Gianguido Baldinozzi

Modeling of line-shape asymmetry in powder diffraction

Theoretical calculation shows that suitable approximations of the line-shape asymmetry in powder diffraction profiles should be represented by functions having limited codomains.

Microstructure and magnetic properties of strained La0.7Sr0.3MnO3 thin films

The lattice deformation of dense strained La0.7Sr0.3MnO3 (LSMO) films is shown to control the easy direction of the magnetization. Optimized pulsed laser deposited conditions allow the fabrication of dense LSMO thin films which present an exceptional flatness with a peak–valley roughness (Rp–v) of 1 A, associated to epitaxial grains as large as 1 μm. Electron microscopy coupled with x-ray diffraction have been used to study the unit cell distortion of both tensile and compressive dense LSMO films as a function of the thickness. No relaxation of the lattice distortion imposed by substrate has been observed in the thickness range 10–60 nm. The Curie temperature is not significantly affected by the nature of the substrate: a TC of 350 K is observed for both SrTiO3 (STO) and LaAlO3 (LAO) substrates, i.e., close to the bulk material (369 K). In contrast, the easy direction of magnetization depends on the substrate. For tensile films deposited on the STO substrate, the unit cell is elongated along the film’s pl...

Neutron Diffraction Study of the in Situ Oxidation of UO2

This paper discusses uranium oxide crystal structure modifications that are observed during the low-temperature oxidation which transforms UO(2) into U(3)O(8). The symmetries and the structural parameters of UO(2), beta-U(4)O(9), beta-U(3)O(7), and U(3)O(8) were determined by refining neutron diffraction patterns on pure single-phase samples. Neutron diffraction patterns were also collected during the in situ oxidation of powder samples at 483 K. The lattice parameters and relative ratios of the four pure phases were measured during the progression of the isothermal oxidation. The transformation of UO(2) into U(3)O(8) involves a complex modification of the oxygen sublattice and the onset of complex superstructures for U(4)O(9) and U(3)O(7), associated with regular stacks of complex defects known as cuboctahedra, which consist of 13 oxygen atoms. The kinetics of the oxidation process are discussed on the basis of the results of the structural analysis.

Investigation on the zirconia phase transition under irradiation

Zirconia, ZrO2, produced by the oxidation of zirconium alloys in nuclear reactors, possesses a high stability under neutron irradiation. No amorphisation of yttrium-stabilised zirconia has been observed even at high dpa values (≈100 dpa). In pure monoclinic zirconia, a phase transition monoclinic → cubic (tetragonal) induced by irradiation has already been observed. The aim of this work is to study in detail the mechanism responsible for this transition. For that purpose, different kinds of irradiations with electrons (to study point defects) and low energetic ions (to study clusters due to collision cascades) have been performed on zirconia samples. A local probe (Raman spectroscopy) and a non-local probe (grazing X-ray diffraction) have been used to characterise the phase formed during irradiation, which is clearly the tetragonal phase. For the ionic implantation, the grazing X-ray diffraction permits to separate effects due to the ballistic collisions and the implantation peak. Using this method, it was possible to show that the profile of the tetragonal phase was only linked to the dpa profile. This result associated to the results obtained by the Raman spectroscopy (broadening of Raman peaks) shows that the phase transition may be induced by clusters formed near the collision cascades.

Density functional theory calculations of UO2 oxidation : Evolution of UO2+x, U4O9−y, U3O7, and U3O8.

Formation of hyperstoichiometric uranium dioxide, UO2+x, derived from the fluorite structure was investigated by means of density functional theory (DFT) calculations. Oxidation was modeled by adding oxygen atoms to UO2 fluorite supercells. For each compound ab initio molecular dynamics simulations were performed to allow the ions to optimize their local geometry. A similar approach was used for studying the reduction of U3O8. In agreement with the experimental phase diagram we identify stable line compounds at the U4O9-y and U3O7 stoichiometries. Although the transition from fluorite to the layered U3O8 structure occurs at U3O7 (UO2.333) or U3O7.333 (UO2.444), our calculated low temperature phase diagram indicates that the fluorite derived compounds are favored up to UO2.5, that is, as long as the charge-compensation for adding oxygen atoms occurs via formation of U(5+) ions, after which the U3O8-y phase becomes more stable. The most stable fluorite UO2+x phases at low temperature (0 K) are based on ordering of split quad-interstitial oxygen clusters. Most existing crystallographic models of U4O9 and U3O7, however, apply the cuboctahedral cluster. To better understand these discrepancies, the new structural models are analyzed in terms of existing neutron diffraction data. DFT calculations were also performed on the experimental cuboctahedral based U4O9-y structure, which enable comparisons between the properties of this phase with the quad-interstitial ones in detail.

