DIFFRACTION STUDIES ON STRONGLY CORRELATED PEROVSKITE OXIDES

In recent years, a great interest has been devoted to the so called strongly correlated systems containing perovskite building blocks. These systems exhibit a complex interplay between charge, spin, orbital and lattice degrees of freedom paving the way for very attractive applications. In this work, entitled \u201cDiffraction Studies on Strongly Correlated Perovskite Oxides\u201d the use of x-ray diffraction techniques to investigate the coupling between the structure and the physical properties of several bulk material based on perovskite structure is presented. The thesis is organized in five chapters. Introduction presents a very general overview on strongly correlated perovskite oxides and the scope of the thesis. The first chapter reports technical details of the diffraction techniques involved in all the structural studies performed during the PhD. Chapter 2 reports an accurate investigation performed on the magnetoresistive cobaltite GdBaCo2O5+d (d=0) using single-crystal and synchrotron powder X-ray diffraction. In this work, we assign the correct space group and we demonstrate that a very small tetragonal-to-orthorhombic lattice distortion is coupled to magnetic phase transition. In Chapter 3, we show the study of the temperature induced insulator-to-metal transition for GdBaCo2O5+d (d>0.5). By using a combined approach between electron paramagnetic resonance and powder diffraction techniques we provide new interesting features about the spin \u2013 lattice interaction occurring in these systems. Chapter 4 presents synchrotron X-ray powder diffraction study on EuTiO3 system. We show for the first time the existence of a new structural phase transition occurring in EuTiO3 below room temperature. In addition, by performing the atomic pair distribution function analysis of the powder diffraction data, we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material and we propose that the lattice disorder is of fundamental importance to understand the EuTiO3 properties. Finally, beyond the scope of the thesis, in Chapter 5, we review the basic procedure to get the differential pair distribution function obtained by applying the anomalous X-ray diffraction technique to total X- ray scattering method. We show an example of the application of this procedure by presenting the case of gadolinium doped ceria electrolytes. This work will show that use of the powder diffraction techniques provides a powerful tool to unveil the coupling between the structure and the physical properties in strongly correlated perovskite oxides

Oxygen transport in nanostructured lanthanum manganites

Methods and models describing oxygen diffusion and desorption in oxides have been developed for slightly defective and well crystallised bulky materials. Does nanostructuring change the mechanism of oxygen mobility? In such a case, models should be properly checked and adapted to take into account new material properties. In order to do so, temperature programmed oxygen desorption and thermogravimetric analysis, either in isothermal or ramp mode, have been used to investigate some nanostructured La1\u2013xAxMnO3 d samples (A = Sr and Ce, 20\u201360 nm particle size) with perovskite-like structure. The experimental data have been elaborated by means of different models to define a set of kinetic parameters able to describe oxygen release properties and oxygen diffusion through the bulk. Different rate-determining steps have been identified, depending on the temperature range and oxygen depletion of the material. In particular, oxygen diffusion was shown to be rate-limiting at low temperature and at low defect concentration, whereas oxygen recombination at the surface seems to be the rate-controlling step at high temperature. However, the oxygen recombination step is characterised by an activation energy much lower than that for diffusion. In the present paper oxygen transport in nanosized materials is quantified by making use of widely diffused experimental techniques and by critically adapting to nanoparticles suitably chosen models developed for bulk materials

Phase Transformations in the CeO2-Sm2O3System : A Multiscale Powder Diffraction Investigation

The structure evolution in the CeO2-Sm2O3system is revisited by combining high resolution synchrotron powder diffraction with pair distribution function (PDF) to inquire about local, mesoscopic, and average structure. The CeO2fluorite structure undergoes two phase transformations by Sm doping, first to a cubic (C-type) and then to a monoclinic (B-type) phase. Whereas the C to B-phase separation occurs completely and on a long-range scale, no miscibility gap is detected between fluorite and C-type phases. The transformation rather occurs by growth of C-type nanodomains embedded in the fluorite matrix, without any long-range phase separation. A side effect of this mechanism is the ordering of the oxygen vacancies, which is detrimental for the application of doped ceria as an electrolyte in fuel cells. The results are discussed in the framework of other Y and Gd dopants, and the relationship between nanostructuring and the above equilibria is also investigated

Relaxor ferroeletric behavior inSr1−xPrxTiO3 : Cooperation between polar and antiferrodistortive instabilities

Chemical doping at the Sr and Ti sites is a feasible way to alter the quantum paraelectric state of SrTiO3 perovskite. Doping with Pr is known to induce relaxor ferroelectricity at room temperature in the Sr1 12xPrxTiO3 solid solution. The relationship between its dielectric properties and structural phase transition has been debated, but no definitive structural argument has been proposed. Here we present a systematic structural study of Sr1 12xPrxTiO3 (0.020 _ x _ 0.150).We establish the structural phase diagram using high-resolution x-ray powder diffraction by finding the antiferrodistortive structural phase transitions for all the compositions studied. By using pair distribution function analysis, we show the mismatch between local and long-range structures in terms of increased local order parameters. Finally, we propose a correlation between the local structural order parameters and the emergence of hard polar modes as found by Raman spectroscopy. Our results are quantitatively consistent with recent theoretical calculations showing that the increase of local tetragonality and local octahedral tilting above a critical value in fact underlie the polar instability. This confirms that structural orders involving both polar and antiferrodistortive characters compete and cooperate at different levels, promoting ferroelectricity in Sr1 12xPrxTiO3

