RESUMEN
The focus collection 'Waste-heat harvestingviathermoelectric conversion: Materials, devices and systems for sustainable energy technologies' collates several research articles and a Roadmap highlighting the most recent advances in the field of thermoelectricity from the viewpoint of both basic and applied research, with a special eye on the work of the Italian community.
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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.
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The evolution of the defect structure and microstructure of heavily Gd-doped ceria (Ce1-µREµO2-y, 0.313 ≤ µ ≤ 0.438) for different synthetic pathways is investigated here to explore the way defects interact with each other in a composition range known to effectively hamper the application of the material as an electrolyte. Synchrotron radiation powder diffraction is exploited by combining conventional Rietveld analysis with the Pair Distribution Function to get a multiscale picture of defect structures, and it is combined with Raman spectroscopy to assess local scale interactions. Samples were prepared via both the sol-gel route and coprecipitation of oxalates by sintering the powders at different temperatures to obtain samples with different defect distributions and crystallite sizes, investigated using electron microscopy and Whole Powder Pattern Modelling from diffraction data. As a general scheme, increasing the doping amount transforms the fluorite structure of ceria into C-type Gd2O3. For samples annealed at and above 900 °C, containing crystals at least â¼100 nm in size, this transformation occurs through a mechanism involving first the formation of distorted Gd-rich droplets on the local scale, then the growth of extended C-type nanodomains. Nanoparticles, resulting from thermal treatments at lower temperature, are less distorted on the local scale and transform abruptly upon doping, without forming larger dopant-rich aggregations, from fluorite to the C-type. The annealing temperature not only acts on the sintering of the crystallites, it is also found to promote a radical change in the microstructure as a consequence of the preferential aggregation of oxygen vacancies.
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Orthorhombic LaErO3 and cubic (La0.5 Ln 0.5)2O3 oxides (Ln: Er, Dy) were examined by neutron powder diffraction between 1.5 K and 15 K in order to investigate their crystallographic and magnetic structures. At 1.5 K both LaErO3 and (La0.5Er0.5)2O3 display a magnetic moments ordering, whereas for (La0.5Dy0.5)2O3 only short range magnetic correlations can be argued, suggesting a possible magnetic moments ordering at lower temperature. LaErO3 is characterized by a magnetic wavevector k = (0, 0, 0) and forms an antiferromagnetic G x C y A z -type structure belonging to the [Formula: see text] Shubnikov group with a total magnetic moment of 6.78(3) µ B. The antiferromagnetic structure of (La0.5Er0.5)2O3 is similar to the one typical of Er2O3, with a total magnetic moment of 4.28(2) µ B at both different magnetic sites; it is characterized by k = (0, 0, 0) and belongs to the magnetic [Formula: see text] Shubnikov group.
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Magnetization measurements and neutron powder diffraction analyses followed by Rietveld refinement have been carried out in order to investigate the magnetic structures of LaYbO3 and LaHoO3. Both compounds exhibit a negative thermal expansion along the a and b axes. In LaYbO3 Yb(3+) spins order at 2.4 K according to a FyGz-type structure, belonging to the Pn'ma' magnetic space group. Conversely, LaHoO3 is paramagnetic down to 1.5 K.