3.2 Experiments on a Mach3 diffractometer
4.1.2 Low dimensional properties
The oxide materials of the series of layered perovskite-related compound AnBnO3n+2 show a large variety of interesting electrical and magnetic properties. For example in
4.1. CRYSTAL CHEMICAL AND PHYSICAL PROPERTIES 33
Figure 4.1: Idealized, non-distorted perovskite-related structures. (a) Crystal structures of then = 5, 6 and ∞ members of the series AnBnO3n+2 in the projection alonga. For each structure the general formula and an example with reference is given. (b) Schematic projection of the zig-zag and chain like connections of the octahedra along b and a, respectively. The black and red colors indicate the differences in height along the direction of the projections.
the strontium niobates the electric character changes completely by a slight change of the oxygen content. While Sr2Nb2O7 (n = 4) is ferroelectric, with one of the highest known ferroelectric transition temperature of TC = 1613 K (Nanamatsu et al., 1975), Sr5Nb5O17 (n = 5) is a quasi-1D metal. It shows highly anisotropic electrical properties with a metallic conductivity along theaaxis (Kuntscher et al., 2002). This metallic character can be related to the chain-like arrangement of the NbO6 octahedra along a along with a partially filled 3d band on the Nb atoms. As a consequence of the layered structure the resistivity is much larger perpendicular to the chains, because the gaps between the slabs hamper the electrical transport.
The varying electrical properties have been studied by three different experi-mental methods, namely DC-resistivity measurement, optical spectroscopy in the infrared range and angle-resolved photoemission spectroscopy (Kuntscher et al., 2004). All methods pointed to the same result, that the difference between the
34 CHAPTER 4. PEROVSKITE-RELATED COMPOUNDS AnBnO3n+2
n = 4 and 5 members can be found in the existence of a central chain of almost undistorted NbO6 octahedra in Sr5Nb5O17. Band structure calculations in the local density approach showed, that a predominant contribution to the density-of-states near the Fermi-energy is attributed to Nb atoms of the least distorted octahedra in the center of the slabs. Nb in octahedra closer to the borders of the slabs contribute significantly less, as these octahedra are more distorted. Such chains of nearly undis-torted octahedra lack in Sr2Nb2O7. Thus the quite large distortion of all octahedra prevent a metallic behavior in the case of Sr2Nb2O7. An alternative explanation can be given by considering the valences of the ions in both compounds. For Sr2Nb2O7 charge neutrality is reached with valences Sr2+, Nb5+ and O2−and an empty 3d band of the Nb atoms. On the other hand charge neutrality for Sr5Nb5O17 leads to the same valences Sr2+ and O2−, but requires Nb4.8+ in average. Therefore, the 3d band consists of 0.2 electrons/Nb atom, which are responsible for the metallic behavior.
Similar differences in the electrical behavior are also found for other niobates and titanates (Kuntscher et al., 2003; 2004; Lichtenberg et al., 2001; 2008).
Similar to the electrical properties the magnetic behavior shows a significant dependence on the crystal structure type. In this series magnetic superstructures can be formed, if the cation sites are occupied by ions having a magnetic moment.
For titanates Ln3+TiOx (Ln = Cr, Pr, Nd, Sm or Eu) the magnetic properties are exclusively determined by the magnetic moments of the Ln3+ ions. In contrast, the magnetic properties of compounds, doped with iron at the B cation sites, are predominated by the iron (Lichtenberg et al., 2008). Examples for a significant change of the magnetic interaction, as a result of a slight variation of the iron content, are the two Lan(Ti1−xFex)nO3n+2 compounds with x = 0.20 for n = 5 and x = 0.33 for n = 6 (Chapter 8). The n = 5 compound displays almost an Curie-Weiss behavior without any clear indications for a magnetic ordering. Whereas the magnetic behavior with temperature of the n = 6 compound is markedly different from that of the n = 5 compound. It shows a strong ferromagnetic interaction at high temperatures with a change to antiferromagnetic interaction at about 280 K (compare figure 8.3). But both compounds remain paramagnetic at all temperatures and no long range order is developed.
The differences in the magnetic correlations are explained in the distinct lower concentration of magnetic Fe3+ ions of the n = 5 compound compared to then= 6 type. Furthermore, two central layers, occupied predominately by Fe ions create a quasi-2D magnetic lattice in the case of La6(Ti0.67Fe0.23)6O20. Within the magnetic lattice ferromagnetic correlated clusters of in average NC= 51.5 Fe ions are formed at
4.2. OVERLOOKED SUPERSTRUCTURE REFLECTIONS 35
low temperature. Within each slab the clusters are linked by only a few inter-cluster connections, therefore their interaction is paramagnetic. Between neighboring slabs the interactions of the clusters are antiferromagnetic and enhanced at least by a factor of NC. At higher temperatures the ferromagnetic interaction in the slabs dominates over the antiferromagnetic interaction between neighboring slabs and the clusters start to dissolve. Therefore the crossover from a ferromagnetic to an antiferromagnetic coupling appears. For La5(Ti0,77Fe0.23)5O17only 1D chains of sites along aexist in the centered layers of the slabs, which are highly occupied by iron.
Moreover, these chains are interrupted by non-magnetic Ti4+ ions and no significant magnetic interaction appears (Chapter 8).
A generalization of these results indicates the possibility of a magnetic super-structure with long range order in the n = 7 compound with the nominal com-position La7Ti4Fe3O23. This compound would have an even higher concentration of magnetic Fe3+ ions in the inner three layers of each slab. Unfortunately the synthesis of this n = 7 compound was not successful yet. On the other hand, the magnetic interaction might be increased by the consideration of different magnetic ions, e.g. Mn3+, on the B cation sites. As a result a (anti-)ferromagnetic ordering might appear.