polymorph of the Bi4V2O10 phase. The reciprocal lattice, as well as the unit cell symmetry and the microstructural features, are in agreement with the P22121 space group and a disordered distribution of anionic vacancies in the perovskite-type layers of the structure.

The hydrothermal synthesis of tetragonal tungsten bronze-based catalysts for the selective oxidation of hydrocarbons

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.

Interplay between humidity, temperature and electrical response of a conductivity sensor based on a La2LiNbO6 double perovskite

The La2LiNbO6 perovskite has been prepared in the polycrystalline form by a solid state reaction. Structural characterization by means of monochromatic X-ray and neutron powder diffraction (XRD and ND) and Rietveld refinement showed that the crystal structure belongs to the group of 1 : 1 B-site rock-salt ordered double perovskites with the most common tilting system amongst them being a−a−b+ (S. G. P21/n, a = 5.61612(3), b = 5.76645(2), c = 7.94107(4) A, β = 90.276(2)°). Scanning Transmission Electron Microscopy (STEM) evidences that there is no cross-substitution between Li and Nb and that a remaining portion of lanthanum is randomly located in the projected positions of lithium. Impedance spectroscopy has been used to analyse the electrical-response properties of the materials. Conductivity is strongly dependent on the relative humidity (RH), changing by about 3 orders of magnitude between 25 and 90% RH. However, no conductivity increase with change in RH% is observed when the lateral surfaces of the sensor are covered with paraffin. This confirms that adsorption of water by the sample plays a crucial role in modulating the conduction mechanism. La2LiNbO6 also exhibits a very good durability, reproducibility, response time, hysteresis and dynamic linearity to be considered as a promising sensing material for a practical humidity sensor.

Correction: On the origin of the spontaneous formation of nanocavities in hexagonal bronzes (W,V)O3

Correction for ‘On the origin of the spontaneous formation of nanocavities in hexagonal bronzes (W,V)O3’ by E. Garcia-Gonzalez et al., Dalton Trans., 2014, 43, 14644–14652.

Local chemical inhomogeneities in NaNb1−xTaxO3 as observed by HRTEM and HAADF-STEM

Sodium niobate NaNbO3, crystallizing with a perovskite-type structure, is an interesting material both because of its structural phase transitions as well as for its electric properties ranging from antiferroelectric to paraelectric behaviour [1]. Compositional variations in the sublattice of both A and B cations have been widely studied in order to stabilize a ferroelectric phase at room temperature [2,3]. The substitution of Ta by Nb leads to the formation of ferroelectric materials for Ta-rich compositions. Remarkably, a noticeable discontinuity of the electric properties occurs in the range 0.4 ≤ x ≤ 0.6 for NaNb1−xTaxO3 and no explanation has been given to justify such behavior [4]. To obtain a deeper insight into the complicated relationship between structure and properties in this system, we prepared and reinvestigated the solid solution NaNb1−xTaxO3 (0 ≤ x ≤ 1.0).