J.M. Gallardo-Amores

A study of Mn-Ti oxide powders and their behaviour in propane oxidation catalysis

Mn-Ti mixed oxides with composition Ti 1–x Mn x O y (x=0, 0.1, 0.2, 0.5, 0.8, 0.9 and 1) have been prepared. A Mn-TiO 2 monolayer type sample has also been prepared by impregnation, for comparison. Manganese is found to speed up the anatase-to-rutile phase transition, more clearly in the impregnated sample, while titanium tends to slightly hinder the thermodynamically reversible hausmannite-to-bixbyite phase transition upon cooling. The catalytic activity of all samples in propene oxidation decreases by increasing the Ti content. Conversely, the catalytic activity in propane oxidation shows a maximum at intermediate composition.

Lattice Dynamics of β-V2O5: Raman Spectroscopic Insight into the Atomistic Structure of a High-Pressure Vanadium Pentoxide Polymorph

We report here the Raman spectrum and lattice dynamics study of a well-crystallized β-V(2)O(5) material prepared via a high-temperature/high-pressure (HT/HP) route, using α-V(2)O(5) as the precursor. Periodic quantum-chemical density functional theory calculations show good agreement with the experimental results and allow one to assign the observed spectral features to specific vibrational modes in the β-V(2)O(5) polymorph. Key structure-spectrum relationships are extracted from comparative analysis of the vibrational states of the β-V(2)O(5) and α-V(2)O(5) structures, and spectral patterns specific to the basic units of the two V(2)O(5) phases are proposed for the first time. Such results open the way for the use of Raman spectroscopy for the structural characterization of vanadium oxide-based host lattices of interest in the field of lithium batteries and help us to greatly understand the atomistic mechanism involved in the α-to-β phase transition of vanadium pentoxide.

Influence of tile synthesis parameters on the structural and textural properties of precipitated manganese oxides

Abstract A systematic study has been conducted in order to compare the structural and morphological properties as well as the genesis course of manganese oxides prepared by a precipitation-calcination method under different conditions. In particular, the roles of the precursor salt, the pH, the precipitating agent and thermal treatment were investigated. It was found that calcination at 873 K of the starting materials gives rise in all cases to crystalline bixbyite. However, the phases formed at lower temperatures are quite different and depend largely on the synthesis conditions. The systems prepared from a Mn(III) ionic salt present maximum specific surface areas according to a greater development medium-sized pores.

Lanthanide substitution by high pressure in the RuSr2GdCu2O8 magnetic superconductor

Abstract A systematic study of the structural and magnetic properties of the family RuSr2RECu2O8 (RE=Er, Ho, Y, Dy, Tb, Gd and Eu) has been performed. All these materials appear to be tetragonal (P4/mmm) and the unit cell volume decreases along with the lanthanide ion dimension. Differences are observed in the magnetic behaviour of these compounds. High pressure and high temperature are needed to synthesize most of the members of this family.

High pressure driven structural and electrochemical modifications in layered lithium transition metal intercalation oxides

High pressure–high temperature (HP/HT) methods are utilized to introduce structural modifications in the layered lithium transition metal oxides LiCoO2 and Li[NixLi1/3−2x/3Mn2/3−x/3]O2 where x = 0.25 and 0.5. The electrochemical property to structure relationship is investigated combining computational and experimental methods. Both methods agree that the substitution of transition metal ions with Li ions in the layered structure affects the compressibility of the materials. We have identified that following high pressure and high temperature treatment up to 8.0 GPa, LiCoO2 did not show drastic structural changes, and accordingly the electrochemical properties of the high pressure treated LiCoO2 remain almost identical to the pristine sample. The high pressure treatment of LiNi0.5Mn0.5O2 (x = 0.5) caused structural modifications that decreased the layered characteristics of the material inhibiting its electrochemical lithium intercalation. For Li[Li1/6Ni1/4Mn7/12]O2 more drastic structural modifications are observed following high pressure treatment, including the formation of a second layered phase with increased Li/Ni mixing and a contracted c/a lattice parameter ratio. The post-treated Li[Li1/6Ni1/4Mn7/12]O2 samples display a good electrochemical response, with clear differences compared to the pristine material in the 4.5 voltage region. Pristine and post-treated Li[Li1/6Ni1/4Mn7/12]O2 deliver capacities upon cycling near 200 mA h g−1, even though additional structural modifications are observed in the post-treated material following electrochemical cycling. The results presented underline the flexibility of the structure of Li[Li1/6Ni1/4Mn7/12]O2; a material able to undergo large structural variations without significant negative impacts on the electrochemical performance as seen in LiNi0.5Mn0.5O2. In that sense, the Li excess materials are superior to LiNi0.5Mn0.5O2, whose electrochemical characteristics are very sensitive to structural modifications.