Defect clustering in Ce1-xGdxO2-x/2 using the PDF Analysis

Aim of this work is to unveil the complex positional disorder induced by gadolinium doping and oxygen vacancies formation in Ce1-xGdxO2-x/2 (CGO) electrolytes for solid oxide fuel cells by means of the Pair Distribution Function (PDF) analysis. The whole range of Gd concentration xGd (0 ~10 \uc5. The same models applied at lower r values fails to reproduce the experimental curves. A clear improvement of the fit quality in the 1.5 < r < ~6 \uc5 range was obtained for all the CGO samples applying a biphasic model encompassing both a fluorite CeO2-like and a C-type Gd2O3-like phases. This provides evidence that extended defects at local scale exist in the CGO. Gd-rich and Ce-rich droplets coexist in the sub-nanometric range

Differential Pair Distribution Function applied to Ce1-xGdxO2-x/2 system

The Pair Distribution Function technique based upon X-ray diffraction data is a powerful tool to unveil disorder on the nanometric scale, which is however element insensi-tive. To overcome this problem, Differential Pair Distribution Functions (DPDF) can be obtained by exploiting the anomalous dispersion of X-rays near the absorption edge of a certain element. In this paper the DPDF method is briefly reviewed and applied to the case of gadolinium doped ceria electrolytes. XRPD data have been collected at the Ce-K edge on the ID31 beamline of the European Synchrotron Radiation Facility (ESRF). The validity of this approach to extract chemical specific information is also briefly discussed

Charge ordering transition in GdBaCo2O5: evidence of reentrant behavior

We present a detailed study on the charge ordering (CO) transition in GdBaCo2O5 system by combining high resolution synchrotron powder/single crystal diffraction with electron paramagnetic resonance (EPR) experiments as a function of temperature. We found a second order structural phase transition at TCO=247 K (Pmmm to Pmma) associated with the onset of long range CO. At Tmin = 1.2TCO, the EPR linewidth rapidly broadens providing evidence of spin fluctuations due to magnetic interactions between Gd3+ ions and antiferromagnetic couplings of Co2+/Co3+ sublattices. This likely indicates that, analogously to manganites, the long-range antiferromagnetic order in GdBaCo2O5 sets in at TCO. Pair distribution function (PDF) analysis of diffraction data revealed signatures of structural inhomogeneities at low temperature. By comparing the average and local bond valences, we found that above TCO the local structure is consistent with a fully random occupation of Co2+ and Co3+ in a 1:1 ratio and with a complete charge ordering below TCO. Below T = 100 K the charge localization is partially melted at the local scale, suggesting a reentrant behavior of CO. This result is supported by the weakening of superstructure reflections and the temperature evolution of EPR linewidth that is consistent with paramagnetic (PM) reentrant behavior reported in the GdBaCo2O5.5 parent compound.Comment: 30 pages, 10 figure

Crystal structure and structural phase transitions in the GdBaCo2O5.0 cobaltite

We present an accurate investigation of the prototypical rare-earth cobaltite GdBaCo2O5.0 by complementary synchrotron powder and conventional source single-crystal x-ray diffraction experiments. The correct space group (Pmmm) and the accurate crystallographic structure of this compound at room temperature (RT) have been determined. By increasing T a second-order structural-phase transition to a tetragonal structure with space group P4/mmm at T 48 331 K is found. Close to the N\ub4eel temperature (TN 48 350 K), anomalies appear in the trend of the lattice constants, suggesting that the structural phase transition is incipient at TN. A possible mechanism for this complex behavior is suggested

Defect engineering over anisotropic brookite toward substrate-specific photo-oxidation of alcohols

Generally adopted strategies for enhancing the photocatalytic activity are aimed at tuning the visible light response, the exposed crystal facets, and the nanocrystal shape. Here, we present a different approach for designing efficient photocatalysts displaying a substrate-specific reactivity upon defect engineering. The platinized, defective anisotropic brookite TiO2 photocatalysts are tested for alcohol photoreforming showing up to an 11-fold increase in methanol oxidation rate, compared with the pristine one, while presenting much lower ethanol or isopropanol specific oxidation rates. We demonstrate that the substrate- specific alcohol oxidation and hydrogen evolution reactions are tightly related, and when the former is increased, the latter is boosted. The reduced anisotropic brookite shows up to 18-fold higher specific photoactivity with respect to anatase and brookite with isotropic nanocrystals. Advanced in situ characterizations and theoretical investigations reveal that controlled engineering over oxygen vacancies and lattice strain produces large electron polarons hosting the substratespecific active sites for alcohol photo-oxidation

Effective Ag doping and resistance to sulphur poisoning of La-Mn perovskites for the catalytic flameless combustion of methane

Perovskite-like structured catalysts showed satisfactory activity for the low temperature flameless combustion of methane. This process reduces the emission of CO, NOx and unburnt hydrocarbons. Partial metal ion substitution in the composition of perovskites may improve catalytic activity and it can modulate their resistance to sulfur poisoning. Silver is an interesting dopant due to its limited solubility in the perovskite structure and suitable activity both in extra-and intra-framework positions. The amount of lattice silver tightly depends on the preparation procedure. Samples with nominal composition La1-xAgxMnO3+/-delta with x = 0; 0.05; 0.10 were prepared by flame spray pyrolysis (FP) and by the so-called sol-gel citrate method (SG). Temperature-programmed analysis, X-ray powder diffraction (XRPD) and Electron Paramagnetic Resonance (EPR) spectroscopy were used as the main characterisation tools. Almost all of the catalysts were very active for the flameless combustion of methane. The activity of the FP-prepared catalysts was always higher than that of the SG-prepared ones with identical nominal composition. Furthermore, partial substitution of Ag for La led to substantially higher activity both for SG- and FP-prepared catalysts and the catalytic activity increased with increasing Ag substitution. The residual activity after poisoning with tetrahydrothiophene, a common odoriser used in the natural gas grid, is also presented, together with the transient response of the samples upon poisoning