Ionic Conduction in Perovskite-Type Compounds

A perovskite structure is tolerant of a certain difference in size or valence of foreign atoms, and it forms various kinds of defect structure according to the kind of inserted atoms and the formation environment such as atmosphere and temperature. Not only the single perovskite type but also various kinds of derivatives, so-called perovskite-related compounds, form different kinds of lattice defects. They have also tolerance to accept foreign atoms as an impurity or to deviate from their stoichiometric composition, giving rise to some kinds of lattice defects. 

Such an adaptable structure of the perovskite-type oxide suggests that some constituent ions in the crystal will be mobile from one site to another if the energy needed to overcome the barrier to jump from one site to the other is small. In fact, several kinds of ions have been found to be mobile in perovskite and perovskite-related compounds during the past four decades. Oxide ions 

Nagoya University, Furo-cho, Chigusaku, Nagoya, 464-8601, Japan e-mail iwahara-h m3.gyao.ne.jp 

Ishihara (ed.), Perovskite Oxide for Solid Oxide Fuel Cells, 

Fuel Cells and Hydrogen Energy, DOI 10.1007/978-0-387-77708-5 3, Springer Science+Business Media, LLC 2009 

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Table 3.1 Examples of ionic conduction in perovskite-type compounds...

Most of the possible combinations of large A cations and smaller B ions, which is needed to form perovskite-type oxides ABO3, had been tried by 1955, as described by F.S. Galasso in his famous book [2] entitled Structure, Properties and Preparation of Perovskite-Type Compounds, published in 1969. This book compiled almost all available data at that time concerning structure, properties, and preparation of perovskite-type compounds. In this book, although lattice defects in the perovskite-type crystal were described, the author did not touch on ionic conduction in the perovskite except for a very brief description of BaTiOs- However, in the 1960s, several pioneering studies on ionic conduction in perovskite-type oxides were performed. 

Ionic species that contribute to high conductivity of perovskite-type compounds are rather limited. They are listed in Table 3.1 with representative compounds, their conductivities, and distinctive features. The mobile ionic species, except hydrogen, are host components of the compounds, whereas protons are unique in that they are incorporated from water vapor or hydrogen gas in the ambient atmosphere at elevated temperature. Of these, oxide ion conductors are best known, and oxide ionic conduction in various kinds of perovskite and perovskite-related oxides has been studied. 

These studies were not intended to seek a good ionic conductor but to confirm the conduction species to clarify the phenomena characteristic to the ferroelectric or pyroelectric materials. It should be noted that the conductivity of ferroelectric materials mentioned above is very low and most of the researchers in those days took no notice of the value of conductivity itself. Studies on highly conductive ionic conductors of perovskite-type compounds were started in the second half of the 1960s to search for a good oxide ion-conducting electrolyte for fuel cells and oxygen sensors. These are described in the following sections. 

As described in the Introduction section, many perovskite-type compounds can deviate from their stoichiometric composition to a considerable extent because of the strong stability of the perovskite-type structure. This deviation results in the formation of electronic defects such as excess electrons or electron holes, which cause n-type or p-type electronic conduction. Thus, it should be noted that, in general, perovskite-type compounds are have facile electronic conduction, and that the ionic conduction is often accompanied by electronic conduction. As the concentration of the electronic defects depends on the deviation of A/B and/or (A + B)/X from their stoichiometric ratio, the content of impurity, atmosphere and temperature, and contribution of electronic conduction varies with those conditions. 

Different kinds of non-oxide perovskite-type compounds have been known in carbides, halides, nitrides, and hydrides [2], Conjecturing from the oxide ion conduction in ABO3, it would be possible to expect anionic conduction, such as halide ionic or nitride ionic, in these non-oxide perovskite compounds ABX3. 

Ti,Nb)-0, and La-deficient La2-02 layers (Fig. 6.7(a)). Two-dimensional lithium cation conduction has also been reported in the orthorhombic layered perovskite-type compound Lao.62Lio.i6Ti03 [55], in which the Li cation exists and migrates only near the La-deficient La2-02 layer. This work has thus revealed that oxide ion diffusion in an ionic conductor with a double perovskite structure is two dimensional. 

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