Requirements for Oxygen Anion and Electronic Conduction within Perovskites

Requirements for Oxygen Anion and Electronic Conduction within Perovskites 

In practice, however, cubic perovskites are observed possessing tolerance factors between 1.04 and 0.95. Stability for the perovskite structure requires that 1.0 S 0.75 and that the A lattice site be stable to twelvefold coordination [18]. This requires that the A-site cation possesses a radius greater than 0.09 nm. Tolerance factors between 0.9 and 0.75 promote buckling of the comer-shared octahedra [19] giving rise to many perovskites with an orthorhombic structure. 

One strategy for increasing the population of mobile oxygen anions in a perovskite lattice, so as to potentially increase ionic conductivity, is to enhance the population of lattice oxygen vacancies. This requirement can be realized in the orthorhombic brownmUlerite stmcture [22, 23] which is derived from the perovskite stmcture by removing one-sixth of the unit cell oxygen atoms to give the empirical 

The use of an oxygen deficient perovskite lattice has two potential advantages. 

there is an intrinsic population of oxygen vacancies within the lattice bulk as a consequence of the oxygen deficiency. This avoids potential trap sites for ionic migration [24] caused by ahovalent dopants frequently introduced into many ioni-cally conducting materials. Secondly, it is generally more difficult to remove additional lattice oxygen from the membrane under the chemically reducing conditions present at its partial oxidation surface. 

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