Marco Scavini

Role of intrinsic disorder in the structural phase transition of magnetoelectric EuTiO3

Up to now the crystallographic structure of the magnetoelectric perovskite EuTiO3 was considered to remain cubic down to low temperature. Here we present high resolution synchrotron X-ray powder diffraction data showing the existence of a structural phase transition, from cubic Pm-3m to tetragonal I4/mcm, involving TiO6 octahedra tilting, in analogy to the case of SrTiO3. The temperature evolution of the tilting angle indicates a second-order phase transition with an estimated Tc=235K. This critical temperature is well below the recent anomaly reported by specific heat measurement at TA\sim282K. By performing atomic pair distribution function analysis on diffraction data we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated Tc, the average model symmetry is fully compatible with the local environment distortion but the former is characterized by a reduced value of the tilting angle compared to the latter. At T=240K data show the presence of local octahedra tilting identical to the low temperature one, while the average crystallographic structure remains cubic. On this basis, we propose intrinsic lattice disorder to be of fundamental importance in the understanding of EuTiO3 properties.

IrO2-based disperse-phase electrocatalysts: a complementary study by means of the cavity-microelectrode and ex-situ X-ray absorption spectroscopy.

In this work, IrO(2)-based powders are screened by cyclic voltammetry for the determination of the electrochemical active sites and for the qualitative evaluation of the iridium atoms speciation. All results are obtained using a cavity-microelectrode as powder holder, thus exploiting the features of this innovative tool, whose best potentialities have been recently introduced by our group. All the studied materials have been prepared by the sol-gel technique and differ in calcination temperature and method of mixing the metal oxide precursors. The electrochemical results are complemented with the information obtained by X-ray absorption spectroscopy (XAS), that give insights on the local structure of each selected sample, confirming the trends found by cyclic voltammetry and give new and unexpected insights on the powder structural features.

Charge ordering transition in GdBaCo2O5: Evidence of reentrant behavior

We present a detailed study on the charge ordering transition in a GdBaCo2O5.0 system by combining highresolution synchrotron powder/single-crystal diffraction with electron paramagnetic resonance experiments as a function of temperature. We found a second-order structural phase transition at T-CO=247 K (Pmmm to Pmma) associated with the onset of long-range charge ordering. At T-min approximate to 1.2T(CO), the electron paramagnetic resonance linewidth rapidly broadens, providing evidence of antiferromagnetic spin fluctuations. This likely indicates that, analogously to manganites, the long-range antiferromagnetic order in GdBaCo2O5.0 sets in at approximate to T-CO. Pair distribution function analysis of diffraction data revealed signatures of structural inhomogeneities at low temperature. By comparing the average and local bond valences, we found that above T-CO 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 T-CO. Below T approximate to 100 K the charge localization is partially melted at the local scale, suggesting a reentrant behavior of charge ordering. This result is supported by the weakening of superstructure reflections and the temperature evolution of electron paramagnetic resonance linewidth that is consistent with paramagnetic reentrant behavior reported in the GdBaCo2O5.5 parent compound.

Rare Earth doped ceria: a combined X-ray and neutron pair distribution function study

Abstract Rare Earth doped ceria materials (Ce1–xRExO2–x/2) are widely studied for their application in solid oxide fuel cell devices. In this work, RE(Yb, Y, Nd, La)-doped ceria samples at constant (x = 0.25) doping rate were subjected to a combined synchrotron radiation and neutron powder diffraction study. The dopants were chosen in order to cover a wide range of dopant-ionic radii. The effect of doping on the average structure is investigated using conventional Rietveld analysis, while the Pair Distribution Function technique is used to explore the spatial extent of disorder as well as the local structure. Two models for mapping the local structure, in terms of oxygen relaxation and nano-phase separation, are presented.

EXAFS-XANES evidence of in situ cesium reduction in Cs-Ru/C catalysts for ammonia synthesis.

