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Abstract: The metallic compound forms a full range solid solution to the insulating phase . At a Na concentration of 0.25mole per formula unit, we found an unexpected contribution to the specific heat at low temperature [K. Yamaura, et al., Chem. Mater. 17 (2005) 359]. To address this issue, specific heat and AC and DC magnetic susceptibilities were additionally measured under a variety of conditions for the sample. A new set of data clearly indicate the additional specific heat is magnetic in origin; however, the magnetic entropy is fairly small ( of Schottky term for a simple splitting doublet), and there is no other evidence to suggest that a magnetic phase transition is responsible for the anomalous specific heat. [Copyright &y& Elsevier]

Artificial photosynthesis constitutes one of the most promising alternatives for harvesting solar energy in the form of fuels, such as hydrogen. Among the different devices that could be developed to achieve efficient water photosplitting, tandem photoelectrochemical cells show more flexibility and offer high theoretical conversion efficiency. The development of these cells depends on finding efficient and stable photoanodes and, particularly, photocathodes, which requires having reliable information on the mechanism of charge transfer at the semiconductor/solution interface. In this context, this work deals with the preparation of thin film calcium ferrite electrodes and their photoelectrochemical characterization for hydrogen generation by means of electrochemical impedance spectroscopy (EIS). A fully theoretical model that includes elementary steps for electron transfer to the electrolyte and surface recombination with photogenerated holes is presented. The model also takes into account the complexity of the semiconductor/solution interface by including the capacitances of the space charge region, the surface states and the Helmholtz layer (as a constant phase element). After illustrating the predicted Nyquist plots in a general manner, the experimental results for calcium ferrite electrodes at different applied potentials and under different illumination intensities are fitted to the model. The excellent agreement between the model and the experimental results is illustrated by the simultaneous fit of both Nyquist and Bode plots. The concordance between both theory and experiments allows us to conclude that a direct transfer of electrons from the conduction band to water prevails for hydrogen photogeneration on calcium ferrite electrodes and that most of the carrier recombination occurs in the material bulk. In more general vein, this study illustrates how the use of EIS may provide important clues about the behavior of photoelectrodes and the main strategies for their improvement.

Three structures of CaMn2O4, CaFe2O4, and CaTi2O4 have been proposed as post-spinel phases. Because these structures are very similar, several ambiguities and inconsistencies appear in high-pressure studies, leading to many problems that are yet to be solved. Systematic powder diffraction studies related to these three phases were conducted under high pressure using synchrotron radiation. All three samples have further high-pressure polymorphs. CaMn2O4 transforms to the CaTi2O4-type structure at about 30 GPa. The MnO6 octahedron in the lower-pressure structure is distorted by the Jahn-Teller effect. A new phase was observed at pressures above 50 GPa during compression of CaFe2O4. Rietveld profile fitting analysis of diffraction data at 63.3 GPa demonstrated that the high-pressure structure, with space group Pnam, is produced via a martensitic transformation by displacing atoms in every third layer perpendicular to the c axis. CaTi2O4 also has a new high-pressure polymorph above 39 GPa with space group Bbmm. The most probable post-spinel candidate in the mantle is the CaTi2O4-type structure. The CaMn2O4-type structure is only formed at high pressure from spinel phases with atoms susceptible to Jahn-Teller distortion. [ABSTRACT FROM AUTHOR]