On the evolution of the DyNiO3 perovskite across the metal-insulator transition though neutron diffraction and Mössbauer spectroscopy studies.

The structural changes of polycrystalline DyNiO3 perovskite across the metal-insulator transition (TMI = 564 K) have been studied by high resolution neutron diffraction techniques together with Mössbauer spectroscopy, in a sample doped with 1.5 at.% 57Fe. In the insulating (semi-conducting) regime, below T(MI), the perovskite is monoclinic, space group (SG) P21/n, and the crystal structure contains two chemically different Ni1 and Ni2 cations, as a result of the charge disproportionation of Ni3+ cations. The beta parameter, characterizing the low-temperature monoclinic distortion, is smaller than 90.04 degrees for T < TMI, indicating a strongly pseudo-orthorhombic symmetry, although the internal monoclinic symmetry, implying the splitting and shifts of oxygen positions around the two Ni sites is perfectly detected by neutrons. Above TMI, DyNiO3 becomes orthorhombic, SG Pbnm. Upon heating across TMI, there is an abrupt convergence of the two sets (Ni1 and Ni2) of three Ni-O bond lengths, in the monoclinic-insulating phase, to three unique Ni-O distances in the orthorhombic-metallic phase upon entering the metallic region. The 57Fe Mössbauer spectra of an iron-doped (1.5 at.%) DyNiO3 sample recorded in the insulating, paramagnetic temperature range (TN < T < TMI) are discussed by supposing that the Fe3+ probe cations replace nickel in the two octahedral Ni1 and Ni2 sites. Electric field gradient calculations have shown that the 57Fe hyperfine parameters of Fe1 and Fe2 subspectra reflect a specificity of local structure corresponding to large (Ni1O6) and small (Ni2O6) octahedra. At T > TMI, the 57Fe spectrum gives clear evidence for the formation of an unique state for iron probe atoms and could, therefore, imply that the charge disproportionation in the (NiO6) subarray completely vanishes at the insulator-->metal transition.

Thermal stabilization of phase and structural state in binary lamellar metallic systems.

The paper proposes and describes a physical model of thermally induced processes in binary lamellar systems. The model has been developed for the theoretical explanation of an experimentally revealed fact of thermal stabilization of intermetallic phases on the surface of a lamellar sample. Based on the model we developed an algorithm for calculations and a computer code that operates with three one-phase and two two-phase regions in the binary alloy state diagram. The computational model includes as inputs changes in concentration boundaries for existing phases with change in temperature, as well as arbitrary temperature-time regimes for the thermal treatment of the lamellar system under investigation. Good agreement between the theoretical calculations and Mössbauer investigations of binary lamellar Fe-Be systems has been achieved.

[High-temperature microbial sulfate reduction can be accompanied by magnetite formation]. / Vysokotemperaturnaia mikrobnaia sulfatreduktsiia mozhet soprovozhdat'sia obrazovaniem magnetita.

The hyperthermophilic sulfate-reducing archaeon Archaeoglobus fulgidus was found to be capable of lithoautotrophic growth on medium containing molecular hydrogen, sulfate, and amorphous Fe(III) oxide. During the growth of this microorganism, amorphous Fe(III) oxide was transformed into black strongly magnetic sediment rich in magnetite, as shown by Mossbauer studies. Experiments involving inhibition of microbial sulfate reduction and abiotic controls revealed that magnetite production resulted from chemical reactions proceeding at elevated temperatures (83 degrees C) between molecular hydrogen, amorphous Fe(III) oxide, and sulfide formed enzymatically in the course of dissimilatory sulfate reduction. It follows that magnetite production in this system can be characterized as biologically mediated mineralization. This is the first report of magnetite formation as a result of activity of sulfate-reducing microorganisms.