Mesoporous α-Fe2O3 thin films synthesized via the sol–gel process for light-driven water oxidation

This work reports a facile and cost-effective method for synthesizing photoactive α-Fe(2)O(3) films as well as their performances when used as photoanodes for water oxidation. Transparent α-Fe(2)O(3) mesoporous films were fabricated by template-directed sol-gel chemistry coupled with the dip-coating approach, followed by annealing at various temperatures from 350 °C to 750 °C in air. α-Fe(2)O(3) films were characterized by X-ray diffraction, XPS, FE-SEM and electrochemical measurements. The photoelectrochemical performance of α-Fe(2)O(3) photoanodes was characterized and optimized through the deposition of Co-based co-catalysts via different methods (impregnation, electro-deposition and photo-electro-deposition). Interestingly, the resulting hematite films heat-treated at relatively low temperature (500 °C), and therefore devoid of any extrinsic dopant, achieve light-driven water oxidation under near-to-neutral (pH = 8) aqueous conditions after decoration with a Co catalyst. The onset potential is 0.75 V vs. the reversible hydrogen electrode (RHE), thus corresponding to 450 mV light-induced underpotential, although modest photocurrent density values (40 μA cm(-2)) are obtained below 1.23 V vs. RHE. These new materials with a very large interfacial area in contact with the electrolyte and allowing for a high loading of water oxidation catalysts open new avenues for the optimization of photo-electrochemical water splitting.

An ab-initio study of the rôle of lone pairs in the structure and insulator–metal transition in SnO and PbO

We have performed density functional calculations on tetragonal SnO and PbO (litharge) in the space group P4/nmm with the specific intention of examining the role played by Sn 5s and Pb 6s lone pairs in stabilizing the structure, and in giving rise to semi-metallic behavior (of SnO at ambient pressure and of PbO in the γ phase). Use of the electron localization function has permitted real-space visualization of the lone pair in these structures. We also discuss the electronic structure of the orthorhombic PbO (massicot, space group Pbma) which again has localized lone pairs, contrary to some earlier expectation.

Crystal structure of the antiferroelectric perovskite Pb2MgWO6

Lead magnesium tungstate, Pb2MgWO6, Mr = 718.54. Phase I: cubic, Z = 4, Fm3m, a = 8.0058 (4)A, V = 513.1 (2) A 3, Dx = 9.30 Mg m -3 at 350 K, final Rwp = 4.5 and 7.7%, RBragg = 2.9 and 5.7% for neutron and Xray powder data, respectively. Phase II: orthorhombic, Pmcn (Pnma), Z = 4, a = 7.9440(4) and 7.9041 (3), b = 5.6866(3) and 5.7035(2), c = 11.4059(5) and 11.4442 (4) A, V= 515.3 (1) and 515.9 (1) A 3 at 294 and 80 K, respectively, Dx = 9.26 Mg m -3 at 294 K. Final Rwp = 4.0 and 8.5%, RBragg = 4.0 and 9.2% at 294 K and Rwp = 4.0 and 7.4%, RBragg = 2.9 and 8.4% at 80 K for neutron and X-ray powder data, respectively. To achieve the determination of the structures, X-ray and neutron powder diffraction data were refined together using the Rietveld profile method. The Pb main displacement in the orthorhombic phase from the ideal cubic positions is almost along the [0121o direction. The O displacements correspond to a weak distortion of the octahedra.

Study of boron carbide evolution under neutron irradiation by Raman spectroscopy

Boron carbide, B12C3, is an absorbing material used to control the reactivity of nuclear reactors by taking advantage of nuclear reactions (e.g. 10 B(n,a) 7 Li), where neutrons are absorbed. During such reactions, radiation damages originating both from these nuclear reactions and from elastic collisions between neutrons and atoms lead to a partial destruction of this material, which gives the main limitation of its lifetime in nuclear reactors. In order to understand the evolution of B12C3 in nuclear plants, the eAect of neutron irradiation in B12C3 has been investigated by Raman and nuclear magnetic resonance (NMR) spectroscopies. Comparisons of B12C3 samples irradiated by 1 MeV electrons, 180 keV helium ions and neutrons are used to study the microstructure evolution of this material by Raman scattering. The analysis of Raman spectra of diAerent B12C3 samples irradiated by neutrons clearly shows that during the cascade displacements, the 485 and 527 cm ˇ1 modes disappear. These characteristic features of Raman spectra of the neutron irradiated samples are interpreted by a microscopic model. This model assumes that the CBC linear chain is destroyed whereas icosahedra are self-healed. 10 B atoms destroyed during the neutron irradiation are replaced in icosahedra by other boron and carbon atoms coming from the linear CBC chain. The 11 B NMR analysis performed on unirradiated and irradiated B4C samples shows the vanishing of a strong quadrupolar interaction associated to the CBC chain during the high neutron irradiation. The 11 B NMR spectroscopy confirms the previous Raman spectroscopy and the proposed microscopic model of B12C3 evolution under neutron irradiation. ” 2000 Elsevier Science B.V. All rights reserved.

Raman study of the structural phase transition in the ordered perovskite Pb2MgWO6

Lead magnesium tungstate Pb2MgWO6 undergoes a structural phase transition at about 310 K. This phase transition has been characterized by Raman spectroscopy. The main features of the Raman spectra, collected between 173 and 523 K, are consistent with group theoretical predictions though unexpected weak and broad signals are observed in the high-temperature phase. From the comparison between these results and the structures previously determined, it is concluded that the phase transition has a behaviour intermediate between pure displacive and order-disorder.