Cristina Artini

Synthesis and characterisation of superconducting RuSr2GdCu2O8

Abstract We report on the structural, electrical and magnetic properties of RuSr2GdCu2O8 samples made by systematic synthesis work so that they differ from one another in number of annealings and sintering temperature. Our aim is to highlight the different behaviours as a consequence of the thermal treatments. In particular, we deal with the onset of superconductivity in relation to the homogenisation process, and its influence on resistivity, structural and magnetic ordering. Finally, the problems related to the magnetic measurements due to the superimposition of different magnetic states are examined.

High temperature structural study of Gd-doped ceria by synchrotron X-ray diffraction (673 K ≤ T ≤ 1073 K).

The crystallographic features of Gd-doped ceria were investigated at the operating temperature of solid oxides fuel cells, where these materials are used as solid electrolytes. (Ce(1-x)Gd(x))O(2-x/2) samples (x = 0.1, 0.3, 0.5, 0.7) were prepared by coprecipitation of mixed oxalates, treated at 1473 K in air, and analyzed by synchrotron X-ray diffraction in the temperature range 673 K ≤ T ≤ 1073 K at the Elettra synchrotron radiation facility located in Trieste, Italy. In the whole temperature span a boundary was found at x ∼ 0.2 between a CeO2-based solid solution (for x ≤ 0.2) and a structure where Gd2O3 microdomains grow within the CeO2 matrix, taking advantage of the similarity between Gd(3+) and Ce(4+) sizes; the existence of the boundary at x ∼ 0.2 was confirmed also by measurements of ionic conductivity performed by impedance spectroscopy. Similar to what observed at room temperature, the trend of the cell parameter shows the presence of a maximum; with increasing temperature, the composition corresponding to the maximum moves toward lower Gd content. This evidence can be explained by analyzing the behavior of the coefficient of thermal expansion as a function of composition.

Synthesis and superconductive characterisation of RuSr2NdxGd1-xCu2O8 compounds (x = 0, 0.09, 0.18, 0.35)

Abstract We prepared various RuSr 2 Nd x Gd 1− x Cu 2 O 8 samples with systematically varying Nd amounts by x =0, 0.09, 0.18 and 0.35. The solid state route involved mixed oxides (Nd x Gd 1− x ) 2 O 3 obtained by coprecipitation. The XRD and microstructural analysis showed that a high phase purity was obtained. The Nd solubility limit can be placed between x =0.18 and x =0.35. The superconducting transition temperature dramatically decreased with increasing Nd content, confirming that Nd entered the lattice. The field dependence of superconducting transitions exhibited a polycrystalline behaviour.

Correlations between Structural and Electronic Properties in the Filled Skutterudite Smy(FexNi1–x)4Sb12

A structural study of the filled skutterudite Smy(FexNi1-x)4Sb12 was performed by means of X-ray powder diffraction and μ-Raman spectroscopy with the aim to unveil the correlations between structural and electronic properties of this material and to favor the improvement of its thermoelectric performance. Samples were prepared by direct reaction of the elements at 1223 K, followed by quenching and subsequent sintering at 873 K; microstructure and composition of the obtained products were determined by SEM-EDS. The position of the boundary separating regions that obey hole- and electron-based conduction mechanisms was found by X-ray diffraction at x ≈ 0.63 and y ≈ 0.30, confirmed by measurements of room-temperature Seebeck coefficient, and discussed on the basis of crystallographic data. The presence of a discontinuity is observed in several structural and spectroscopic parameters at the p/n crossover; it is interpreted as associated with the change in the conduction mechanism. The role of the rare earth filling fraction in driving the structural response of the material is investigated too. The advantage of using X-ray diffraction and μ-Raman spectroscopy as aids in the study of electronic properties of this material is highlighted, as well as the complementarity of the two techniques.

Synthesis Effects on the Magnetic and Superconducting Properties of RuSr2GdCu2O8

A systematic study on the synthesis of the Ru-1212 compound by preparing a series of samples that were annealed at increasing temperatures and then quenched has been performed. It results that the optimal temperature for the annealing lies around 1060–1065°C; a further temperature increase worsens the phase formation. Structural order is very important and the subsequent grinding and annealing improves it. Even if from the structural point of view the samples appear substantially similar, the physical characterizations highlight great differences both in electrical and magnetic properties related to intrinsic properties of the phase as well as to the connection between the grains as inferred from the resistive and the Curie-Weiss behaviour at high temperature as well as in the visibility of field cooled and zero field cooled magnetic signals.

Search for new superconducting oxides in M4Pn2O phases (M=Ca, Sr, Ba and Pn=As, Sb, Bi)

Abstract All the high- T c oxide superconducors known to date are copper-based oxides (cuprates) in the layered perovskite crystal structures that contain a planar network of copper and oxygen (CuO 2 plane). In searching for possible new superconducting metal oxides without Cu ions, we have examined the layered tetragonal pnictides M 4 Pn 2 O (M=Ca, Sr, Ba and Pn=As, Sb, Bi) because their structures are considered as an anti-perovskite with alternate stacking of two kinds of layers; conducting M 2 Pn 2 block layers sandwiched by insulating perovskite-based M 2 O layers. Compounds were prepared by melting stoichiometric amounts of the components in tantalum tube all sealed under an argon atmosphere. The samples have been characterised by scanning electron microscopy, powder X-ray diffraction (XRD), metallographic examination and electrical resistivity. For these stoichiometric compounds no superconductive transition is observed.

Ionic conductivity and local structural features in Ce1-xSmxO2-x/2

Sm-Doped ceria is one of the most promising materials to be used as electrolyte in solid oxide fuel cells due to its remarkable ionic conductivity values in the intermediate temperature range. Transport properties and local structural features of Ce1-xSmxO2-x/2 (0.1 ≤ x ≤ 0.7) were studied by an impedance/μ-Raman spectroscopy coupled approach up to 1073 K. Results suggest that C-based nanosized defect clusters are responsible for the drop in ionic conductivity observed even at x = 0.2, i.e. at a Sm content lower than necessary to allow C domains to reach the percolation threshold through crystallites. Moreover, within the fluorite-type compositional region, with increasing the Sm content, defect clusters undergo a rearrangement resulting in the enlargement of C-based domains rather than in the increase of their number; at higher x, on the contrary, both the size and amount of C domains increase in parallel.

Rare-Earth-Doped Ceria Systems and Their Performance as Solid Electrolytes: A Puzzling Tangle of Structural Issues at the Average and Local Scale

Rare-earth (RE)-doped ceria systems, in particular when RE ≡ Nd, Sm, or Gd, are well-known to be characterized by high values of ionic conductivity in the intermediate temperature range, which, in principle, makes them ideal solid electrolytes in solid oxide fuel and electrolysis cells. Defect chemistry turns out to be a pivotal issue in this framework because ionic conductivity is driven by the ability of oxygen vacancies to move through the lattice, and any form of defect clustering tends to depress the efficiency of oxygen transport. In this viewpoint, not only are factors at the average scale assessed, such as the compositional extent of the CeO2-like solid solution, but also the occurrence of local inhomogeneities due to vacancy-dopant association is discussed in correlation with its central role in hindering the migration of vacancies. The relationship between the stability of the hybrid phase and the RE3+ ionic size is presented, and the highly complementary role of Raman spectroscopy toward X-ray diffraction is described in detail. The key points of the whole discussion are finally used to identify the most relevant structure-related parameters affecting ionic conductivity in the studied material.