Oxygen Nonstoichiometry, Defect Structure and Chemical Expansion of Advanced Perovskite and Double Perovskite Oxides

Perovskites and double perovskites based upon substituted oxides LnBO_3-δ and LnBaB₂O_6-δ, respectively, where Ln=lanthonoid, B=3d-transition metal, are the state-of-the-art materials for a variety of different devices for moderate high temperature applications such as solid oxide fuel cells (SOFCs) and mixed ionic and electronic conducting (MIEC) membranes. The unique feature of the oxides is their ability to undergo both thermal expansion and that induced by the defects of oxygen nonstoichiometry in the oxide lattice. The latter is chemical or defect-induced expansion. This property is extremely sensitive to the defect structure of the oxide material. Despite it is generally recognized that point defects are responsible for chemical expansion  its mechanism is still controversial topic. Different reasons such as changing Coulomb forces, atomic packing, local structure, preferred coordination, association between dopants and vacancies and others are discussed in this respect. If electronic defects have localized nature then another reason may lead to chemical expansivity. Within the framework of such assumption the oxygen vacancy formation is accepted to be accompanied by the reduction of 3d-metal cations. As a result, average size of the B-site cations increases due to the apparent substitution of “large” B^(z-1)+ for smaller B^z+. Oxygen vacancy formation may also contribute to chemical expansion observed due to a change of coulomb interaction between ions. If oxide lattice chemical expansion is caused by a change of mean ionic radius due to reduction of most reducible cation then a model allowing its calculation can be developed. In order to do it the following assumptions should be accepted. (i) A closely packed lattice of oxide is formed by ions with rigid spheres. (ii) Expansion in each of three space directions is of equal value. The latter seems to be valid only for oxides with pseudo-cubic structure such as perovskites but this is not the case for layered double perovskite.

The model proposed for pseudo-cubic oxides was shown in this work to coincide completely with experimental data on chemical expansion for the perovskite La_0.7Sr_0.3Co_0.9Fe_0.1FeO_3-δ. On the contrary, chemical expansion of PrBaCo₂O_6-δ along a axis and contraction along c axis were found to compensate each other completely in the double perovskite and, therefore, its volume (overall) chemical expansion becomes negligible. As a result the cell volume linearly increases with temperature in air contrary to simple cubic perovskites such as La_0.7Sr_0.3Co_0.9Fe_0.1O_3-δ.