A.R. Landa-Cánovas

On the Non-Stoichiometry in Rutile-Type »SbVO4

Heating equimolar mixtures of Sb{sub 2}O{sub 3} and V{sub 2}O{sub 5} at 800{degrees}C in flowing gas with varying O{sub 2}/N{sub 2} ratios produces a continuous nonstoichiometric series of rutile type, i.e., Sb{sub 0.9}V{sub 0.9+x}{open_square}{sub 0.2-x}O{sub 4}, Sb{sub 0.9}{sub 0.9}{open_square}{sub 0.2}O{sub 4}, a = 4.63, c = 3.03 {angstrom} (X-ray powder data, XRD), is formed in pure oxygen and exhibits a modulated structure with an approximate supercell: 2{radical}2a, 2{radical}2b, 4c (electron diffraction, (ED)). In pure nitrogen, reduced Sb{sub 0.9}V{sub 1.1}O{sub 4}, a = 4.60, c = 3.08 {angstrom} (XRD), with the supercell {radical}2a, {radical}2b, 2c (ED), is produced. Heating at intermediate partial pressures of oxygen give phases with the basic rutile cell a = b, c (XRD, ED). The formulation of this series is supported by data obtained by Fourier transform infrared spectroscopy. Under reducing conditions (in pure nitrogen), a solid solution series of Sb{sub 0.9}V{sub 1.1}O{sub 4} and VO{sub 2} is observed, i.e., Sb{sub 0.9-y}V{sub 1.1+y}O{sub 4}, 0 < y < 0.7. Vanadium-rich Sb{sub 0.2}V{sub 1.8}O{sub 4}, with a = 4.56, c = 2.99 {angstrom} (XRD), exhibits a basic rutile lattice with diffuse intensity between Bragg spots (ED).

On the nonstoichiometry in rutile-type {approximately}SbVO{sub 4}

Heating equimolar mixtures of Sb{sub 2}O{sub 3} and V{sub 2}O{sub 5} at 800{degrees}C in flowing gas with varying O{sub 2}/N{sub 2} ratios produces a continuous nonstoichiometric series of rutile type, i.e., Sb{sub 0.9}V{sub 0.9+x}{open_square}{sub 0.2-x}O{sub 4}, Sb{sub 0.9}{sub 0.9}{open_square}{sub 0.2}O{sub 4}, a = 4.63, c = 3.03 {angstrom} (X-ray powder data, XRD), is formed in pure oxygen and exhibits a modulated structure with an approximate supercell: 2{radical}2a, 2{radical}2b, 4c (electron diffraction, (ED)). In pure nitrogen, reduced Sb{sub 0.9}V{sub 1.1}O{sub 4}, a = 4.60, c = 3.08 {angstrom} (XRD), with the supercell {radical}2a, {radical}2b, 2c (ED), is produced. Heating at intermediate partial pressures of oxygen give phases with the basic rutile cell a = b, c (XRD, ED). The formulation of this series is supported by data obtained by Fourier transform infrared spectroscopy. Under reducing conditions (in pure nitrogen), a solid solution series of Sb{sub 0.9}V{sub 1.1}O{sub 4} and VO{sub 2} is observed, i.e., Sb{sub 0.9-y}V{sub 1.1+y}O{sub 4}, 0 < y < 0.7. Vanadium-rich Sb{sub 0.2}V{sub 1.8}O{sub 4}, with a = 4.56, c = 2.99 {angstrom} (XRD), exhibits a basic rutile lattice with diffuse intensity between Bragg spots (ED).

