Perovskite structure chemical substitution

The ample diversity of properties that these compounds exhibit, is derived from the fact that over 90% of the natural metallic elements of the periodic table are known to be stable in a perovskite oxide structure and also from the possibility of synthesis of multicomponent perovskites by partial substitution of cations in positions A and B giving rise to compounds of formula (AjfA i- )(ByB i-J,)03. This accounts for the variety of reactions in which they have been used as catalysts. Other interesting characteristics of perovskites are related to the stability of mixed oxidation states or unusual oxidation states in the structure. In this respect, the studies of Michel et al. (12) on a new metallic Cu2+-Cu3+ mixed-valence Ba-La-Cu oxide greatly favored the development of perovskites exhibiting superconductivity above liquid N2 temperature (13). In addition, these isomorphic compounds, because of their controllable physical and chemical properties, were used as model systems for basic research (14). 

The dielectric materials important for many electronic and optical applications are acentric and many possess perovskite and perovskite-like structures. These structures are characterized by BO octahedra and a wide variety of phases can be obtained by virtue of the fact that the octahedra can be attached in several ways in building up a macroscopic phase. These structures afford considerable latitude for chemical substitution and thus for alteration of properties... 

An effective approach to modulate the Tc is chemical substitution in such films. Moreover, it is also known that adding more kinds of cations to the -site in perovskite structure (ABO3) usually results in diffuse phase transition, which can broaden the transition... 

The perovskite-type oxides have unique characteristics in response to a wide range of properties that are assigned to the cation substitution capacity in its structure, generating isostructural solid with formula Ai- AyBi-yByOsi s. These substitutions can lead to the stabilization of the stmcture with an unusual oxidation state for one of the cations and the creation of anionic and cationic vacancies. This has a significant influence on the catalytic activity of these materials compared to the typical supported materials. Another important feature is the thermal stability of these materials and mechanical and chemically stable reaction conditions [41,148]. 

La2/3Ti03 is a perovskite-type oxide in which one third of the A-site cations is deficient. When lithium is partially substituted for La in A sites of this oxide, lithium ions become mobile [46]. The lithium ion conductivity of Lao.51Lio.34TiO2.94 is about 10 S cm at room temperature [47]. This value belongs to the highest among lithium ion conductors that are chemically stable in an atmospheric environment. As La and Li ions are randomly distributed in the A-site position in the perovskite-type structure and, therefore, A-site vacancies are also distributed randomly, it is considered that the lithium ions can easily move through the vacancies. The relationship between the conductivity and content of lithium ions obeys so-called percolation theory [48]. 

Perovskite and perovskite-like structures are exhibited by a very large number of compounds and they offer wide crystal-chemical latitude for alteration of crystal structure and dielectric, transport, and electrical properties by suitable solid solution substitutions. For a fairly comprehensive tabulation of ternary and quaternary perovskites, some properties, and preparation of materials through about 1968, the reader is referred to Ref. 223. 

---