Control of Copper and Iron Oxidation States in Some Triple- and Double-Perovskite Oxides

摘要

This thesis consists of eight publications and a summary of the obtained experimental results, reviewed together with the most essential literature related to the topic. The work deals with ways of tuning and analyzing the transition metal oxidation states in the superconductive Cu(Ba,Sr)2(Yb,Ca)Cu2O6+z triple perovskite and the BaRE(Fe,Cu)O5+ double perovskite, which has aroused interest as a structure potentially exhibiting both superconductivity and magnetoresistivity. These triple- and double-perovskite oxides are compared with regard to their charge distribution at different cation and oxygen stoichiometries. Complementary methods of analysis, including both chemical and physical techniques, are utilized for the determination of the charge distribution in these layered oxide structures with several transition metal ions. Multivariate data analysis is introduced as a novel way for examining the structure-property correlations of complex oxide structures based on the crystallographic data obtained by neutron diffraction. The Ca(II)-for-RE(III) substituted and oxygen-doped Cu(Ba0.8Sr0.2)2(Yb1-xCax)Cu2O6+z triple perovskites (0 x 0.35 with z 0 and 0 < z < 1 with x = 0) are described for their fine structures and the charge distribution over the unit cell is determined by two different methods. Bond-valence-sum (BVS) calculations based on structural data obtained by neutron diffraction are shown to be a convenient means to study small gradual changes in the fine structure and the charge distribution. O K-edge and Cu L2,3-edge XANES results independently showed same trends in the charge distribution as did the BVS calculations, i.e. stronger oxidation of the superconductive part of the structure by Ca(II)-for-RE(III) substitution than by oxygen doping. The oxygen stoichiometry of the BaRE(Fe0.5Cu0.5)2O5+ double perovskite is shown to depend on the size of the RE3+ ion and on the oxygen partial pressure during the final heat treatment of the synthesis. Under ambient pressure, the excess oxygen atom can be inserted only in the structures with largest REs (Nd and Pr) whereas high-pressure treatment (p = 5 GPa) was found to stabilize also in the structures with smaller REs. The charge distribution between copper and iron is studied by means of coulometric titration and 57Fe Mossbauer spectroscopy. In the normal-pressure oxygenated samples only Fe(III) is oxidized to higher oxidation states, but high-pressure heat treatment enables oxidation of Cu(II) to Cu(III) as well. The excess oxygen can be removed from all compounds by deoxidative post-annealing performed in a controlled way in a thermobalance.