Unheated natural mixtures of manganite and secondary pyrolusite, from the same lateritic manganiferous sequence, were studied in different orientations by high-resolution transmission electron microscopy (HRTEM), electron diffraction, and energy-dispersive X-ray analysis (EDX) to determine the fine structure of these phases, their possible crystallographic relations, and the genetic processes that led to the formation of the pyrolusite. Typical palisadic texture was observed for both minerals. Characteristic cracks parallel t. (010) of the pyrolusite structure and in particular <210> microfissures in manganite were noted as signs of structural accommodation accompanying the transformation phenomenon between these two minerals. A previously unreported manganese oxide of the spinel-type (γ-Mn2O3 or Mn3O4) was also identified in the original mixture. This oxide gave pure microdomains as intergrowths with pyrolusite adjacent to manganite. This is the first report of a natural occurrence of γ-Mn2O3. The manganite-pyrolusite transformation process and an unsuspected γ-Mn2O3 (Mn3O4)-pyrolusite transition were directly illustrated in detail for the first time. Interfaces between the concerned phases were not sharp or smooth, but exhibited strong strain contrasts and interferential periodicities. Lattice images and microdiffraction patterns proved that both transformations were oriented, suggestive of topotactic relations. In addition, the principal minerals in the matrix (illite, kaolinite, and goethite) were examined for a better understanding of their role in Mn-oxyhydroxides transformations.

Investigations were conducted to determine the hydrothermal transformations of synthetic birnessite exchanged with different metal ions. Autoclaving in a Teflon-lined stainless steel pressure vessel at 155°C for 24 hr of Mg-, Ca-, La-, and Co-saturated birnessite yielded manganese minerals having 10-Å X-ray powder diffraction (XRD) spacings. The autoclaved Mg-birnessite yielded a mineral identical to natural todorokite in its infrared (IR) spectrum and XRD patterns. High-resolution transmission electron microscopy (HRTEM) provided images having 10-, 12.5-, 15-, and 20-Å wide fringes indicating heterogeneous channel widths in the crystallographic a direction, and IR spectroscopy produced bands at 757, 635, 552, 515, 460, and 435 cm-1, confirming the product obtained by autoclaving Mg-birnessite to be todorokite. Prolonged autoclaving of Mg-birnessite yielded manganite (γ-MnOOH) as a by-product; manganite did not form when the autoclaving time was shortened to 8 hr. Also, when Ca-saturated samples were autoclaved, the product gave d-values of 10 Å, but the XRD lines were broad and heterogeneity of the channel sizes was evident from HRTEM observations. The Ca-derivative had an IR spectrum similar to that of natural todorokite. Images showing 10-Å lattice fringes were observed by HRTEM for the Ni-saturated sample, which also produced an XRD pattern similar to that of the Mg-saturated sample. Co- and Lasaturated samples did not form todorokite, although HRTEM of La-saturated samples indicated some 10-Å lattice fringes that were unstable in the electron beam. Birnessite saturated with Na, K, NH4, Cs, Ba, or Mn(II) gave products having 7-Å spacings upon autoclaving.

Phenol (benzenol) oxidation by three synthetic manganese oxides (buserite, manganite, and feitknechtite) has been studied in aerated, aqueous, acidified suspensions. The rate of reaction was pH dependent. Oxidation was greatly enhanced below pH 4, when diphenoquinone and p-benzoquinone were identified as the first products. Initial reaction rate was correlated with standard reduction potential (E°) of the oxides following the order: feitknechtite > manganite > buserite. A more gradual process of phenol oxidation after the initial reaction was influenced by electrochemical properties of the solution. High soluble manganese activity and increase in pH adversely affected reaction rates. Thus, the reactivity of the oxides was related to their stability and possibly the ability to readsorb Mn(II), following the order: buserite > manganite > feitknechtite. The results indicate that thermodynamic and electrochemical data for oxides and phenols are useful in predicting under which conditions phenols can be oxidized by a given system.