Structural and morphological characterization of Mn–Zr mixed oxides prepared by a sol–gel method

Abstract Mn–Zr mixed oxide materials have been prepared by a sol–gel method from a Mn (III) precursor salt and a solution of zirconium acetate in acetic acid, being characterized from a structural and morphological point of view. Conventional solid state techniques show that as obtained powders are formed by mixtures of manganese and zirconium acetates and (hydro) oxides, which evolve to bixbyite, hausmannite, as well as tetragonal and monoclinic zirconia above 873 K. Zirconium enhances the formation of the non-thermodynamically stable manganese compounds instead of the thermodynamically stable ones. This effect was not observed in other samples previously prepared by coprecipitation. In general, mixed systems are thermally stable up to 1073 K due to the formation of a solid solution phase of 10% Mn into zirconia, and display higher specific surface areas than those prepared by the coprecipitation method. Their activities in the combustion of heavy aromatic molecules are higher than those of the pure manganese oxides, very likely due to a synergic effect of zirconium. Moreover, the catalytic properties of hausmannite and bixbyite phases for burning this type of hydrocarbons seem to be very similar.

XRD study of ZrW2O8versus temperature and pressure

Abstract ZrW 2 O 8 samples have been prepared from ZrO 2 and WO 3 precursors by a ceramic method. Different portions of this material were subjected to non cumulative pressure treatments within the 2–65 Kbar range; the metastable high-pressure phase, γ-ZrW 2 O 8 , has been isolated by quenching from 6 Kbar. Further pressure increases up to 10 Kbar produced the volume reduction of the γ-phase; a minimum volume was found even after having released the pressure. Otherwise, heating above 600°C, followed by quenching at 77 K leads to the high-temperature phase, β-ZrW 2 O 8 , which is stable at room temperature. Finally, when γ-ZrW 2 O 8 was subjected to pressure and temperature, γ-ZrW 2 O 8 could also be quenched with a still larger volume reduction at 6 Kbar and 600°C.

New Members of the RuSr 2 RECu 2 O 8 Family of Phases (RE=Rare Earth) Obtained at High Pressure and Temperature

At room pressure, Sm, Eu and Gd seem to be the only RE elements that accept to enter into the structure of RuSr 2 GdCu 2 O 8 . However high pressure and high temperatures allow one to replace gadolinium by other lanthanide cations. We have been successful in replacing Gd by Y, La, Pr, Nd, Tb, Dy, Ho and Er. X-ray diffraction patterns fitted by the Rietveld method show that the average crystal structure of RuSr 2 RECu 2 O 8 is tetragonal with P 4/ mmm space group symmetry. Micro-Raman experiments have been performed on RuSr 2 GdCu 2 O 8 samples synthesized at both high and room pressure; the resulting spectra are consistent with the same structure, since there are no major changes in the low-frequency range.

Oxygen deficiency and formation of a new ordered perovskite-based structure Sr(Sr0.5Sb0.5)O3−y

Abstract A new oxygen-deficient and ordered perovskite-type structure with the formula Sr(Sr 0.5 Sb 0.5 )O 3− y has been synthesized readily by a solid state reaction in air at 1200°C. The structure of the Sr(Sr 0.5 Sb 0.5 )O 3− y phase as determined by Rietveld analysis using X-ray diffraction data, corresponds to an ordered perovskite with face-centered cubic symmetry, space group Fm 3 m (no. 225) and lattice parameters, a =8.3136(3) A, V =574.602(3) A 3 and Z =8. An important structural feature of Sr(Sr 0.5 Sb 0.5 )O 3− y is the oxygen deficiency and the displacements of oxygen atoms from their ideal positions toward the Sb 5+ cations. This fact leads to an alternating arrangement of larger [SrO 6 ] and smaller [SbO 6 ] octahedra (B-sites), the remaining Sr 2+ being in the larger 12-fold coordinated A-sites. The thermal stability of the Sr(Sr 0.5 Sb 0.5 )O 3− y phase is pointed out, its melting point being 1480°C, and the subsequent quenching giving rise to a glass material.

Synthesis and Structural Characterization of a New Phase of GaAsO 4

A new polymorph of GaAsO 4 has been synthesised at high pressure and temperature (60 kbar and 1273 K). Samples were characterised by powder X-ray diffraction, scanning electron microscopy and EDAX spectroscopy. This material shows an hexagonal symmetry with cell parameters, a =8.1931(4) Å and c =4.3744(2) Å; particles are of hexagonal shape with a narrow size distribution around 2-3 w m and composition close to the Ga:As atomic ratio=1:1. This new high pressure phase of GaAsO 4 represents a new transition path for the ABO 4 compounds.