Oxide crystals

Perovskite-related oxides display many exciting electronic and structural properties. Sr₂RuO₄, for example, shows an interesting story with many aspects from the physical and materials research point of view [1]. It has the same layered K₂NiF₄ structure as (La,Ba)₂CuO₄, the parent compound of the high-T_c superconductors, discovered by J.G. Bednorz and K.A. Müller in 1986. By floating zone melting, single crystals of Sr₂RuO₄ could be grown and investigated in detail. They revealed a highly metallic behavior along the layers and it could be shown that Sr₂RuO₄ is potentially useful as a metallic substrate for the epitaxial growth of high-T_c superconducting thin films [2]. Furthermore, it was discovered that these Sr₂RuO₄ crystals are superconductors having a critical temperature T_c ~ 1 K [3]. Up to now Sr₂RuO₄ represents the only Cu-free superconductor isostructural to (La,Ba)₂CuO₄. Investigations of the properties of Sr₂RuO₄ are of interest to understand the mechanisms of superconductivity in these compounds. Moreover, the superconductivity in Sr₂RuO₄ was found to be of an unconventional spin-triplett type [4,5]. Therefore, Sr₂RuO₄ is even today an active field of research [4,5].

In many cases, the physical and structural properties of perovskite-related materials can be tuned by substitution or by modifying their oxygen content. A notable example for such materials is given by the LaTiO_x system. By varying the oxygen content x from 3.00 to 3.50, layered structures as well as three-dimensional ones can be obtained and ferroelectric, antiferromagnetic, semiconducting, metallic and quasi-1D metallic behavior is observed in single-phase materials [6,7]. Furthermore, temperature-driven metal-semiconductor transitions occur [6,7]. The LaTiO_x compounds belong to a homologous series A_nB_nO_3n+2 with a perovskite-related layered crystal structure. The thickness of the layers increases with increasing n and for n = ∞ the three-dimensional perovskite structure is realized. Our current research is focused on electrical conducting niobates and titanates of the A_nB_nO_3n+2 type [7-13]. These materials represent a new group of quasi-1D metals. Many of these compounds can be prepared as single crystals by the floating zone melting technique.

Finally, it should be mentioned that in the system LaTiO_x , which initiated the research on the titanates and niobates of the A_nB_nO_3n+2 type, the both end members gained recently a revived attention. In the case of the antiferromagnetic Mott insulator LaTiO₃ (n = ∞) there are distinct indications for the presence of an orbital ordering at temperatures below T_N ~ 150 K [14-16]. The ferroelectric insulator LaTiO_3.5 (n = 4) was studied in form of thin films and devices in capacitor configuration were realized which permit a switching between two different voltage states [17].