High Pressure X-ray Diffraction as a Tool for Designing Doped Ceria Thin Films Electrolytes

Abstract

Rare earth-doped ceria thin films are currently thoroughly studied to be used in miniaturized solid oxide cells, memristive devices and gas sensors. The employment in such different application fields derives from the most remarkable property of this material, namely ionic conductivity, occurring through the mobility of oxygen ions above a certain threshold temperature. This feature is in turn limited by the association of defects, which hinders the movement of ions through the lattice. In addition to these issues, ionic conductivity in thin films is dominated by the presence of the film/substrate interface, where a strain can arise as a consequence of lattice mismatch. A tensile strain, in particular, when not released through the occurrence of dislocations, enhances ionic conduction through the reduction of activation energy. Within this complex framework, high pressure X-ray diffraction investigations performed on the bulk material are of great help in estimating the bulk modulus of the material, and hence its compressibility, namely its tolerance toward the application of a compressive/tensile stress. In this review, an overview is given about the correlation between structure and transport properties in rare earth-doped ceria films, and the role of high pressure X-ray diffraction studies in the selection of the most proper compositions for the design of thin films.

Chemical lattice strain in nonstoichiometric oxides: an overview

“…This includes the variation of coordination environments of ionic species, formation of defect clusters, nanodomains embedded in the matrix of the main phase, etc. For example, recent studies of ceria solid solutions [97][98][99][100][101][102][103][104] show that all these aspects may signicantly affect the behavior of the material including, of course, its chemical expansion. In this regard, it is clear that chemical strain should be reanalyzed with the emphasis on the real spatial arrangement of the constituent ionic species and not only on the idealized uorite structure as was done in the past.…”

“…In this respect it should be noted that the real microscopic picture of ceria solid solutions on the local scale may be much more complex than that assumed by Bishop et al 84 in his analysis as shown recently in the excellent studies of Artini et al 97–104 They found that the clusterization of a rare-earth dopant with oxygen vacancies leads to the formation of microheterogeneity which at some point results in crystallization of the Ia 3̄ cubic C-phase, typical of sesquioxides of the rare earths from Gd to Lu, where the latter are six-coordinated by oxygen. For larger rare-earth dopant-cations formation of a hybrid H-phase intermediate between the fluorite (F) and C phases was observed, where the F-based solid solution mainly hosts randomly dispersed C clusters.…”

“…All these circumstances cast doubt on the results of geometrical interpretations of chemical lattice strain described above, and the problem should be reexamined taking into account the new data on the local structure of ceria solid solutions. [97][98][99][100][101][102][103][104] 3.2. Perovskites 3.2.1.…”