Structural Considerations and High Resolution Electron Microscopy Observations on LanTin-δO3n (n ≥ 4δ)

doi:10.1006/jssc.1994.1048

Journal of Solid State Chemistry

Volume 108, Issue 2, February 1994, Pages 314-335

https://doi.org/10.1006/jssc.1994.1048 Get rights and content

Abstract

Crystals of the compounds La_nTi_n-δO_3n are shown, by means of electron diffraction and high resolution electron microscopy, to form a homologous series of hexagonal or rhombohedral structures. The structures can be described as resulting from the perovskite structure by the periodic introduction of intrinsic stacking faults in a cubic stacking of close packed LaO₃ layers. The titanium ions, which have mixed valencies, occupy the octahedral interstices between the close packed LaO₃ layers, whereas the interstices in the fault planes remain vacant, making up for the nonstoichiometry. The corner-sharing TiO₆ octahedra are found to be cooperatively tilted about the normal to the layer planes. The "cut and projection" method is used to analyze the electron diffraction patterns and to determine the stacking sequences from geometrical features of the diffraction patterns. Six different stacking sequences are confirmed by direct imaging. High resolution images support the tilt scheme of the TiO₆ octahedra. The formation mechanism of the regular block sequences is discussed and the defects which allow this mechanism to operate are modeled and visualized.

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The 5H-type hexagonal perovskite Ba₅Nb₄O₁₅ was synthesized by solid-state reaction and the Pechini method and acceptor-doped phases Ba₅Nb_3.9M_0.1O_15-δ (M = Ti, Zr) by the former route. Monophasic doped and undoped material was obtained on final sintering at 1450 °C for 4 h with the M-doped samples leading to a reduced c-parameter. Scanning electron microscopy revealed a morphology of interlacing elongated plaques for all compositions, but with reduced grain size for the doped compositions. The highest density (95.1%) was observed for the Pechini-synthesized phase, whereas Zr doping had a deleterious effect on densification. The electrical transport properties were measured in H₂O- and D₂O-humidified air in the temperature range 300–550 °C by impedance spectroscopy. The presence of a H⁺/D⁺ isotope effect confirms the presence of proton conductivity in this structure type. Ba₅Nb₄O₁₅ synthesized by the Pechini method exhibits a conductivity of 8.4 × 10⁻⁶ S.cm⁻¹ at 550 °C in humidified air with an activation energy of 0.75 eV. The M-doped samples presented lower total conductivities, most probably due to the larger grain-boundary volume associated with their smaller grain size.
Synthesis, structural, magnetic and transport properties of layered perovskite-related titanates, niobates and tantalates of the type A<inf>n</inf>B<inf>n</inf>O<inf>3n+2</inf>, A′A<inf>k-1</inf>B<inf>k</inf>O<inf>3k+1</inf> and A<inf>m</inf>B<inf>m-1</inf>O<inf>3m</inf>
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This article represents a continuation of a paper on A_nB_nO_3n+2 = ABO_x compounds which was published in 2001 in this journal. This work reports also on oxides of the type A′A_k₋₁B_kO_3k+1 (Dion–Jacobson type phases) and hexagonal A_mB_m₋₁O_3m. The title materials have in common a layered perovskite-related structure whose layers are formed by corner-shared BO₆ octahedra. The three homologous series differ structurally in their orientation of the BO₆ octahedra with respect to the c-axis. This can be considered as a result from cutting the cubic perovskite ABO₃ structure along different directions followed by an insertion of additional oxygen, namely along the [100], [110] and [111] direction for A′A_k₋₁B_kO_3k+1, A_nB_nO_3n+2 and A_mB_m₋₁O_3m, respectively. The materials, with emphasis on electrical conductors, were prepared by floating zone melting and characterized by thermogravimetric analysis, X-ray powder diffraction and magnetic measurements. On crystals of five different compounds the resistivity was measured along the distinct crystallographic directions. Concerning A_nB_nO_3n+2 this work is focussed on two topics. The first are materials with paramagnetic rare earth ions at the A site or transition metal ions such as Fe³⁺ at the B site. The second are non-stoichiometric compounds. Furthermore, we discuss issues like occupational order at the B site, the proximity of some materials to the pyrochlore structure, potential magnetic ordering, and a possible coupling between magnetic and dielectric properties. The oxides A′A_k₋₁B_kO_3k+1 gained attention during a study of the reduced Ba–(Ca,La)–Nb–O system which lead to conducting Dion–Jacobson type phases without alkali metals. Concerning hexagonal A_mB_m₋₁O_3m the emphasis of this work are conducting niobates in the system Sr–Nb–O. The title materials have in common a quasi-2D (layered) structure and they are mainly known as insulators. In the case of electrical conductors, however, their transport properties cover a quasi-1D, quasi-2D and anisotropic 3D metallic behavior. Also temperature-driven metal-to-semiconductor transitions occur. A special feature of the quasi-1D metals of the type A_nB_nO_3n+2 is their compositional, structural and electronic proximity to non-conducting (anti)ferroelectrics. We speculate that these quasi-1D metals may have the potential to create new (high-T_c) superconductors, especially when they are viewed from the perspective of the excitonic type of superconductivity. Referring to literature and results from this work, a comprehensive overview on the title oxides and their properties is presented.
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2005, Journal of Solid State Chemistry
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Citation Excerpt :
They can be described using the different stacking sequences. First sequence can be generally expressed as (hhc…c) and the structures contain completely empty B-site between two face-sharing layers [4–21]. In the second case the positions in FSO are partially occupied and the stacking sequence can be written as (hc…c) [12, 22–25].
A new hexagonal perovskite-type oxide Ba₈Ta₄Ru_8/3Co_2/3O₂₄ was synthesized by the solid-state method at 1573 K and characterized by electron diffraction (ED), time-of-flight (TOF) neutron powder diffraction, and magnetic susceptibility. Structure parameters of Ba₈Ta₄Ru_8/3Co_2/3O₂₄ were refined by the Rietveld method from the TOF neutron powder diffraction data on the basis of space group P6₃/mcm and lattice parameters a=10.0075(1) Å and c=18.9248(2) Å as obtained from the ED data (Z=3). The crystal structure of Ba₈Ta₄Ru_8/3Co_2/3O₂₄ consists of 8-layered (cchc)₂ close-packed stacking of BaO₃ layers along the c-axis. Corner-shared octahedra are filled by Ta only and face-shared octahedra are statistically occupied by Ru, Co, and vacancies. Similar compounds Ba₈Ta₄Ru_8/3M_2/3O₂₄ with M=Ni and Zn were also prepared. Magnetic susceptibility measurements showed no magnetic ordering down to 5 K.

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Regular ArticleStructural Considerations and High Resolution Electron Microscopy Observations on LanTin-δO3n (n ≥ 4δ)

Abstract

Regular Article
Structural Considerations and High Resolution Electron Microscopy Observations on La_nTi_n-δO_3n (n ≥ 4δ)