Perovskites, crystal structure structures

Perovskites are compounds of the ABC3- type where C is often oxygen, but not always. Figure 11.6 shows two versions of the perovskite crystal structure... 

Figure 11.6 Views of perovskite crystal structure. Top—conventional cubic unit cell white circles = oxygen black circle = transition metal gray circles = alkali or alkaline earth metal. Bottom—extended unit cell to show the cage formed by the oxygen octa-hedra. Adapted from Bragg et al. (1965).

The latter indicates that the dominant bonding type is covalent. This was also observed for CaTi03 and BaTi03, both of which have the perovskite crystal structure, but are considerably softer than MgSi03.The Mg perovskite is about twice as hard as crystobalite (quartz). However, hydration converts MgSi03 to talc, which is very soft. 

The perovskite crystal structure is exhibited by a large number of compounds because numerous metals can form the octahedral sub-units, and other metal ions can lie between the sets of eight octahedra. The main constraint is on the sizes of the ions that can be chosen to fit compactly together. 

Figure 1.42 The perovskite crystal structure of CaTiOs. From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Figure 9.3 The perovskite crystal structure, (a) Ideal cubic phase showing comer-shared octahedra surrounding the twelve-coordinated cuboctahedral site (b) projection of the orthorhombic perovskite MgSi03 structure at 6.7 GPa along the c axis (from Kudoh etal., 1987). Rotation and tilting of [Si06] octahedra relative to the cubic structure are shown. The eight shorter bonds (Mg-0 distances = 199 to 244 pm) are indicated by solid lines and the four longer bonds (Mg-0 = 277 to 315 pm) by dashed lines.

The much larger dielectric constant of a BST film (>200), compared to that of a polyerystailine Ta O film on Si (20-25) or amorphous SiO/SiN bi-layer (6 - 7), is due to its perovskite crystal structure, in which a large dipole moment is induced when the electric field is applied. Therefore, deposition of BST films with good crystalline quality is extremely important to obtain a large capacitance. In most deposition methods, the higher the deposition temperature the better the crystalline quality of the film as long as the stoichiometric composition is maintained. 

To control the extent of intermixing and stoichiometry of the cation species, there have been a number of investigations to synthesize stoichiometric mixed-metal precursors with structures similar to the perovskite crystal structure. The motivation behind these efforts is that stoichiometric precursors with structures similar to the desired crystaUine phase should undergo crystallization at lower heat treatment temperatures. Most attempts in this area, however, have resulted in mixed-metal species with a cation stoichiometry different than perovskite." " The formation of such compounds indicates the importance of thermodynamic sinks in the synthesis of mixed-metal precmsors. ... 

Fig. 7 Cubic perovskite crystal structure of high-symmetry parent compound KZnFa. Large spheres are cations, K+, medium-size dark spheres are anions, F, and small black spheres are metal ions, Zn. Metal-ligand octahedra share common vertices. (From [18])...

Fig. 12 Hexagonal perovskite crystal structure of the parent compound CsNiCfi. Black circles represent transition metal atoms, Ni in this case. White circles are ligands. Shaded circles are atoms of Cs . Face-sharing octahedrons [NiCle] are packed in linear chains (From [57])...

Ferroelectric materials in this family have the perovskite crystal structure, where Ti+4(Zr+4) is in the B site at the center of the unit cell within an octrahedral coordination of O-2, and Pb+2 (La+3) occupies the A site at cube corners. Considerable discussion and experimentation have occurred to decide what charge-compensating defects are created by the... 

Figure 2.4 The cubic perovskite crystal structure of a compound of stoichiometry ABXj. The larger A cations sit at the center of eight corner-linked BX octahedra.

Modules of perovskite-type (abbreviated to perovskite) crystal structure alternating with other structural modules occur in several crystalline materials that are known as hybrid or intergrowth perovskites. Three-dimensional (3D - the octahedra share corners in three non-coplanar directions such that layers of thickness many octahedra are formed), two-dimensional (2D - the octahedra share corners in two directions such that layers of thickness one octahedron are formed), onedimensional (ID - the sharing of octahedral corners develops along one direction only such that rows of octahedra result) and zero-dimensional (OD - only isolated octahedra occur) perovskite layers are known. In the OD and ID layers, the positions of the isolated octahedra (OD) and rows of octahedra (ID) are supposed to match those of the octahedral framework in the perovskite structure. 

