Browsing by Author "Hoelzel, M"

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    Lattice dynamics of hydrogenated austenitic steels
    (Australian Institute of Physics, 2005-01-31) Danilkin, SA; Hoelzel, M; Udovic, TJ; Rameriz-Cuesta, T; Parker, SF; Wipf, H; Fuess, H
    We investigated hydrogen vibrations in of Fe 18Cr-10Ni and Fe-25Cr-20Ni austenitic steels doped in H gas atmosphere at pressures up to 7 GPa. Measurements were performed with neutron spectrometers FANS at NIST and TOSCA at ISIS. Experiments show that vibrational energy of H atoms in studied steels decreases from 132 meV at H/Me=0.0033 to 111 meV at H/Me=0.9 due to lattice dilatation. The hydrogen peaks are broadened. At H contents from 0.003 to 0.4-where the single broad peak is observed-the broadening is most probably connected with the Me-H force constant disorder. At H/Me>0.4-0.5-where H-peak has the two-component structure-the H-H interaction becomes important resulting in the dispersion of the optical phonon branches.
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    Magnetic structures and phase transitions in PrMn2-xFexGe2
    (American Institute of Physics, 2008-11-19) Wang, JL; Campbell, SJ; Studer, AJ; Avdeev, M; Hofmann, M; Hoelzel, M; Dou, SX
    The magnetic properties and magnetic structures of PrMn2-xFexGe2 compounds (space group I4/mmm) have been investigated using magnetic, Fe-57 Mossbauer effect (x=1.0,1.3,1.6), and neutron diffraction measurements (x=0.4,0.6,0.8,1.3) over the temperature range of 3-410 K. This has enabled the existing magnetic phase diagram for PrMn2-xFexGe2 to be extended from Fe concentration x=0-1 to the full range x=0-2 in terms of concentration and d(Mn-Mn), the intralayer distance. Analysis of the Mossbauer spectra (4.5-300 K) using a model which takes nearest-neighbor environments into account confirms the nonmagnetic nature of Fe atoms in these compounds, and leads to hyperfine parameters which deviate around the magnetic transition temperatures derived from the magnetic and neutron investigations while also enabling the Debye temperatures of PrMn2-xFexGe2 (x=0.4-1.6) to be determined. The experimental values for T-C(inter) are found to decrease rapidly with increasing Fe concentration in the range x=0.0-0.6 compared with calculated T-C(inter) values due to pressure (and therefore geometric) effects only. This behavior demonstrates that electronic effects and replacement of the magnetic Mn atoms with nonmagnetic Fe atoms contribute to the overall magnetic behavior of PrMn2-xFexGe2 compounds. Compared with intralayer Mn-Mn interactions, the interlayer Mn-Mn interactions play the major role in the anomalous thermal expansion observed at magnetic transition in these layered systems, with the interlayer Mn-Mn interactions governing the significant magnetovolume effects. © 2008, American Institute of Physics
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    Mechanosynthesis of nanocrystalline MgFe2O4—neutron diffraction and Mössbauer spectroscopy
    (Springer Nature, 2010-12-22) Šepelák, V; Bergmann, I; Feldhoff, A; Litterst, FJ; Becker, KD; Cadogan, JM; Hofmann, M; Hoelzel, M; Wang, JL; Avdeev, M; Campbell, SJ
    The evolution of nanocrystalline n-MgFe2O4 by high-energy milling a mixture of MgO and α-Fe2O3 for periods of between 0 h and 12 h has been investigated by neutron diffraction in addition to previous Mössbauer, XRD and HRTEM measurements. Complete transformation of the milled products to n-MgFe2O4 only occurs on milling to ∼8 h even though the average particle size decreases to < ∼10 nm after milling for 2 h. The applied field Mössbauer spectra of n-MgFe2O4 can be well described by two subspectra representing core and shell regions with different cation distributions and spin canting angles. The neutron pattern of nanocrystalline MgFe2O4 is described well by two components comprising nanoparticles of core and shell dimensions ∼7(1) nm and ∼0.7(1) nm, respectively, in support of the Mössbauer core-shell model. © 2021 Springer Nature Switzerland AG.
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    Neutron diffraction study of MnNiGa2—structural and magnetic behaviour
    (AIP Publishing, 2014-01-07) Wang, JL; Ma, L; Hofmann, M; Avdeev, M; Kennedy, SJ; Campbell, SJ; Din, MFM; Hoelzel, M; Wu, GH; Dou, SX
    MnNiGa2 crystallizes in the L21 (Heusler) structure and has a ferromagnetic ordering temperature TC ∼ 192 K. Rietveld refinement of the neutron diffraction patterns indicates that the Ga atoms occupy the equivalent 8c position, while Mn and Ni share the 4a (0, 0, 0) and 4b (0.5, 0.5, 0.5) sites with a mixed occupancy of Mn and Ni atoms. It is found that that ∼83% of Mn and ∼17% Ni are located at the 4a site while ∼83% of Ni and ∼17% Mn occupy the 4b site. There is no evidence of a magneto-volume effect around TC. In agreement with this finding, our detailed critical exponent analyses of isothermal magnetization curves and the related Arrott plots confirm that the magnetic phase transition at TC is second order. © 2014,AIP Publishing LLC.
