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ABSTRACT: A series of layered oxides of nominal composition SrFe(1-x)Mn(x)O(2) (x = 0, 0.1, 0.2, 0.3) have been prepared by the reduction of three-dimensional perovskites SrFe(1-x)Mn(x)O(3-δ) with CaH(2) under mild temperature conditions of 583 K for 2 days. The samples with x = 0, 0.1, and 0.2 exhibit an infinite-layer crystal structure where all of the apical O atoms have been selectively removed upon reduction. A selected sample (x = 0.2) has been studied by neutron powder diffraction (NPD) and X-ray absorption spectroscopy. Both techniques indicate that Fe and Mn adopt a divalent oxidation state, although Fe(2+) ions are under tensile stress whereas Mn(2+) ions undergo compressive stress in the structure. The unit-cell parameters progressively evolve from a = 3.9932(4) Å and c = 3.4790(4) Å for x = 0 to a = 4.00861(15) Å and c = 3.46769(16) Å for x = 0.2; the cell volume presents an expansion across the series from V = 55.47(1) to 55.722(4) Å(3) for x = 0 and 0.2, respectively, because of the larger effective ionic radius of Mn(2+) versus Fe(2+) in four-fold coordination. Attempts to prepare Mn-rich compositions beyond x = 0.2 were unsuccessful. For SrFe(0.8)Mn(0.2)O(2), the magnetic properties indicate a strong magnetic coupling between Fe(2+) and Mn(2+) magnetic moments, with an antiferromagnetic temperature T(N) above room temperature, between 453 and 523 K, according to temperature-dependent NPD data. The NPD data include Bragg reflections of magnetic origin, accounted for with a propagation vector k = ((1)/(2), (1)/(2), (1)/(2)). A G-type antiferromagnetic structure was modeled with magnetic moments at the Fe/Mn position. The refined ordered magnetic moment at this position is 1.71(3) μ(B)/f.u. at 295 K. This is an extraordinary example where Mn(2+) and Fe(2+) ions are stabilized in a square-planar oxygen coordination within an infinite-layer structure. The layered SrFe(1-x)Mn(x)O(2) oxides are kinetically stable at room temperature, but in air at ~170 °C, they reoxidize and form the perovskites SrFe(1-x)Mn(x)O(3-δ). A cubic phase is obtained upon reoxidation of the layered compound, whereas the starting precursor SrFeO(2.875) (Sr(8)Fe(8)O(23)) was a tetragonal superstructure of perovskite.
Inorganic Chemistry 11/2011; 50(21):10929-36. · 4.60 Impact Factor
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ABSTRACT: The synthesis, crystal structure, and dielectric properties of four novel members of the family of double perovskites Pb(2)LnSbO(6) are described. The room-temperature crystal structures were refined from neutron powder diffraction (NPD) data in the monoclinic C2/c (No. 15) space group. They contain a completely ordered array of alternating LnO(6) and SbO(6) octahedra sharing corners, tilted in antiphase along the three pseudocubic axes, with a a(-)b(-)b(-) tilting scheme, which is very unusual in the crystallochemistry of perovskites. The lead atoms occupy highly asymmetric voids with 8-fold coordination due to the stereoactivity of the Pb(2+) electron lone-pair. Several trends are observed for the entire family of compounds upon heating. The Ln = Lu, Yb, and Er oxides display three successive phase transitions in a narrow temperature range, as shown by differential scanning calorimetry (DSC) data, while the Ln = Ho shows only two transitions. Different crystal structure evolutions have been found from temperature-dependent NPD and DSC, following the space-group sequence C2/c → P2(1)/n → R ̅3 → Fm ̅3m for Ln = Lu and Yb, the sequence C2/c → unknown → P2(1)/n → Fm ̅3m for Ln = Er, and C2/c → P2(1)/n → Fm ̅3m for Ln = Ho. The Ln/Sb long-range ordering is preserved across the consecutive phase transitions. Dielectric permittivity measurements indicate the presence of a paraelectric/antiferroelectric transition (associated with the last structural transition), as suggested by the negative Curie temperature from the Curie-Weiss fit of the reciprocal permittivity.
Inorganic Chemistry 06/2011; 50(12):5545-57. · 4.60 Impact Factor
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ABSTRACT: The synthesis, crystal structure, and dielectric properties of the novel double perovskite Pb(2)TmSbO(6) are described. The room-temperature crystal structure was determined by ab initio procedures from neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (SXRPD) data in the monoclinic C2/c (No. 15) space group. This double perovskite contains a completely ordered array of alternating TmO(6) and SbO(6) octahedra sharing corners, tilted in antiphase along the three pseudocubic axes, with an a(-)b(-)b(-) tilting scheme, which is very unusual in the crystallochemistry of perovskites. The lead atoms occupy a highly asymmetric void with 8-fold coordination due to the stereoactivity of the Pb(2+) lone electron pair. This compound presents three successive phase transitions in a narrow temperature range (at T1 = 385 K, T2 = 444 K, and T3 = 460 K in the heating run) as shown by differential scanning calorimetry (DSC) data. The crystal structure and temperature-dependent NPD follow the space-group sequence C2/c → P2(1)/n → R3 → Fm3m. This is a novel polymorph succession in the high-temperature evolution of perovskite-type oxides. The Tm/Sb long-range ordering is preserved across the consecutive phase transitions. Dielectric permittivity measurements indicate the presence of a paraelectric/antiferroelectric transition (associated with the last structural transition), as suggested by the negative Curie temperature obtained from the Curie-Weiss fit of the reciprocal permittivity.