Catalysis and structure of the SbVO4/Sb2O4 system for propane ammoxidation

Pure Sb0.9V0.9O4 and various preparations with excess of either vanadia or antimony oxide, including mechanical mixtures, have been investigated for propane ammoxidation to acrylonitrile. The catalysts were characterized before and after use in catalysis by various methods, including electron microscopy, infrared spectroscopy and XPS. The catalytic data show that preparations with approximate to SbVO4 and alpha-Sb2O4, compared with the single phases, are more selective to acrylonitrile formation on the condition that the excess antimony oxide is present while synthesising the approximate to SbVO, phase. Considering the catalytic data together with the results from the characterisations, various possibilities are discussed to explain the role of excess alpha-Sb2O4 in propane ammoxidation. Possibilities that can be excluded on rational grounds are catalysis on two phases, or, at grain boundaries, an influence on the morphology of approximate to SbVO4 from alpha-Sb2O4, the formation of alpha-Sb2O4 containing vanadium, defect formation, creation of active sites by the spillover of oxygen, and formation of VSb2O5. Instead, the observed synergy effect is due to the formation of approximate to SbVO4 enriched with antimony at the surface, creating isolation to a suitable level of the V-centres. The explanation is consistent with several observations including catalytic data for a series of vanadium compounds with different vanadium content, showing that structural isolation of the vanadium is necessary for obtaining high selectivity to acrylonitrile. (Less)

Differences in n-type doping efficiency between Al- and Ga-ZnO films

A careful and wide comparison between Al and Ga as substitutional dopants in the ZnO wurtzite structure is presented. Both cations behave as n-type dopants and their inclusion improves the optical and electrical properties of the ZnO matrix, making it more transparent in the visible range and rising up its electrical conductivity. However, the same dopant/Zn ratio leads to a very different doping efficiency when comparing Al and Ga, being the Ga cation a more effective dopant of the ZnO film. The measured differences between Al- and Ga-doped films are explained with the hypothesis that different quantities of these dopant cations are able to enter substitutionally in the ZnO matrix. Ga cations seem to behave as perfect substitutional dopants, while Al cation might occupy either substitutional or interstitial sites. Moreover, the subsequent charge balance after doping appear to be related with the formation of different intrinsic defects that depends on the dopant cation. The knowledge of the doped-ZnO films microstructure is a crucial step to optimize the deposition of transparent conducting electrodes for solar cells, displays, and other photoelectronic devices.

Elucidation of the crystal structure of oxyapatite by high-resolution electron microscopy.

High-resolution electron-microscopy (HREM) images from different hydroxyapatite (OHAp) samples showed p3 projection symmetry along [001] instead of the p6 projection symmetry compatible with the space group P6_3/m of OHAp. Image processing was used to establish without ambiguity that threefold symmetry dominates the images along [001]. OHAp crystals decompose in the transmission electron microscope and it is concluded that the threefold symmetry observed corresponds to an early step in the decomposition process, the dehydration of OHAp to oxyapatite (OAp). A structural model for OAp where every second O atom along the 6(3) axis in OHAp is removed has the maximal space-group symmetry P{\bar 6}. This is compatible with the p3 projection symmetry observed. Atomic shifts in this OAp model compared to the OHAp structure were estimated using the HREM images and geometric optimizations of the atomic structure. No refinements of the atomic coordinates against diffraction data were possible but the simulated HREM images of this crude model fit well with the experimental images.

On the crystal chemistry of NaP zeolites

Abstract Multiphase samples containing a series of well-crystallized gismondine-type NaP zeolites, unit cell content Na x Al x Si 16−x O 32 ·yH 2 O with 3 ⩽ x ⩽ 8, were prepared at 90–95°C or 150°C from aluminosilicate glasses and aqueous sodium hydroxide. Powder X-ray diffraction using a Guinier-Hagg camera and energy dispersive X-ray microanalysis performed in a scanning electron microscope revealed three NaP phases with increasing silicon content, i.e., low-silica P (8–10 Si atoms per unit cell), orthorhombic medium-silica P (10–12 Si/cell), and tetragonal high-silica P (12–13 Si/cell). NaP1 (pseudocubic lattice geometry) probably represents an end-member low-silica P with 10 Si/cell, whereas NaP2 (pseudotetragonal lattice geometry) is identical to medium-silica P.

