05.01.2009 Synthesis, structural, magnetic and transport properties of layered perovskite-related titanates, niobates and tantalates of the type AnBnO3n+2, A′Ak−1BkO3k+1 and AmBm−1O3m
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F. Lichtenberg , a,  , 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 A′Ak−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 A′Ak−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 A′Ak−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
Fig. 1. Sketch of the idealized crystal structure of the j = 1, 2, 3 and ∞ members of the perovskite-related layered homologous series Aj+1BjO3j+1 (Ruddlesden–Popper phases) projected along the a- (or b-) axis. The layers along the ab-plane are formed by corner-shared BO6 octahedra. Along the c-axis the layers are j BO6 octahedra thick. Light and heavy drawing of the BO6 octahedra as well as filled and open circles indicates a height difference perpendicular to the drawing plane of about 2 Å, the B–O bond length and the half of the octahedron body diagonal. The compositional examples from the Sr–Ti–O system are Ti4+ (3d0) insulators.
Fig. 2. Sketch of the idealized type I crystal structure of the k = 2, 3, 4 and ∞ members of the perovskite-related layered homologous series A′Ak−1BkO3k+1 (Dion–Jacobson phases) projected along the a- (or b-) axis. The layers along the ab-plane are formed by corner-shared BO6 octahedra. Along the c-axis the layers are k BO6 octahedra thick. Perpendicular to the drawing plane there is a height difference between the BO6 octahedra and the A cations of about 2 Å, the B–O bond length and the half of the octahedron body diagonal. Compositional examples are taken from the Rb–(Na,Ca,La)–Nb–O system. The type I structure is realized for very large A′ cations like Rb+ or Cs+.
Fig. 3. Sketch of the idealized type II crystal structure of the k = 2, 3 and ∞ members of the perovskite-related layered homologous series A′Ak–1BkO3k+1 (Dion–Jacobson phases) projected along the b-axis. The layers along the ab-plane are formed by corner-shared BO6 octahedra. Along the c-axis the layers are k BO6 octahedra thick. Light and heavy drawing of the BO6 octahedra as well as filled and open circles indicates a height difference perpendicular to the drawing plane of about 2 Å, the B–O bond length and the half of the octahedron body diagonal. Compositional examples are taken from the K–(Ca,La)–Nb–O system. The type II structure is realized for large A′ cations like K+ or Ba2+.
Fig. 4. Sketch of the idealized crystal structure of the n = 2, 3 and 4 members of the perovskite-related layered homologous series AnBnO3n+2 = ABOx projected along the a-axis. In the formula ABOx the ideal oxygen content x = 3 + 2/n is specified. Within the layers the corner-shared BO6 octahedra extend zigzag-like along the b-direction and chain-like along the a-axis, see Fig. 6. Along the c-axis the layers are n BO6 octahedra thick. The n = 3 (II) member represents the ordered stacking sequence n = 2, 4, 2, 4, … Light and heavy drawing of the BO6 octahedra as well as filled and open circles indicate a height difference perpendicular to the drawing plane of about 2 Å, the B–O bond length and the half of the octahedron body diagonal. The compositional examples from the (La,Sr)–(Ta,Ti)–O system are Ta5+ (5d0)/Ti4+ (3d0) insulators which are, apart from the n = 3 (I) compound, ferroelectric.
Fig. 5. Sketch of the idealized crystal structure of the n = 4.5, 5, 6 and ∞ members of the perovskite-related layered homologous series AnBnO3n+2 = ABOx projected along the a-axis. In the formula ABOx the ideal oxygen content x = 3 + 2/n is specified. Within the layers the corner-shared BO6 octahedra extend zigzag-like along the b-direction and chain-like along the a-axis, see Fig. 6. Along the c-axis the layers are n BO6 octahedra thick. The n = 4.5 member represents the ordered stacking sequence n = 5, 4, 5, 4, … Light and heavy drawing of the BO6 octahedra as well as filled and open circles indicate a height difference perpendicular to the drawing plane of about 2 Å, the B–O bond length and the half of the octahedron body diagonal. Compositional examples are taken from the (La,Ca)–Ti–O system.
