Crystal perovskite

Certain glass-ceramic materials also exhibit potentially useful electro-optic effects. These include glasses with microcrystaUites of Cd-sulfoselenides, which show a strong nonlinear response to an electric field (9), as well as glass-ceramics based on ferroelectric perovskite crystals such as niobates, titanates, or zkconates (10—12). Such crystals permit electric control of scattering and other optical properties. 

Perovskites have the chemical formula ABO, where A is an 8- to 12-coordinated cation such as an alkaU or alkaline earth, and B is a small, octahedraHy coordinated high valence metal such as Ti, Zr, Nb, or Ta. Glass-ceramics based on perovskite crystals ate characteri2ed by their unusual dielectric and electrooptic properties. Examples include highly crystalline niobate glass-ceramics which exhibit nonlinear optical properties (12), as well as titanate and niobate glass-ceramics with very high dielectric constants (11,14). 

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. 

Three LaCoOs samples (1,11, and 111) with different specific surface areas were prepared by reactive grinding. In the case of LaCoOs (1), only one step of grinding was performed. This step allowed us to obtain a erystalline LaCoOs phase. LaCoOs (11) and LaCoOs (111) were prepared in two grinding steps a first step to obtain perovskite crystallization and a second step with additive to enhanee speeific surface area. The obtained compounds (perovskite + additive) were washed repeatedly (with water or solvent) to free samples from any traee of additive. The physical properties of the three catalysts are presented in Table 10. LaCoOs (1) was designed to present a very low specific surface area for comparison purposes. NaCl used as the additive in the case of LaCoOs (11) led to a lower surface area than ZnO used for LaCoOs (111), even if the crystallite size calculated with the Sherrer equation led to similar values for the three catalysts. The three catalysts prepared were perovskites having specific surface areas between 4.2, 10.9 and 17.2 m /g after calcination at 550 °C. A second milling step was performed in the presence of an additive, yielding an enhanced specific surface area. 

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.

So far, the bonding and surface structure aspects of electrocatalysis have been presented in a somewhat abstract sort of way. In order to make electrocatalysis a little more real, it is helpful to go through an example—that of the catalysis of the evolution of oxygen from alkaline solutions onto substances called perovskites. Such materials are given by the general formula RT03, where R is a rare earth element such as lanthanum, and T is a transition metal such as nickel. In the electron catalysis studied, the lattice of the perovskite crystal was replicated with various transition metals, i.e., Ni, Co, Fe, Mn, and Cr, the R remaining always La. 

Nishihata et al. (2002) reported the re-dispersion of Pd in a Perovskite-type oxide. They investigated the oxidation state and the local structure of Pd by using X-ray absorption analysis. Pd occupies the -site in La2PdCo06 in the oxidized sample. For the reduced catalyst, the XAD and XANES measurements suggested the segregation of metallic Pd from the perovskite crystal. They imply that Pd also moves back and forth between the -site in the perovskite structure and sites within the lattice of Pd metal clusters dispersed on perovskite surface when the catalyst is exposed to fluctuations in the redox characteristics of the emission exhaust. 

Perovskite crystallizes in the cubic space group — Pm3m. The Ti4+ ions are located at the corners of the unit cell, a Ca2+ ion at the body center, and O2- ions at the mid-points of the edges this so-called A-type cell is shown in Fig. 10.4.1(a). When the origin of the cubic unit cell is taken at the Ca2+ ion, the Ti4+ ion occupies the body center and the O2- ions are located at the face centers this B-type unit cell is shown in Fig. 10.4.1(b). 

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. 

Servoin JL, Gervais F, Quittet AM, Luspin Y (1980) Infrared and Raman responses in ferroelectric perovskite crystals apparent inconsistencies. Phys Rev B 21 2038 Salje EKH, Bismayer U (1997) Hard mode spectroscopy the concept and applications. Phase Transitions 63 1... 

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. ... 

BaTi03, which has a perovskite crystal lattice, is a ferroelectric material i.e. a material in which the change in polarization P with varying applied electric field E traces a dielectric hysteresis loop analogous to the hysteresis loop exhibited by ferromagnetic materials. 

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]. 

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