RESUMO
A room temperature structural study has been performed through the whole compositional range of the (Ce1-xLux)O2-x/2 system by synchrotron X-ray diffraction and µ-Raman spectroscopy. Samples were synthesized by thermal treatment in air at 1373 K of coprecipitated mixed oxalates. A CeO2-based solid solution with a fluorite-type structure (F) was found to be stable up to x = 0.4, while at higher Lu content a (F + C) biphasic region was observed, with C being the cubic atomic arrangement typical of sesquioxides of the heaviest rare earths. A comparative approach including also results deriving from other (Ce1-xREx)O2-x/2 systems (RE ≡ Gd and Sm) allowed us to conclude that the compositional extent of the F solid solution is a complex function of RE3+ size and RE compressibility. On this basis, the dependence of ionic conductivity on the RE identity was interpreted as related both to the Ce4+/RE3+ size closeness and to RE compressibility. Ce4+/RE3+ dimensional issues were also revealed to rule the appearance of the hybrid structure observed in the two aforementioned systems, consisting of the intimate intergrowth of C microdomains within the F-based host lattice. Moreover, a more extended definition of F-based solid solution, including also the hybrid structure, is formulated; the latter is meant as a modification of the former, occurring when mainly RE-vacancy aggregates are incorporated into the host lattice in spite of isolated RE ions. By µ-Raman spectroscopy it was possible to demonstrate that the mechanism of oxygen vacancy formation is common to all the systems studied, provided that the structure of the F-based solid solution, also including the hybrid structure, is retained.
RESUMO
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.
RESUMO
A structural study of Sm- and Gd-doped ceria was performed with the aim to clarify some unexplained structural features. (Ce1-xREx)O2-x/2 samples (RE ≡ Sm, Gd; x = 0, 0.1, ..., 1) were prepared by coprecipitation of mixed oxalates and subsequent thermal treatment at 1473, 1173, or 1073 K in air; they were then analyzed at room temperature both by synchrotron X-ray diffraction and µ-Raman spectroscopy. Two structural models were adopted to fit the experimental data, namely, a fluoritic one, resembling the CeO2 structure at low RE content, and a hybrid one at higher RE content, intermediate between the CeO2 and the RE2O3 structures. Two main transitions were detected along the compositional range: (a) an RE-dependent transition at the boundary between the fluoritic and the hybrid regions, of a chemical nature; (b) an RE-independent transition within the hybrid region at â¼0.5, having a purely geometrical nature. The presence of two finely interlaced F- and C-based structures within the hybrid region was confirmed, and hints of their composition were obtained by µ-Raman spectroscopy. The obtained results indicate a possible explanation for the non-Vegard behavioral trend of the cell parameters.
RESUMO
A series of luminescent rare-earth ion-doped hexagonal II-type Gd oxycarbonate phosphors Gd2-xRExO2CO3 (RE = Eu(3+), Yb(3+), Dy(3+)) have been successfully synthesized by thermal decomposition of the corresponding mixed oxalates. The Yb(3+) doped Gd-oxycarbonate has evidenced a high persistent luminescence in the NIR region, that is independent from the temperature and makes this materials particular attractive as optical probes for bioimaging.