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    Structures, phase transitions, hydration, and ionic conductivity of Ba4Nb2O9
    (American Chemical Society, 2009-08-25) Ling, CD; Avdeev, M; Kutteh, R; Kharton, VV; Yaremchenko, AA; Fialkova, S; Sharma, N; Macquart, RB; Hoelzel, M; Gutmann, MJ
    Ba4Nb2O9 is shown to have two basic polymorphs: a high-temperature γ phase, which represents an entirely new structure typed and a low-temperature (x phase, which has the rare Sr4Ru2O9 structure type. The phases are separated by a reconstructive phase transition at similar to 1370 K, the kinetics of which are sufficiently slow that the γ phase can easily be quenched to room temperature. Below similar to 950 K, both (α and γ phases absorb significant amounts of water. In the case of the γ phase, protons from absorbed water occupy ordered positions in the structure, giving rise to a stoichiometric phase γ-III-Ba4Nb2O9.1/3H(2)O at room temperature. γ-III-Ba4Nb2O9-1/3H(2)O partially dehydrates, at similar to 760 K to give another stoichiometric phase γ-II-Ba4Nb2O9.1/3H(2)O, which completely dehydrates at similar to 950 K to γ-I- Ba4Nb2O9. The hydrated γ phases exhibit faster protonic and oxide ionic transport than the hydrated (x phases because of the presence in the γ phases of 2D layers containing Nb5+ cations with unusually low oxygen coordination numbers (4 or 5) separated by discrete OH groups. Hydration appears to play an important role in stabilizing the γ phases at low temperatures, with the γ -> α transition oil reheating a quenched sample occurring at higher temperatures in humid atmospheres. © 2009, American Chemical Society
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    Structures, phase transitions, hydration, and ionic conductivity of Ba4Ta2O9
    (American Chemical Society, 2010-01-26) Ling, CD; Avdeev, M; Kharton, VV; Yaremchenko, AA; Macquart, RB; Hoelzel, M
    Low-temperature α-Ba4Ta2O9 is isostructural with α-Ba4Nb2O9 (Sr4Ru2O9 type), and it undergoes a reconstructive phase transition at approximately the same temperature (1400 K) to a γ form that can easily be quenched to room temperature. However, the γ forms of the two compounds are completely different. Whereas γ-Ba4Nb2O9 represents a unique structure type, γ-Ba4Ta2O9 adopts a more conventional 6H-perovskite type. The α→γ transition is virtually irreversible in the tantalate, unlike the niobate, which can be converted back to the α form by annealing slightly below the transition temperature. Quenched γ-Ba4Ta2O9 is highly strained due to the extreme size mismatch between Ba2+ (1.35 Å) and Ta5+ (0.64 Å) cations in perovskite B-sites, and undergoes a series of symmetry-lowering distortions from P63/mmc→P63/m→P21/c; the second of these transitions has not previously been observed in a 6H perovskite. Below 950 K, both α-Ba4Ta2O9 and γ-Ba4Ta2O9 hydrate to a greater extent than the corresponding phases of Ba4Nb2O9. Both hydrated forms show significant mixed protonic and oxide ionic conductivity, and electronic conductivity upon dehydration. © 2010, American Chemical Society
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    Two stage magnetic ordering and spin idle behavior of the coordination polymer Co-3(OH)(2)(C4O4)(2)center dot 3H(2)O determined using neutron diffraction
    (American Chemical Society, 2011-03-01) Mole, RA; Stride, JA; Henry, PF; Hoelzel, M; Senyshyn, A; Alberola, A; Garcia, CJG; Raithby, PR; Wood, PT
    We report the magnetic structure of two of the magnetically ordered phases of Co-3(OH)(2)(C4O4)2 center dot 3H(2)O, a coordination polymer that consists of a triangular framework decorated with anisotropic Co(II) ions. The combination of neutron diffraction experiments and magnetic susceptibility data allows us to identify one phase as displaying spin idle behavior, where only a fraction of the moments order at intermediate temperatures, while at the lowest temperatures the system orders fully. This novel magnetic behavior is discussed within the framework of a simple Hamiltonian and representational analysis and rationalizes this multiphase behavior by considering the combination of frustration and anisotropy. © 2011, American Chemical Society