Perovskite oxides ABO3 exhibit a wide range of properties which are of interest both in basic research and in applications. This is largely due to the possibility to incorporate almost any element of the periodic table into the perovskite structure. Partial substitution of elements is also possible, and can lead to chemical ordering of the elements: in case of B-site cation substitution, a so-called B-site ordered double perovskite A2B'B"O6 may form. Such double perovskites offer novel properties and applications due the cation order-disorder phenomenon and the combination of two different B-site cations. The aim of the present thesis was to gain a greater understanding of the structure, redox behavior and electronic and magnetic properties of A2B'B"O6 double-perovskite oxides, and the chemical and physical principles behind them. Among the possible applications for double perovskites, compounds of the type Sr2B'MoO6 have been considered as anode materials for solid-oxide fuel cells. In this work, Sr2B'MoO6 perovskites with B' = Mg, Mn, Fe, Co or Ni were examined. Of these, Sr2MgMoO6 was found to be redox stable, whereas the compounds with a transition metal at the B' site were not. However, partial substitution of Mg in Sr2MgMoO6 by a transition metal could provide materials with improved performance and good redox stability. Partial substitution of Mo in Sr2MgMoO6 by Nb or W was also studied. In both cases the compounds remained redox stable, but their electrical conductivities were impaired by the substitution. In case of the Nb-for-Mo substitution, strong correlations between oxygen-vacancy concentration and B-site cation ordering was found. Finally, a concept of using an all-ceramic composite material consisting of Sr2MgMoO6 and SrMoO4 as an SOFC anode was presented. The A2B'B"O6 perovskites also exhibit various magnetic properties, which are of interest in basic research. Compounds with the composition A2CuB"O6 are especially intriguing, as they may show low-dimensional and/or frustrated magnetic behavior depending on their composition. It is thus useful to examine this group of compounds as a whole, as they can provide information on magnetic properties in solids in general. In the present work two new compounds of this family, Sr2CuMoO6 and Sr2CuIrO6, were synthesized using a high-pressure synthesis method. In addition, magnetic ground state properties of the quasi-low-dimensional Sr2CuWO6 and Sr2CuMoO6 were determined. The Sr2CuB"O6 family of compounds were found to exhibit a transition from quasi-low-dimensional to a more typical antiferromagnetic behavior with increasing d-orbital occupancy of the B" cation. Similar transition was noted in a series of Ba2Cu(W1-xUx)O6 compounds with increasing x, with the distinction that both W and U are diamagnetic.
|Translated title of the contribution||A2B'B"O6-tyyppisten perovskiittien ominaisuudet ja käyttökohteet: polttokennoista kvasimatalaulotteiseen magnetismiin|
|Publication status||Published - 2014|
|MoE publication type||G5 Doctoral dissertation (article)|
- double perovskite
- solid-oxide fuel-cell