Recent advances in perovskites: Processing and properties
Introduction
The perovskite structure is one of the most wonderful to exist. It belongs to the ternary family of crystalline structures and has a formula ABX3. It is built from a dense packing of X anions (oxygen preferentially), with two types of sites, one with coordination six and the other with coordination eight or twelve. The octahedral sites can host small cations of one, two, three, four, five or six valence oxidation states, whereas in eight or twelve coordination sites, mono-, di- or tri-valent large-sized cations can be located. The resulting compounds have a wide range of chemical formulas, properties and applications, as summarized in Table 1. Each A cation is surrounded by twelve X anions in a cubic-octahedral coordination, and each B cation is surrounded by six X anions, in an octahedral coordination. Each X anion is surrounded by two B-site cations and four A-site cations (Fig. 1). Several reviews are devoted to the different aspects of the crystalline perovskite-type compounds [1], [2], [3], [4].
Only a few perovskite-type compounds exist in nature. Some of these compounds, their original scientific names and their discovery sites are listed in the following:
The mineral perovskite was discovered in 1839 in the Ural Mountains, Russia, by the German mineralogist and chemist Gustav Rose. It was named in honor of the dignitary, mineralogist and Russian military officer Lev Alexeievitch Perovsky.
In an ideal perovskite structure, as shown in Fig. 2 for the cubic SrTiO3 oxide, the A cation (Sr) is coordinated to 12 oxygen anions whereas the B (Ti) cations are coordinated to 6 oxygen anions. The A cation is larger than the B cation; the oxygen anions are coordinated to 2 B cations and four A cations. In this way, there is contact between the A, B and O ions. (RA + RO) should be equal to √2(RB + RO), where RA, RB and RO correspond to the crystal radii for cations A and B and the anion O, respectively.
The stability of a perovskite-type phase for a particular group of cations and anions is related to the so-called “tolerance t” factor. This parameter defines the symmetry of the system and significantly affects its dielectric properties. The tolerance is a variable factor with regard to the limits for the cations size and allows the formation of a perovskite-type phase [1]. It is given by the expression t = (RA + RO)/√2(RB+ RO). When the t value is close to 1, the perovskite phase is formed, although some perovskite structures can form in the range between 0.90 and 1.10, as the cases of BaZrO3 (t = 1.01, cubic) and CaTiO3 (t = 0.97, pseudo-orthorhombic monoclinic). In the case of B-type complex perovskites A(B0.5B′0.5)O3, the above relation is modified to t = (RA + RO)√2[(RB + RB′)/2 + RO].
When the tolerance factor is less than 1, the system is of low symmetry; t larger than 1 corresponds to big A cations and small B cations, so the B cations have a larger space to move. When t is less than 1, the B cations have a bigger size.
The properties of the perovskite-like compounds as a function of their composition and crystalline structures and symmetries, are very wide. Many perovskite-type compounds are considered ionic compounds, but the bond type is commonly a mixing of ionic and covalent. With regard to the electrical and magnetic features, they may be insulators, ferroelectric compounds, semiconductors, superconductors, metallic conductors, ionic conductors, antiferro-, ferro- or ferrimagnetic compounds, multiferroics. Some examples are given in Table 1.
Section snippets
Crystalline structure
Perovskites can crystallize in all possible symmetries, from cubic (high symmetry) to triclinic (very low symmetry). The most typical and important examples are summarized in the following.
Processing
Solid-state reaction from oxides, sol–gel, hydrothermal synthesis, high-pressure synthesis, mechanically-activated synthesis and others have been used to prepare perovskite-like compounds. It is important to notice that different procedures may lead to compounds with the same chemical formulation but rather different crystalline symmetry and even different structures. In addition, most of the perovskite-like compounds are polyphasic as a function of temperature, in particular their oxygen
Proton conducting perovskites
Nowick and Du reviewed the various perovskite-structured oxides found to be high temperature protonic conductors [22]. Authors classified these oxides into two groups: the simple or ABO3 types and the complex (or mixed) types of formula A2B′B″O6 and A3B′B2″O9. Based on TEM and electron diffraction experiments, the authors pointed out some microstructural features and forwarded a simple “first-order” model for consideration of various fundamental aspects, in particular, the proton incorporation
Conclusions
Oxides with perovskite-type structure have shown a great potential in a wide range of applications. Their versatility with regard to the crystalline symmetry and their ability to incorporate many modifying cations, lead to a series of new properties; the facility for tailoring composition, properties and shaping (bulk, thick and thin films, nanoparticles) allows to think that their future will be as important as their splendid past.
Some of the compounds and materials exposed in this review,
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