Browsing by Author "Caspi, EN"

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    Crystallographic and magnetic properties of CaLaMnMoO6 double perovskite
    (Elsevier, 2006-07-01) Lin, Q; Greenblatt, M; Caspi, EN; Avdeev, M
    Powder neutron diffraction studies show that CaLaMnMoO6 double perovskite crystallizes in monoclinic P21/n, with , , Å and . Mn and Mo occupy the 2c and 2d positions, respectively, with 6.0(4)% Mn/Mo anti-site mixing. Temperature-dependent magnetic susceptibility measurements reveal that CaLaMnMoO6 is ferrimagnetic, with TN=92(3) K, below which large magnetic frustration is detected. The zero-field magnetic moment measured at 5 K is about 1.2 μB, comparable to that of ALaMnMoO6 (A=Ba and Sr), but much lower than expected for antiparallel ordering of formally Mn2+ (d5) and Mo5+ (d1). Moreover, no long-range magnetic ordering is observed in neutron diffraction data down to 4 K. The magnetic frustration is discussed in the framework of nearest-neighbors next-nearest-neighbors magnetic frustration. © 2006 Elsevier Inc.
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    The effect of non-magnetic dilution of the Tb sublattice in TbCo3B2
    (Institute of Physics, 2010-01-20) Wolfson, EJ; Caspi, EN; Ettedgui, H; Shaked, H; Avdeev, M
    Solid solutions of Tb1-xYxCo3B2 (x = 0.05, 0.1, 0.25, 0.4 and 0.5) were studied by neutron powder diffraction, x-ray diffraction, AC susceptibility and SQUID magnetization measurements. Their magnetic and crystallographic properties were deduced and examined together with those previously published for the end compounds (x = 0, 1). These solid solutions have hexagonal symmetry and are paramagnetic at RT, and undergo a magnetic ordering transition of the Co sublattice, with the magnetic moments along the hexagonal axis, at T-Co similar to 150(15) K, independent of Y concentration. A second magnetic ordering transition of the Tb sublattice T-Tb <= 30 K accompanied by the rotation of the magnetic moments towards the basal plane, was observed for solid solutions with Y concentration x <= 0.25. This transition was also found to be accompanied by a crystallographic symmetry decrease. Unexpectedly, neutron powder diffraction showed that the magnitude of the ordered magnetic moment of the Tb ion decreases with Tb concentration. © 2010, Institute of Physics
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    Magnetic and crystallographic properties of TbCo4B
    (Elsevier, 2006-11-15) Caspi, EN; Dubman, M; Ettedgui, H; Shaked, H; Melamud, M; Keller, L; Avdeev, M
    The compound TbCo4B was studied using measurements of AC magnetic susceptibility, magnetization and neutron powder diffraction. It is found that this compound orders at TC≈450 K. Each of the two Tb and two Co sublattices orders ferromagnetically. The coupling is ferromagnetic within each pair and ferrimagnetic between pairs. A compensation point at ∼300 K and a spin reorientation between 400 and 430 K were found. © 2006 Elsevier B.V.
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    Magnetic and crystallographic study of Tb0.75Y0.25Co3B2
    (Elsevier, 2008-03-02) Wolfson, EJ; Caspi, EN; Ettedgui, H; Shaked, H; Avdeev, M
    Tb0.75Y0.25Co3B2 was studied as a function of temperature by neutron powder diffraction, ac susceptibility and SQUID magnetization measurements. The solid solution, which is of hexagonal symmetry and is paramagnetic at 300 K, undergoes a magnetic Co-Co ordering transition at similar to 150 K, and a second magnetic Tb-Tb ordering transition at similar to 17 K. The latter induces a spin-reorientation transition, in which the magnetic axis rotates from the c-axis toward the basal plane. The component of the magnetic axis, which is perpendicular to c, leads to a crystal symmetry reduction from hexagonal to monoclinic. The observed magnitude of the magnetic moment of the Tb ion is 1.5 mu(B), unusually small relative to the free ion and parent compound (TbCo3B2) values. These magnetic and crystal properties are discussed and compared with what was previously published for the parent compound. © 2008, Elsevier Ltd.
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    On the polyhedral volume ratios VA/VB in perovskites ABX³
    (International Union of Crystallography, 2007-06) Avdeev, M; Caspi, EN; Yakovlev, SO
    This paper presents analytical expressions for the calculation of ratios of cation coordination polyhedra volumes (VA/VB) for perovskites ABX³ of the Stokes–Howard diagram directly from atomic coordinates. We show the advantages of quantifying perovskite structure distortion with polyhedral volume ratios rather than with tilting angles, and discuss why space groups with multiple crystallographically inequivalent A or B sites (I4/mmm, Immm, P42/nmcetc.) are much less common than those with a single A and B site (I4/mcm, R c, Pnmaetc.). Analysis of crystallographic data for approximately 1300 perovskite structures of oxides, halides and chalcogenides from the Inorganic Crystal Structure Database revealed that the most highly distorted perovskites belong to the space group Pnma and formally lower-symmetry perovskites (I2/m, I2/a) are less distorted geometrically. Critical values of the VA/VB ratios for the most common phase transitions Pnma↔I4/mcm and Pnma↔R c are estimated to be 4.85 with the possible intermediate space group Imma stable in the very narrow range of VA/VB 4.8–4.9. Transitions to post-perovskite CaIrO3-type structures may be expected for VA/VB < 3.8. © 2007, International Union of Crystallography