Journal of the American Chemical Society 10/2010; 132(41):14470-80. · 9.91 Impact Factor
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Berichte der deutschen chemischen Gesellschaft 03/2007; 2007(14):1972 - 1979. · 2.94 Impact Factor
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Berichte der deutschen chemischen Gesellschaft 06/2005; 2005(13):2600 - 2606. · 2.94 Impact Factor
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ABSTRACT: A2NiMoO6 (A = Sr, Ba) perovskites have been prepared in polycrystalline form by thermal treatment, in air, of previously decomposed citrate precursors. These materials have been studied by X-ray (XRD) and neutron powder diffraction (NPD) data. At room temperature, the crystal structure is tetragonal, space group I4/m, for Sr2NiMoO6 and for Ba2NiMoO6 the crystal structure is cubic, space group Fmm. Both perovskites contain divalent Ni cations. The low temperature antiferromagnetic ordering has been followed from sequential neutron diffraction data. Peaks of magnetic origin appear in the NPD patterns below temperatures of TN = 81.2 K and TN = 64.3 K for the Sr and Ba compounds, respectively. The magnetic structures are defined by a propagation vector k = (1/2,0,1/2) for Sr2NiMoO6 and k = (1/2,1/2,1/2) for Ba2NiMoO6. The refined magnitude of the Ni magnetic moments also suggests a divalent oxidation state with S = 1. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)
Berichte der deutschen chemischen Gesellschaft 07/2003; 2003(15):2839 - 2844. · 2.94 Impact Factor
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ABSTRACT: Ba2CoBO6 (B = Mo, W) perovskites have been prepared in polycrystalline form by thermal treatment, in air, of previously decomposed citrate precursors. These materials have been characterized by X-ray (XRD) and neutron powder diffraction (NPD) data and magnetization measurements. At room temperature, the crystal structure is cubic, space group Fmm, with a = 8.08623(3) and 8.10799(3) Å for the Mo and W compounds, respectively. The crystal contains alternating CoO6 and Mo(W)O6 octahedra, with an almost negligible anti-site disordering. The low temperature antiferromagnetic ordering has been followed from sequential neutron diffraction data. Peaks of magnetic origin appear at the NPD patterns below temperatures of TN = 27 K and TN = 19 K for the Mo and W compounds, respectively. The magnetic structures are both defined by a propagation vector k = (1/2, 1/2, 1/2). They can be described as an array of ferromagnetic layers of Co moments, perpendicular to the [111] directions, coupled antiferromagnetically. The refined magnitude of the Co2+ magnetic moments suggests a high-spin electronic configuration (S = 3/2); covalence effects account for the slight reduction in the low-temperature ordered moments, although this effect is minimized for the W compound. Structural and magnetic features are discussed in relation to the chemical nature of the species present in the solids. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)
Berichte der deutschen chemischen Gesellschaft 08/2002; 2002(9):2463 - 2469. · 2.94 Impact Factor
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ABSTRACT: RNiO3 nickelates have been prepared under high oxygen pressure (R = Sm, Eu, Gd) or high hydrostatic pressure (R = Dy, Ho, Y) in the presence of KClO4. The samples have been investigated at room temperature (RT) by synchrotron X-ray powder diffraction to follow the evolution of the crystal structures and microstructures along the series. The distortion of the orthorhombic (space group Pbnm) perovskite progressively increases along the series, leading for the smallest Ho3+ and Y3+ cations to a subtle monoclinic distortion (space group P21/n) which implies the splitting of the Ni positions in the crystal. This symmetry was confirmed by neutron powder diffraction; the crystal structures for RHo and Y were refined simultaneously from RT synchrotron and neutron powder diffraction data. In both perovskites the oxygen octahedra around Ni1 and Ni2 positions are significantly distorted, suggesting the manifestation of Jahn−Teller effect, which is almost absent in the nickelates of lighter rare earths. The very distinct mean Ni−O bond distances observed for Ni1 and Ni2 atoms at RT, in the insulating regime, suggest the presence of a charge disproportionation effect, considered as driving force for the splitting of the Ni positions. The metal−insulator (MI) transitions for RNiO3 (R = Gd, Dy, Ho, Y), above room temperature, have been characterized by DSC. The transition temperatures for Gd, Dy, Ho, and Y oxides in the heating runs are 510.7, 563.9, 572.7, and 581.9 K, respectively. The increasing rate of TMI for Dy, Ho, and Y materials is lower than that expected from the variation of TMI for the larger rare earth perovskites. This is probably related to the subtle monoclinic distortion found for Ho and Y nickelates. The high-resolution synchrotron X-ray powder patterns have revealed changes in the microstructure along the series. Powder patterns for orthorhombic RNiO3 (R = Sm, Eu, Gd, Dy) display asymmetric tails for some reflections which are due to structural mistakes such as stacking faults or regular intergrowths. These mistakes are not present in monoclinic RNiO3 (RHo, Y) nickelates.
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