RESUMEN
Li3Fe3Te2O12 adopts a crystal structure, described in space group Pnnm, related to that of LiSbO3, in which Te6+, Fe3+, and Li+ cations reside in a partially ordered configuration within an hcp array of oxide ions. Chemical or electrochemical insertion of lithium is accompanied by a fully reversible migration of some of the Fe cations with an initial capacity of 120 mA h g-1 (2.85 Li per formula unit). Long-term cycling stability is limited by the facile reduction of Te6+ to elemental Te, which leads to cathode decomposition. Partial substitution of Fe by In suppresses Te6+ reduction, such that Li3Fe2InTe2O12 shows no sign of this cathode decomposition pathway, even after 100 cycles. In contrast, Al-for-Fe substitution is chemically limited to Li3Fe2.6Al0.4Te2O12 and appears to have almost no influence on cathode longevity. These features of the Li3Fe3-xMxTe2O12 system are discussed on the basis of a detailed structural analysis performed using neutron and synchrotron X-ray diffraction.
RESUMEN
Layered perovskites of the Gd0.8-xBa0.8Ca0.4+xFe2O5+δ system show oxygen reduction reaction (ORR) activity. The layered crystal structure of these oxides is established by the interplay of the Gd3+, Ba2+, and Ca2+ locations with the ordering of the coordination polyhedra of the Fe3+ cations. Substitution of Gd3+ by Ca2+ increases the oxygen deficiency that is accommodated by the formation of layers of FeO5-squared pyramids intercalated with A-O layers containing mainly Gd3+. The presence of FeO5-squared pyramids in the crystal structure promotes the oxygen diffusion and then the ORR activity. Therefore, GdBa2Ca2Fe5O13 is the oxide of the system which presents lower area specific resistance (ASR) values when it is applied as an electrode in symmetrical cells using Ce0.9Gd0.1O2-δ as an electrolyte.
RESUMEN
B-site-layer-ordered double perovskite Ca2FeMnO6 with unusually high valence Fe4+ was found to exhibit unusual oxygen-release behaviors, contrasting with those of the B-site-disordered perovskite having the identical chemical composition. During heating, the B-site-layer-ordered compound shows a stepwise oxygen release with successive valence changes from Fe4+ to Fe3+ through an intermediate Fe3.5+, whereas the B-site-disordered compound releases oxygen in a single step. The oxygen in Ca2FeMnO6 is released only from the two-dimensional Fe layers, and this selective oxygen release stabilizes the intermediate Fe3.5+ phase with in-plane-oxygen-vacancy ordering. Therefore, the B-site order/disorder strongly affects the oxygen-release behaviors associated with the oxygen-vacancy ordering.
RESUMEN
Ozone oxidation has allowed the stabilization of a very high iron oxidation state in the FeSr2YCu2O7.85 cuprate, in which a long-range magnetic ordering of the high valent iron cations coexists with the superconducting interactions (magnetic ordering temperature TN = 110 K > superconducting critical temperature Tc = 70 K). The somewhat unexpected A-type AFM structure, with a µ(Fe) â¼ 2 µB magnetic saturation moment associated with the hypervalent iron sublattice, suggests an unusual low spin state for the iron cations, while the low dimensionality of the magnetic structure results in a soft switching toward ferromagnetism under small external magnetic fields. The role of the crystal structure and of the high charge concentration in the stabilization of this unusual electronic configuration for the iron cations is discussed.
RESUMEN
The influence of rare earth (RE) elements on superconducting properties of the transition element (TE)-substituted TE xCu1- xSr2RECu2O y cuprates has not been sufficiently emphasized so far. In the case of molibdo-cuprates with the general formula Mo0.3Cu0.3Sr2RECu2O y, all the RE element containing compounds except La, Ce, and Lu can be prepared at room pressure. The influence of the crystal structure on the superconducting properties after ozone oxidation of the present system is reported selecting three groups of RE elements attending to their different atom sizes: small (Yb and Tm), medium (Gd), and big (Nd and Pr). Advanced transmission electron microscopy, various diffraction techniques, and spectroscopic analysis have been used to demonstrate that the increase of structural disorder complemented with a decrease in the hole content play a major role in the vanishing of superconductivity within the present system.