We present here a X-ray absorption spectroscopy (XAS) investigation on the local chemical order and electronic structure of Cs and Ba, promoters of the Ru/C catalysts for ammonia synthesis that attracted interest because of highly increased productivity. The role of the promoters is still largely unclear, although indirect evidence for Cs partial reduction has been obtained by this and other groups. Our XAS analysis with in situ H(2) reduction directly supports the partial Cs reduction in the promoted Ru/C catalysts, depending on the presence of Ru and on the graphitization degree of the support. Higher coordination of Ba was observed with respect to Cs in the reduced samples, without evidence of heavy atoms (Ru, Cs, and Ba) in the surroundings. Because of the strong electropositive nature of Cs, direct experimental evidence of its partial reduction is of outstanding significance also for other applications.

Devitrification kinetics of PbGeO3

The devitrification of glassy PbGeO3 was studied and interpreted by means of isothermal and non-isothermal Johnson-Mehl-Avrami equations. In the case of the non-isothermal approach, several approximated equations proposed by various authors were considered in order to obtain both the activation energy Ea and the Avrami morphological coefficient n of the crystallisation process. A critical discussion of the Avrami coefficient on the basis of experimental morphological evidence is also presented.

Stable and Metastable Phases within the GeO2-Rich Part of the Binary PbO–GeO2 System

Glasses of composition (1-x)PbO · xGeO2 with 0.50 ≤ x ≤ 1.00 were produced by melt quenching. Aliquots of each sample were thermally treated in air for various times at 660°C and then characterized by X-ray powder diffraction (XRPD) and differential thermal analysis (DTA). The XRPD patterns of devitrified samples show the presence of one or more crystalline phases, depending on x. For 0.50 ≤ x ≤ 0.75, short treatments (2 h) resulted in the presence of monoclinic PbGeO3, accompanied by phases of PbGe4O9 stoichiometry. On the contrary, prolonged thermal treatments (360 h) produced a slight decrease in the intensity of the XRPD peaks of the PbGeO3 phase and the transformation of the remaining material into orthorhombic PbGe3O7. For 0.80 ≤ x ≤ 0.95, short treatments (2 h) resulted in the formation of hexagonal GeO2, accompanied by orthorhombic PbGe3O7 and by PbGe4O9 phases. In this case, prolonged thermal treatments (360 h) do not affect strongly the XRPD patterns.

Germanium K edge in GeO2 polymorphs. Correlation between local coordination and electronic structure of germanium

The electronic properties of two room temperature persistent phases of germanium dioxide have been studied by means of experimental and theoretical techniques. We collected the Ge-K edge XANES spectra of these materials at the GILDA beamline of ESRF. The density of states of the two crystal phases, obtained from fully periodic Hartree–Fock and density functional calculations, is taken as the reference term to rationalise and assign the manifolds of the XANES spectra. Although this scheme requires a number of severe approximations, we obtained a good overall agreement between experiment and theory. The topological analysis of the theoretical electron density distribution in the crystals gave further information regarding the electronic properties of germanium dioxide.

Percolating hierarchical defect structures drive phase transformation in Ce1−xGdxO2−x/2: a total scattering study

Pair distribution function analysis up to tens of nanometres allows probing of the structural changes in Ce1−xGdxO2−x/2 solid solutions at varying gadolinium concentrations. Dopant ions and oxygen vacancies form extended Gd2O3-like clusters (droplets) and nanodomains which, on increasing the Gd concentration, percolate and cause a long-range phase transformation. A general crystallographic rationale is presented which could be adopted to describe phase transformations in highly doped materials.

Local chemical environment of Nd3+, Eu3+, and Er3+ luminescent centers in lead germanate glasses

The local chemical environment of Nd3+, Eu3+, and Er3+ luminescent centers in various lead germanate glasses has been investigated by means of extended x-ray absorption fine structure at the Ln LIII edges. The main result of this investigation is that for all the Ln3+ ions the first coordination sphere is more symmetrical in glasses with high lead concentration than in glasses with low Pb content. This result is compatible with differential scanning calorimetry measurements and enlightens previous luminescence investigations carried out by some of the present authors.