05.01.2009
РОССИЙСКАЯ АКАДЕМИЯ НАУК

УРАЛЬСКОЕ ОТДЕЛЕНИЕ

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05.01.2009

Synthesis, structural, magnetic and transport properties of layered perovskite-related titanates, niobates and tantalates of the type AnBnO3n+2, AAk−1BkO3k+1 and AmBm−1O3m




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F. LichtenbergCorresponding Author Contact Information, a, E-mail The Corresponding Author, A. Herrnbergera and K. Wiedenmanna






aExperimentalphysik VI, Center for Electronic Correlations and Magnetism (EKM), Institute of Physics, Augsburg University, D-86135 Augsburg, Germany






Available online 26 October 2008.






Abstract


This article represents a continuation of a paper on AnBnO3n+2 = ABOx compounds which was published in 2001 in this journal. This work reports also on oxides of the type AAk−1BkO3k+1 (Dion–Jacobson type phases) and hexagonal AmBm−1O3m. 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 AAk−1BkO3k+1, AnBnO3n+2 and AmBm−1O3m, 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 AnBnO3n+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 AAk−1BkO3k+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 AmBm−1O3m 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 AnBnO3n+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-Tc) 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.





Keywords: Titanates; Niobates; Tantalates; Perovskite-related crystal structures; Layered materials; Crystal growth; Floating zone melting; Low-dimensional conductors; Resistivity; Magnetic susceptibility; Magnetic ordering; Ferroelectrics; Antiferroelectrics; Superconductivity; Excitonic superconductivity





Article Outline



1. Introduction and overview

1.1. Preliminaries and general survey
1.2. AnBnO3n+2 = ABOx
1.3. Dion–Jacobson type phases AAk−1BkO3k+1
1.4. Hexagonal AmBm−1O3m

2. Experimental

2.1. Sample preparation
2.2. Powder X-ray diffraction
2.3. Magnetic measurements
2.4. Resistivity measurements

3. Results and discussion: sample preparation
4. Results and discussion: Dion–Jacobson phases AAk−1BkO3k+1 without alkali metals
5. Results and discussion: AnBnO3n+2 = ABOx

5.1. Structural properties

5.1.1. AnBnO3n+2 and pyrochlore
5.1.2. Non-stoichiometric compounds
5.1.3. Occupational order at the B site

5.2. Resistivity
5.3. Magnetic susceptibility

5.3.1. Data evaluation in the case of Curie–Weiss behavior
5.3.2. Results and discussion in the case of Curie–Weiss behavior
5.3.3. The n = 5 titanates LnTiO3.4 with Ln = La, Ce, Pr, Nd or Sm

5.4. Speculations about the potential for (high-Tc) superconductivity

5.4.1. View from the perspective of excitonic superconductivity

Electronic polarizability
Ionic (or lattice) polarizability
Dipolar polarizability
Interfacial polarizability

5.4.2. The system Na–W–O

6. Results and discussion: hexagonal AmBm−1O3m
7. Summary

7.1. AnBnO3n+2 = ABOx
7.2. Dion–Jacobson type phases A' Ak−1BkO3k+1 without alkali metals
7.3. Hexagonal AmBm−1O3m

Acknowledgements
Tables of powder XRD data of some compounds
References






















































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