Search

A comparative study of the electrochemical intercalation of Ca2+ and Mg2+ in layered TiS2 using alkylcarbonate-based electrolytes is reported, and for the first time, reversible electrochemical Ca2+ insertion is proved in this compound using both X-ray diffraction and differential absorption X-ray tomography at the Ca L2 edge. Different new phases are formed upon M2+ insertion that are structurally characterized, their amount and composition being dependent on M2+ and the experimental conditions. The first phase formed upon reduction is found to be the result of an ion-solvated intercalation mechanism, with solvent molecule(s) being cointercalated with the M2+ cation. Upon further reduction, new non-cointercalated calcium-containing phases seem to form at the expense of unreacted TiS2. The calculated activation energy barriers for Ca2+ migration in TiS2 (0.75 eV) are lower than those previously reported for Mg (1.14 eV) at the dilute limit and within the CdI2 structural type. DFT results indicate that the...

By means of high resolution synchrotron X-ray and neutron powder diffraction we have investigated the structural evolution of Pr 0.50 Ca 0.50 Mn 0.99 Ti 0.01 O 3 . We have found two structural transitions on cooling; the first takes place at T co ≃ 240 K and the second in a wide temperature range around 100-140 K. Below the first transition two phases coexist; one presents robust charge/orbital ordering, characterized by strong lattice distortions and low strain, and the second presents charge/orbital order characterized by smaller lattice distortions with remarkable spatial fluctuations and short ranged orbital ordering. This second phase is metastable giving rise to thermal hysteresis of structural and macroscopic properties in the cooling-heating cycle. We have observed that thermal hysteresis is due to (i) the transformation of a huge region of short-ranged order into long-ranged ordered regions and (ii) a hardening of the remaining short-ranged regions in which the lattice distortion grows and the strain diminishes. We also report a detailed characterization of the low temperature (ground state) coexisting phases including their structural details, ordered magnetic structures, coherence lengths, microstructural details, and so forth. A great degree of magnetic disorder characterizes the low temperature ground state of the investigated highly diluted manganite.

The crystal and magnetic structures of orthorhombic e-Fe2O3 have been studied by simultaneous Rietveld refinement of X-ray and neutron powder-diffraction data in combination with Mossbauer spectroscopy, as well as magnetization and heat-capacity measurements. It has been found that above 150 K, the e-Fe2O3 polymorph is a collinear ferrimagnet with magnetic moments directed along the a axis, whereas the magnetic ordering below 80 K is characterized by a square-wave incommensurate structure. The transformation between these two states is a second-order phase transition and involves subtle structural changes mostly affecting the coordination of the tetrahedral and one of the octahedral Fe sites. The temperature dependence of the e-Fe2O3 magnetic properties is discussed in light of these results.

The LnBaCo2O5+δ cobaltites with ordered A-site cations exhibit a very rich phase diagram as a function of the oxygen content, which determines the doping of CoO2 planes. By means of thermogravimetry analysis (TGA), we study the appropriate way to tailor oxygen contents in PrBaCo2O5+δ compounds. Starting from an air-synthesized sample, we study the evolution of δ during different heat treatments with “in situ” TGA measurements. We find that δ can be tailored from δ ≃ 0.2 to δ ≃ 0.9. Thermogravimetry measurements also show that oxygen ions can flow through (0 0 1) PrOδ planes at relatively low temperatures (200 °C, this temperature increases in compounds with smaller rare earths). By means of neutron powder diffraction at room temperature, we determine the structural details and study the eventual order of oxygen vacancies for different values of δ.

La5/8-xPrxCa3/8MnO3 (x = 0, 0.1, 0.2, 0.25, 0.3, 0.35, 0.375, 0.4, 0.412, 0.425, 0.45, 0.5, 5/8) series has been synthesized by the ceramic method. All manganites are single phase crystallizing in ...

Ultrahigh-resolution synchrotron diffraction for Bi 0.15 Ca 0.85 MnO 3 at 300 K shows a unique pattern broadened by microstrains. The joint Rietveld refinement of synchrotron and neutron data at RT (room temperature) indicates that this phase is nonstoichiometric due to calcium vacancies. However, the synchrotron pattern at 10 K shows two phases one being monoclinic and the other orthorhombic. The refined weight fractions were 67.7(6) and 28.8(3)%, respectively. The joint Rietveld refinement at low temperature has established that this macroscopic phase separation/segregation is due to different thermal evolutions of the heterogeneous material. Orthorhombic and monoclinic low-temperature phases have different calcium contents. The sample at RT is formed by small domains (2000 A) with minor variations in the cation stoichiometry, but it displays a single orthorhombic GdFeO 3 -type powder pattern. The structural transition driven by the orbital ordering unmasks these domains, and the macroscopic phase segregation comes suddenly into sight. The associated structural changes and magnetic structures are reported. A unique pattern above the orbital ordering temperature (RT in most cases) is not any longer a proof of chemical homogeneity, or single phase existence, in manganites. So, the discussion of many previously reported data for manganites should be revisited.

Yeelimite, Ca4[Al6O12]SO4, is outstanding as an aluminate sodalite, being the framework of these type of materials flexible and dependent on ion sizes and anion ordering/disordering. On the other hand, yeelimite is also important from an applied perspective as it is the most important phase in calcium sulfoaluminate cements. However, its crystal structure is not well studied. Here, we characterize the room temperature crystal structure of stoichiometric yeelimite through joint Rietveld refinement using neutron and X-ray powder diffraction data coupled with chemical soft-constraints. Our structural study shows that yeelimite has a lower symmetry than that of the previously reported tetragonal system, which we establish to likely be the acentric orthorhombic space group Pcc2, with a √2a × √2a × a superstructure based on the cubic sodalite structure. Final unit cell values were a = 13.0356(7) Å, b = 13.0350(7) Å, and c = 9.1677(2) Å. We determine several structures using density functional theory calculations, with the lowest energy structure being Pcc2 in agreement with our experimental result. Yeelimite undergoes a reversible phase transition to a higher-symmetry phase which has been characterized to occur at 470 C by thermodiffractometry. The higher-symmetry phase is likely cubic or pseudocubic possessing an incommensurate superstructure, as suggested by our theoretical calculations which show a phase transition from an orthorhombic to a tetragonal structure. Our theoretical study also predicts a pressure-induced phase transition to a cubic structure of space group I43m. Finally, we show that our reported crystal structure of yeelimite enables better mineralogical phase analysis of commercial calcium sulfoaluminate cements, as shown by RF values for this phase, 6.9% and 4.8% for the previously published orthorhombic structure and for the one reported in this study, respectively. © 2013 American Chemical Society., The work at UMA was funded by MAT2010-16213 research grant (Spain) which is cofunded by FEDER. The work at the DIPC was supported by the Basque Departamento de Educación and the UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio de Innovación, Ciencia y Tecnología (Grant Nos. TEC2007-68065-C03-03 and FIS2010-19609-C02-02), and the ETORTEK research program (NANO-IKER Grant No. IE11-304) funded by the Basque Departamento de Industria and the Diputación Foral de Guipuzcoa.