Data for samples calcined at lower temperature (800°C) are usually more scattered, even if they confirm the decrease of a with increasing content of the doping metals. These results suggest that the noble metals are incorporated at different oxidation states resulting in some defects of the perovskite crystal structure. In tha case of the Pd-containing sample, this hypothesis is confirmed by a small shift of XRD peaks and decreasing peak sharpness observed at high palladixun content [4]. 

Fig. 8.3 Perovskite crystal structure of oxide ceramic materials used to fabricate composite membranes. A very large fraction of the metals in the periodic table can be substituted into the A and B lattice sites. A-sites contain larger cations such as alkaline earth and rare earths, including Ca, Sr and La, whereas the B-sites contain smaller transition metal cations such as H, Nb, V, Fe, Cr, Cu and Co. A near infinite variety of materials can be synthesized...

FIGURE 7.2 The perovskite crystal structure. The lattice Is simple cubic with several cations able to occupy the central octahedron, (a) Atomic model (b) the polyhedron. 

Figure 18.2 Perovskite crystal structure, (a) The nonpolar paraelectric cubic phase (b) A polar ferroelectric phase due to tetragonal distortion. Data for PbTiOs stem from Ref [10].

FIGURE 2.2 LaGaOj forms in the cubic perovskite crystal structure with La atoms at the cube comers, O atoms at the cube faces, and Ga at the cube center. 

The flexibility of the perovskite crystal structure and the opportunity to accommodate various dopants offer the possibihty to tailor material properties. The design of catalytic or electronic properties, such as ionic or electronic conductivity, is in the focus of solid oxide fuel cell (SOFC) research activities. Perovskite-type oxides are therefore a well-investigated class of materials and commonly applied as functional layers in SOFCs, as porous microstructured cathode layer on the air electrode side [32-36] and very recently as anode on the fuel side [35]. On a research level, perovskite-type oxides are also apphed as gas-tight electrolyte to separate anode and cathode compartments [37,38] or as an interconnector material [39,40]. Beside the stoichiometry and crystal structure, processing... 

Total oxidation of other light hydrocarbons reported YFeOs, and LaFeOs catalysts also revealed the importance of the network stability [41]. The superior activity of LaFeOs compared to that of YFeOs was explained by the contribution of La that stabilizes the perovskite crystal structure, thus promoting a partial oxidation or reduction of Fe cations allowing a fraction of oxygen ions to be removed from/or incorporated back to the lattice. On the contrary, the presence of Y in the perovskite structure does not permit an intensive conversion of iron ions between the different oxidation states that corresponded in a lower activity. 

LaFeOs perovskites were also used as nanocatalysts for the heterogeneous Fenton-like reactions of phenol and methyl tert-butyl ether as model contaminants and H2O2 as oxidant [47]. The collected experimental results suggested that the performances of the AFeOs perovskite catalysts are influenced by the intrinsic properties of the perovskite (crystal structure, A site cation, or structural impurities) and processes that occur under the Fenton conditions. 

Figure 22. The octahedral fragment of the perovskite crystal structure ABO, with the transition metal atom B at the center and six oxygen atoms (shadowed) at the apexes of the octahedron. The letters a,b,c,. .. denote the off-center positions of the atom B in the eight wells of the APES induced by the PJTE.

Sketch/describe unit cells for sodium chloride, cesium chloride, zinc blende, diamond cubic, fluorite, and perovskite crystal structures. Do likewise for the atomic structures of graphite and a silica glass. 

It is also possible for ceramic compoimds to have more than one type of cation for two types of cations (represented by A and B), their chemical formula may be designated as A ,B Xp. Barium titanate (BaTiOj), having both Ba and Ti cations, falls into this classification. This material has a perovskite crystal structure and rather interesting electromechanical properties to be discussed later. At temperatures above 120°C (248°F), the crystal structure is cubic. A unit cell of this structure is shown in Figure 12.6 Ba ... 

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