Mechanochemical Synthesis of Nonstoichiometric Fluorite Ca{sub 1-x} La{sub x} F{sub 2+x} Nanocrystals from CaF{sub 2} and LaF{sub 3} Single Crystals

The nonstoichiometric Ca1−xLaxF2+x phase (x ≥ 0.1) is obtained by mechanochemical synthesis from CaF2 and LaF3 single crystals. This phase is the first representative of fluorite fluorides obtained by mechanochemical synthesis in the MFm-RFn systems (m < n ≤ 4). The average grain size ranges within 10–30 nm. The temperature dependence of ionic conductivity of the mechanochemically synthesized phase pressurized at 600 MPa (at its high-temperature portion at temperatures exceeding 200–250°C) coincides with the conductivity of the single crystals of the same composition (Ca0.8La0.2F2.2). The activation energy of ionic conductivity (0.95 eV) corresponds to migration of interstitial fluoride ions in the crystal bulk. Mechanochemical synthesis of a multicomponent fluoride material with nanometer grains opens a new chapter in the chemistry of inorganic fluorides. A decrease of the sintering temperature of the powders with nanometer grains is very important for preparing dense fluoride ceramics of complicated compositions and other polycrystalline forms of fluoride materials.

Electron microscopy study of tubular crystals (BiS)1+δ(NbS2)n

Big crystals (mm-sized) with anomalous tubular morphology were synthesised in the Bi-Nb-S system. Their structural study by electron microscopy and related techniques revealed that they can be described as misfit layer structures, a type of composite modulated structures. The misfit monolayer approximately BiNbS3 and bilayer approximately BiNb2S5 phases appear as well in the preparation but with lamellar morphology as it is typical for these kind of compounds. They consist of periodical intergrowth of pseudotetragonal layers BiS (Q) with pseudohexagonal layers NbS2 (H). approximately BiNbS3 has a stacking sequence ...Q,H,Q,H,... and approximately BiNb2S5 has a stacking sequence ...Q,H,H,Q,H,H,... Backscattered electron imaging, wavelength dispersive X-ray spectroscopy and transmission electron microscopy of transversal cross-sections of single tubular crystals showed that these crystals present a strong compositional and inter-laminar stacking disorder along the tube radius. This disorder is suggested to be the cause for the original wrapping of the layers that gives rise to the tubes. Besides the disordered areas, some ordered slabs have been found with stacking sequences corresponding to binary 3R-NbS2 (approximately 6 A), approximately BiNb2S5 (approximately 17.4 A) (which dominate in the crystal) and to a new related phase approximately BiNb4S9 with a stacking sequence ...Q,H,H,H,H,Q,H,H,H,H,... and a periodicity of approximately 29.2 A.

A study of the system Yb + S, mainly by electron diffraction/microscopy

An electron diffraction study of various ytterbium sulfide preparations reveals the presence of several phases: Yb{sub 3}S{sub 4} in two orthorhombic forms, Ccmm at high temperatures and Pnma, the latter sometimes incommensurately modulated, sometimes not; B1-related types, both cubic and rhombohedral, again the latter sometimes incommensurately modulated. The results are generally consistent with those previously reported. They reveal the absence of long-range order in some of the B1-related phases, and the consequent difficulty in identifying and delineating the phase relations between the cubic and rhombohedral Yb{sub 1-{delta}}S types.

Formation of active phases in the Sb-V-, Al-Sb-V-, and Al-Sb-V-W-oxide systems for propane ammoxidation

The Sb-V-O, Al-Sb-V-O and Al-Sb-V-W-O systems were investigated for the ammoxidation of propane to acrylonitrile. Compared with pure approximate to SbVO4, the approximate to SbVO4 that was obtained by co-calcination of pure approximate to SbVO4 with alpha-Sb2O4 at 800 degrees C and subsequent sieving was found to be much more selective to acrylonitrile. XRD data showed no variation of the rutile unit cell for synthesis ratios Sb:V greater than or equal to 1, indicating that the active phase is approximate to SbVO4 with a surface enriched with Sb. Characterisation of syntheses in the Al-Sb-V-O system allowed the identification of a trirutile-like phase with the composition Al1-xSbVxO4 (0 < x < 0.5). The synthesis of this phase, which is active and selective to acrylonitrile, requires excess of aluminium. Over a fresh preparation with Al, Sb, V and W the activity and the selectivity to acrylonitrile increased considerably with time-on-stream. This behaviour shows that the active rutile phase is formed in situ, and EDX analyses gave the average composition Al0.1Sb0.8V0.7W0.4O4. The highest yield to acrylonitrile that was observed for the three systems was 37 % and was obtained over the Al-Sb-V-W-oxide. (Less)