Microstructural Characterization of Gamma Ferric Oxides of Different Genesis

Using X-ray powder diffraction and high resolution electron microscopy the peculiarities of the structural arrangement of γ-ferric oxide microcrystals prepared with the thermal treatment of acircular magnetite and plasmochemistry were studied. Both samples were found to have a complete structural equivalence : distortion of the basic spinel-like unit cell is not observed, whereas ordered distribution of the cation vacancies on the octahedral positions results in the formation of the tetragonal superlattice. Different magnetic properties of the materials in question seem to be associated with their microstructural features, i.e. the orientation of the multiple twin domains of those every particles of both samples are composed with respect to the developed planes of oxide microcrystals

Structure stabilizing effect of tungsten in mixed molybdenum oxides with Mo5O14Mo_5O_{14}-type structure

Bulk structural properties of single phase crystalline (Mo0.91V0.09)5O14 and (Mo0.68V0.23W0.09)5O14 materials were investigated using in situ X-ray diffraction and in situ X-ray absorption spectroscopy at three metal edges. Temperature programmed experiments in reducing (propene) and oxidizing (oxygen) atmosphere and isothermal redox experiments at 773 K revealed differences in the bulk properties of the two phases studied. A structure stabilizing effect of tungsten in (MoVW)5O14 under oxidizing conditions was found. Moreover, tungsten centers in a (MoVW) dioxide material exert a structure-directing effect towards re-oxidation to a Mo5O14-type structure

The structural genesis of a complex (MoVW)5O14(MoVW)_5O_{14} oxide during thermal treatments and its redox behavior at elevated temperatures

The structural genesis of a Mo0.68V0.23W0.09 oxide with Mo5O14-like structure has been examined. A precursor prepared by spray-drying of mixed aqueous metal salt solutions was calcined in air and subsequently treated in helium at different temperatures. X-ray diffraction, HRTEM, 51V MAS NMR, ESR, UV/Vis DR spectroscopy and oxygen and hydrogen adsorption measurements have been applied to monitor the preparation procedure. It was found that a structure closely related to that of Mo5O14 already appears at nano-scale level after calcination of the spray-dried precursor in air at 350°C. At this stage, the material comprises of crystalline particles less than 3 nm in size stabilized by an amorphous matrix. Further heating causes nano-structural rearrangements that lead to the formation of the final Mo0.68V0.23W0.09 oxide with phase-pure polycrystalline structure. Molybdenum and tungsten ions are hexavalent and coordinated in an octahedral environment. Furthermore, vanadium is present as V4+ and V5+ ions which partially occupy octahedral sites, whereas highly distorted trigonal pyramidal sites could be accommodated in pentagonal bipyramids of the Mo5O14 structure, however, displaced away from the center. According to the results of H2 and O2 adsorption the crystalline ternary oxide does not possess accessible micropores. Oxygen pulses at 450oC and reductive treatment with pure hydrogen at 300oC did not cause noticeable changes of the bulk structure thus indicating a remarkable structural stability of the complex MoVW oxide under redox conditions at elevated temperature