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Toward Sustainable Energy: The Unique Role of Rare Earth Oxides

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Sustainable energy is a wide and nowadays widely used concept that encompasses not only renewable energies, but also all the technologies aimed at enhancing energy efficiency. Within this scenario, rare earth (RE) oxides occupy a unique place. Due to two main features, namely the polymorphism of oxides and ...

Sustainable energy is a wide and nowadays widely used concept that encompasses not only renewable energies, but also all the technologies aimed at enhancing energy efficiency. Within this scenario, rare earth (RE) oxides occupy a unique place. Due to two main features, namely the polymorphism of oxides and the electronic configuration of lanthanide atoms (characterized by the presence of deep-lying 4f electrons screened by outer electrons), RE oxides can be in fact employed in a large variety of devices exploiting their diverse magnetic, electric and optical properties. An incomplete list of applications utilizing RE oxides spans from fuel cells to high temperature superconductivity, thermoelectricity, catalysts, as well as photovoltaic and luminescent materials.

Regarding the structural properties of trivalent RE oxides, below 2000°C they can crystallize in three distinct structural forms, namely hexagonal (A: sp. gr. P63/mmm), monoclinic (B: sp. gr. C2/m) and cubic (C: sp. gr. Ia-3), depending on temperature and on the RE ionic size. In particular, at room temperature, oxides of the largest rare earths (from La2O3 to Nd2O3) adopt the A structure, Sm2O3 and Eu2O3 the B structure, while oxides from Gd2O3 to Lu2O3 crystallize in the C form. The transitions C→B and B→A are promoted by a temperature increase or an increase in the mean ionic size. CeO2, on the contrary, crystallizes in the fluorite cubic structure belonging to the Fm-3m space group, containing four formula units per cell. In this highly symmetric atomic arrangement, only two atomic positions exist: the (0,0,0) atomic site, occupied by Ce, and the (1/4, 1/4, 1/4) site, occupied by O. RE ions can also assume the perovskite structure, when coupled either to a further RE or to another cation of proper size.

This Research Topic aims to collect a series of original research papers and reviews capable of illustrating the manifold variety of applications of RE oxides as a direct consequence of their structural and electronic properties. To this purpose, we welcome articles dealing not only with general aspects of RE oxides (crystallographic features, thermodynamic studies, etc.), but also with the following topics:

 - solid state ionics: RE oxides are involved in fuel cell technology, as solid electrolytes (thanks to the ionic conduction properties of oxides such as RE-doped ceria or Li-La-Zr-O garnets or (La,Li)TiO3 perovskites) and also as electrodes (for instance La-Sr-Co-O and La-Sr-Mn-O perovskites).

 - high-temperature superconductivity: distorted triple perovskite RE oxides, REBa2Cu3Ox, represent the most common and applied class of high-Tc superconductors.

 - thermoelectricity: RE-containing perovskites—such as RE-single and double-doped SrTiO3 and CaMnO3—within the framework of oxide-based thermoelectric materials.

 - phosphors: YAG garnets doped by different RE oxides are among the most studied luminescent materials, similar to RE-doped sesquioxides Y2O3, Lu2O3 and Sc2O3.

 - catalysts: the application of lighter RE oxides as the main component of catalysts is a steadily growing research field.


Keywords: Rare earth oxides, fuel cells, high temperature superconductors, thermoelectrics, luminescent materials, catalysts


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