Fine-Tuning the Physical Properties of Perovskite Related La-Ti-Oxides by Merely Altering the Oxygen Stoichiometry

摘要

The isostructural oxides LaTiO_(0.35), SrNbO_(3.5) and CaNbO_(3.5) adopt layered structures made up of perovskite related slabs intermitted by layers of non-linking BO_6-octahedra (B=Ti, Nb). In these phases the transition metals Ti and Nb are in their highest oxidation states +IV and +V, respectively. They are insulators, in the case of the Ti phase a ferroelectric insulator with the highest known T_c (above 1700°C). By changing the oxygen Stoichiometry, i.e. by controlled reduction or re-oxidation processes in the range of 2ABO3.S <=> 2ABO_3 + 1/2 O_2 (A=La or Sr, B=Ti or Nb) mixed valence phases are obtained. Accordingly, the physical properties of the different phases alter from insulating to semiconducting or to conducting. Detailed studies on the structural changes reveal, however, that the sublattice of the metal cations is basically conserved. In principle the reduction corresponds to a condensation of the perovskite layers leading to intermediate phases such as the semiconducting LaTiO_(3.4), in which five TiO_6-octahedra thick perovskite slabs constitute the structural framework and 20% of the Ti~(+Ⅳ) cations are reduced to Ti~(+Ⅲ). Conductivity measurements using single crystals of the corresponding Ti and Nb phases reveal that these mixed valence oxides must be considered as one-dimensional conductors. The fully reduced phases La~(+Ⅲ)Ti~(+Ⅲ)O_3 and Sr~(+Ⅱ)Nb~(+Ⅳ)O_3 adopt distorted perovskite structures. Extensive high resolution electron microscopic and light microscopic investigations have been carried out in order to characterize the structural mechanism of the reversible, highly topotactic reduction and re-oxidation processes. Thermogravimetric measurements in reducing and oxidizing atmospheres have been performed for the identification of the temperature ranges wherein the decisive mass changes take place. The results of the described experiments support that the properties of such metal oxides can be finely and reversibly tuned by merely changing the oxygen stoichiometries.