As one of the strongest inorganic oxidizers in natural environments, manganese oxides participate in the oxidation processes of dissolved sulfides, affecting their migration, transformation, and toxicity. The amount of and sites for Mn(III) influence significantly the oxidation activity of Mn(IV) oxides. As an easily formed Mn oxide in supergene environments, manganite consists of Mn(III)O6 octahedra; further study is needed of the interaction processes of manganite and dissolved sulfide. In the present study, the interaction mechanisms of dissolved sulfide and manganite were studied systematically. The influences of pH, temperature, and oxygen atmosphere were also investigated in detail. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to characterize the crystal structures, compositions, and micromorphologies of manganite and the intermediate products. The sulfide species were identified by visible spectroscopy, high-performance liquid chromatography, UV-visible (UV-Vis) spectroscopy, and ion chromatography during the reaction process. The results indicated that in a nitrogen atmosphere, elemental sulfur was formed as the main oxidation product of dissolved sulfide by manganite at the initial stage, and polysulfide ions were observed as the intermediates. Elemental sulfur was further oxidized slowly to S2O32−. The initial oxidation rate of dissolved sulfide by manganite increased with temperature from 20 to 40°C. The reaction rate increased at first and then decreased as the pH changed from 4.0 to 12.0, and the greatest oxidation rate was achieved at pH 8.0. In the presence of oxygen, S2O32− was the main product. The oxidation rate of dissolved sulfide decreased, and manganite exhibited significant catalytic activity and stability with respect to the oxidation of dissolved sulfide in the oxygenated aqueous systems. These findings are of fundamental significance in understanding the interaction and transformation of dissolved sulfide and manganese oxides in nature.

We report on the analysis of virtual powder-diffraction patterns from serial femtosecond crystallography (SFX) data collected at an X-ray free-electron laser. Different approaches to binning and normalizing these patterns are discussed with respect to the microstructural characteristics which each highlights. Analysis of SFX data from a powder of Pr0.5Ca0.5MnO3 in this way finds evidence of other trace phases in its microstructure which was not detectable in a standard powder-diffraction measurement. Furthermore, a comparison between two virtual powder pattern integration strategies is shown to yield different diffraction peak broadening, indicating sensitivity to different types of microstrain. This paper is a first step in developing new data analysis methods for microstructure characterization from serial crystallography data.

Powder X-ray diffraction (XRD) data were collected for La0.65Sr0.35MnO3 prepared through an alternative method from a stoichiometric mixture of Mn2O3, La2O3, and SrO2, fired at 1300 °C for 16 h. XRD analysis using the Rietveld method was carried out and it was found that manganite has rhombohedral symmetry (space group R3c). The lattice parameters are found to be a=5.5032 Å and c=13.3674 Å. The bond valence computation indicates that the initial inclusion of Sr occurs at higher temperature.

Powder X-ray diffraction (XRD) data were collected for Pr0.7Ba0.3MnO3. This sample was prepared using the conventional solid state reaction by mixing Pr6O11, Mn2O3, and BaCO3 up to 99.9% purity at 1400 °C in air for 60 h. XRD analysis using the Rietveld method was carried out and it was found that this manganite sample has orthorhombic symmetry with Pnma space group. The lattice parameters are found to be a=5.4900 Å, b=7.7578 Å, and c=5.5227 Å.

Single-phase (Ho0.50Ca0.50)MnO3 has been successfully prepared by means of solid state reaction at high temperature. Its crystal structure, as well as those of Cr-doped samples(Ho1−xCax)(Mn1−yCry)O3, has been refined by the Rietveld method, using X-ray powder diffraction data. All the examined compounds crystallize in the Pnma space group with a distorted perovskite-type structure. The distortion of the BO6 octahedra, due to the Jahn–Teller Mn3+ ionic species, decreases with the Cr content. Bond valence sum calculations were carried out using the refined results.

Reported results for four new cubic compounds NdMMgMn2O6 (M=Li, Na, K, Cs) are questionable and cast doubt on the existence of such compounds. The lattice constants and relative intensities are essentially the same regardless of the nature of M; the X-ray diffraction patterns may be indexed with the lattice constants divided by √8, which result in much better figures of merit and a=0.3861 to 0.3883 nm, close to the subcell parameter of non-stoichiometric pseudocubic NdMnO3.

The low temperature properties of double exchange model in triangular lattice are investigated via truncated polynomial expansion method (TPEM), which reduces the computational complexity and enables parallel computation. We found that for the half-filling case a stable 120° spin configuration phase occurs owing to the frustration of triangular lattice and is further stabilized by antiferromagnetic (AF) su-perexchange interaction, while a transition between a stable ferromagnetic (FM) phase and a unique flux phase with small finite-size effect is induced by AF superexchange interaction for the quarter-filling case.