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

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.