Daniel Niermann

Publications (3)11.89 Total impact

• Article: Dielectric properties of charge ordered LuFe2O4 revisited: The apparent influence of contacts

  Daniel Niermann, Florian Waschkowski, Joost de Groot, Manuel Angst, Joachim Hemberger
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  ABSTRACT: We show results of broadband dielectric measurements on the charge ordered, proposed to be mul- tiferroic material LuFe2O4. The temperature and frequency dependence of the complex permittivity as investigated for temperatures above and below the charge-oder transition near T_CO ~ 320 K and for frequencies up to 1 GHz can be well described by a standard equivalent-circuit model considering Maxwell-Wagner-type contacts and hopping induced AC-conductivity. No pronounced contribution of intrinsic dipolar polarization could be found and thus the ferroelectric character of the charge order in LuFe2O4 has to be questioned.
  03/2012;
• Source

  Available from: Artem Abakumov

  Article: Antiferroelectric (Pb,Bi)1-xFe1+xO3-y Perovskites Modulated by Crystallographic Shear Planes

  Artem M Abakumov, Dmitry Batuk, Joke Hadermann, Marina G Rozova, Denis V Sheptyakov, Alexander A Tsirlin, Daniel Niermann, Florian Waschkowski, Joachim Hemberger, Gustaaf, Van Tendeloo, Evgeny V Antipov
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  ABSTRACT: We demonstrate for the first time a possibility to vary the anion content in perovskites over a wide range through a long-range-ordered arrangement of crystallographic shear (CS) planes. Anion-deficient perovskites (Pb,Bi) 1-x Fe 1þx O 3-y with incommensurately modulated structures were pre-pared as single phases in the compositional range from Pb 0.857 Bi 0.094 Fe 1.049 O 2.572 to Pb 0.409 Bi 0.567-Fe 1.025 O 2.796 . Using a combination of electron diffraction and high-resolution scanning transmission electron microscopy, we constructed a superspace model describing a periodic arrangement of the CS planes. The model was verified by refinement of the Pb 0.64 Bi 0.32 Fe 1.04 O 2.675 crystal structure from neutron powder diffraction data ((3 þ 1)D S.G. X2/m(R0γ), X = [1/2,1/2,1/2,1/2], a = 3.9082(1) Å , b = 3.90333(8) Å , c = 4.0900(1) Å , β = 91.936(2)°, q = 0.05013(4)a* þ 0.09170(3)c* at T = 700 K, R P = 0.036, R wP = 0.048). The (Pb,Bi) 1-x Fe 1þx O 3-y structures consist of perovskite blocks separated by CS planes confined to nearly the (509) p perovskite plane. Along the CS planes, the perovskite blocks are shifted with respect to each other over the 1/2[110] p vector that transforms the corner-sharing connectivity of the FeO 6 octahedra in the perovskite framework to an edge-sharing connectivity of the FeO 5 pyramids at the CS plane, thus reducing the oxygen content. Variation of the chemical composition in the (Pb,Bi) 1-x Fe 1þx O 3-y series occurs mainly because of a changing thickness of the perovskite block between the interfaces, that can be expressed through the components of the q vector as Pb 6γþ2R Bi 1-7γ-R Fe 1þγ-R O 3-3γ-R . The Pb, Bi, and Fe atoms are subjected to strong displace-ments occurring in antiparallel directions on both sides of the perovskite blocks, resulting in an antiferroelectric-type structure. This is corroborated by the temperature-, frequency-, and field-dependent complex permittivity measurements. Pb 0.64 Bi 0.32 Fe 1.04 O 2.675 demonstrates a remark-ably high resistivity >0.1 T Ω cm at room temperature and orders antiferromagnetically below T N = 608(10) K.
  Chemistry of Materials 01/2011; 23:255-265. · 7.29 Impact Factor
• Source

  Available from: Artem Abakumov

  Article: Slicing the perovskite structure with crystallographic shear planes: the A(n)B(n)O(3n-2) homologous series.

  Artem M Abakumov, Joke Hadermann, Maria Batuk, Hans D'Hondt, Oleg A Tyablikov, Marina G Rozova, Konstantin V Pokholok, Dmitry S Filimonov, Denis V Sheptyakov, Alexander A Tsirlin, Daniel Niermann, Joachim Hemberger, Gustaaf Van Tendeloo, Evgeny V Antipov
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  ABSTRACT: A new A(n)B(n)O(3n-2) homologous series of anion-deficient perovskites has been evidenced by preparation of the members with n = 5 (Pb(2.9)Ba(2.1)Fe(4)TiO(13)) and n = 6 (Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16)) in a single phase form. The crystal structures of these compounds were determined using a combination of transmission electron microscopy and X-ray and neutron powder diffraction (S.G. Ammm, a = 5.74313(7), b = 3.98402(4), c = 26.8378(4) Å, R(I) = 0.035, R(P) = 0.042 for Pb(2.9)Ba(2.1)Fe(4)TiO(13) and S.G. Imma, a = 5.7199(1), b = 3.97066(7), c = 32.5245(8) Å, R(I) = 0.032, R(P) = 0.037 for Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16)). The crystal structures of the A(n)B(n)O(3n-2) homologues are formed by slicing the perovskite structure with (101)(p) crystallographic shear (CS) planes. The shear planes remove a layer of oxygen atoms and displace the perovskite blocks with respect to each other by the 1/2[110](p) vector. The CS planes introduce edge-sharing connections of the transition metal-oxygen polyhedra at the interface between the perovskite blocks. This results in intrinsically frustrated magnetic couplings between the perovskite blocks due to a competition of the exchange interactions between the edge- and the corner-sharing metal-oxygen polyhedra. Despite the magnetic frustration, neutron powder diffraction and Mössbauer spectroscopy reveal that Pb(2.9)Ba(2.1)Fe(4)TiO(13) and Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16) are antiferromagnetically ordered below T(N) = 407 and 343 K, respectively. The Pb(2.9)Ba(2.1)Fe(4)TiO(13) and Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16) compounds are in a paraelectric state in the 5-300 K temperature range.
  Inorganic Chemistry 10/2010; 49(20):9508-16. · 4.60 Impact Factor