Synthesis of perovskite-related layered AnBnO3n+2 = ABOx type niobates and titanates and study of their structural, electric and magnetic properties

ABO(X) niobates and titanates belonging to the homologous series A(n)B(n)O(3n+2) are a special group of perovskite-related layered materials. These oxides comprise the highest-T-c ferroelectrics ics such as CaNbO3.50 and LaTiO3.50, as well as thermodynamically stable bulk compounds involving well-ordered stacking sequences of layers with different thickness such as SrNbO3.45. An extensive overview on many ABO(X) compositions of the A(n)B(n)O(3n+2). family and its properties is presented, The crystal structure type is given by n and can be tuned by adjusting the oxygen content X. The charge carrier concentration of the electrical conducting oxides can be varied by appropriate substitutions at the A or B site. To investigate the properties of these systems, more than 150 different compositions were prepared. Most of them were grown by floating zone melting, of which many were fabricated as single crystals with precise control of the oxygen content X. For these crystalline compounds, the synthesis, structural, electric and magnetic features arc discussed. Attempts to prepare series members beyond the known structure types n=4, 4.33, 4.5, 5 and 6 were not successful. For some of the known structures types n, however, pronounced non-stoichiometric homogeneity ranges with respect to the oxygen content X and cation ratio A/B were found. Thus, these systems offer many possibilities to vary the compositional, structural, chemical and physical properties. Further, measurements of the resistivity as a function of temperature T are reported for crystals of the n=4 type Sr0.8La0.2NbO3.50, n=4.5 type Sr0.96Ba0.04NbO3.45 and n=5 types Sr1-yLayNbO3.41 (Y=0, 0.035, 0.1), Sr0.95NbO3.37, CaNbO3.41 and LaTiO3.41. Those measurements, which were performed in the temperature range 4 K less than or equal toT less than or equal to 290 K and along the a-, b- and c-axis, revealed a highly anisotropic conductivity and intricate behavior. In parts of the temperature range, these materials are quasi-ID metals which display temperature-driven metal-semiconductor transitions at lower temperatures. The niobates and titanates investigated represent a new group of quasi-ID metals which are in compositional, structural and electronical proximity to nonconducting layered (anti)ferroelectrics. Furthermore, measurements of the magnetic susceptibility as a function of temperature are reported for many compounds. As a typical property at elevated temperatures, it was observed that the magnetic susceptibility rises with increasing temperature. (C) 2001 Published by Elsevier Science Ltd.