Fig. 6. Sketch of the idealized crystal structure of the perovskite-related layered homologous series AnBnO3n+2 = ABOx projected along the a- and b-axis using the n = 5 member as a representative example. In contrast to Fig. 4 and Fig. 5 the projection along the b-axis clearly shows the chain-like array of the corner-shared BO6 octahedra along the a-axis. Light and heavy drawing of the BO6 octahedra as well as filled and open circles indicate a height difference perpendicular to the drawing plane of about 2 Å, the B–O bond length and the half of the octahedron body diagonal.
Fig. 7. Sketch of the idealized crystal structure of LaTaO4 and La2RuO5 projected along the a-axis. Light and heavy drawing of the BO6 octahedra as well as filled and open circles indicates a height difference perpendicular to the drawing plane. LaTaO4 is an n = 2 member of AnBnO3n+2 = ABOx, see Fig. 4. La2RuO5 is structurally similar but its interlayer region is occupied by La3+ and O2− ions. To our knowledge La2RuO5 is the only compound with this type of structure. Nevertheless, it can be considered as an l = 2 member of the hypothetical series (LaO)22+(A′lBlO3l+2)2− = Al+2BlO3l+4. The structure of La2RuO5 was determined by Boullay et al. [21] as well as by Ebbinghaus [44].
Fig. 8. Special projections of the cubic perovskite structure ABO3 along the z-axis of a fixed x–y–z reference frame. These projections show how Fig. 9 and Fig. 10 come about. ap ≈ 4 Å is the lattice parameter of the cubic perovskite. The BO6 octahedra are accentuated in grey. (1) View of the cubic perovskite structure along its c-axis. (2) The cube stands on one of its corners. This picture results from (1) by tilting it  = 35.3° back around the x-axis.  is the angle between the space and face diagonal of the cube. (3) This picture results from (2) by turning it 30° to the left around the y-axis. This kind of view is used in Fig. 9 and Fig. 10. The height h of the BO6 octahedra along the [111] perovskite direction is given by  , i.e. h ≈ 2.3 Å for ap = 4 Å.
Fig. 9. Sketch of the idealized crystal structure of the m = 4, 5, 6, 7 and ∞ members of the hexagonal perovskite-related layered homologous series AmBm−1O3m projected along the a-axis. How this kind of view comes about is indicated in Fig. 8. Shown are the basis units which are m − 1 BO6 octahedra thick along the c-axis. If the vacant octahedron is taken into account, then the basis units are m octahedra thick. For m = ∞ the three-dimensional perovskite structure ABO3 is realized. The A cations are located at a height difference perpendicular to the drawing plane. Also shown is an example of an ordered intergrowth of two different types, namely m = 5 + 6 which has the formula A11B9O33. Compositional examples are presented from the (Sr,La)–Nb–O system.
Fig. 10. More detailed sketch of the idealized crystal structure of the hexagonal perovskite-related layered homologous series AmBm−1O3m projected along the a-axis using the m = 6 member as a representative example. How this kind of view comes about is shown in Fig. 8. The A cations are located at a height difference perpendicular to the drawing plane. ap ≈ 4 Å is the lattice parameter of the cubic perovskite structure. The bold letters A, B and C indicate the stacking sequence of AO3 sheets along the c-axis andccp (hcp) stands for the corresponding cubic (hexagonal) close-packed arrangement. There are r = 3 different stacking sequences along the c-axis which are separated by horizontal bars, see also Table 1. Therefore the length of the unit cell along the c-axis is given by  . 6 × h is the height of the basis unit consisting of m = 6 BO6 octahedra including the vacant octahedra.
Fig. 11. Features of the BO6 octahedra of k = 2, 3 and 4 members of A′Ak−1BkO3k+1 (Dion–Jacobson phases) and a j = 3 member of Aj+1BjO3j+1 (Ruddlesden–Popper phases). Sketched in the same way as in Fig. 1, Fig. 2 and Fig. 3 is the idealized structure of the layers which are k or j BO6 octahedra thick along the c-axis. For the sake of simplicity the A′ and A cations are omitted. Shown are the percentage values of the octahedra distortions after Eq. (1) in bold numbers, the number of different B–O bond lengths per octahedron in parenthesis, and the experimentally determined B site occupancies. They were calculated or taken from the crystallographic data presented in Refs. [118] and [182] (k = 2), [230] (j = 3), [37], [53] and [109] (lower k = 3), [76] (upper k = 3), and [185] (k = 4). If two adjacent BO6 octahedra along the a-axis are not equivalent, then two columns are used.
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