A-Site Vacancies

Since the BO3 array in the perovskite structure forms a stable network, the large A cations can be missing either partly or wholly. Cation vacancies on the cubo-octahedral A sites can be generated by increasing the valence of the octahedral B cation species that is, A + B + C 

This sequence is illustrated by, for example, the compounds LaGa03 TiOa - Lai/3Nb03 Re03 [64-66]. 

Tungsten bronzes are metallic conductors, and have a metallic luster and colors that go from gold to black depending on composition. They are very resistant chemically, and serve as industrial catalysts and as pigments in bronze colors. Besides bronzes, A-site-defective perovsldte oxides are known to be formed when B =Ti, Nb, Ta, and so on. One interesting example of a similar phase is Cuo.5Ta03, where copper atoms are ordered at the A sites these compounds will exhibit metallic properties if the B atom occurs in a low oxidation state. 

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Figure 7.14 (a) The insertion of a propylene molecule into a site vacancy in the Ziegler-Natta catalyst, (b) The... 

Fig. 2. Triangular plot of Ca, Na, and A-site vacancies in pyrochlore from Vishnevogorskii calculated from structural formulae normalized to two B-site cations. Compositions of primary pyrochlore plot near the Ca-Na join (solid symbols). Compositions of the altered areas move toward the Ca-vacancy join, then along this join toward the top of the triangle.

Defects in perovskite oxides can be due to cation vacancies (A or B site), amon vacancies or anion excess. Cation-deficient oxides such as A,WOj give rise to oxide bronze structures, W03 itself representing the limiting case of the A-sile deficient oxide A-site vacancies are seldom ordered in these metallic systems. B-site vacancies are favoured in hexagonal perovskites and ordering of these vacancies gives rise to superstructures in some of the oxides. 

Much deeper insights into the water sorption properties of polyelectrolytes can be provided by evaluating the (nNa)p term directly. By the use of the activity coefficient of Na+ ions in the PA A polyelectrolyte phase, (yNa)p, (aNa)p can be expressed as (aNa)p = (yNa)P [Na]p. Two concentration terms, i.e., (1) Na+ ions present in the polyelectrolyte phase to neutralize free car-boxylate groups and (2) Na+ ions imbibed in the polyelectrolyte phase in the form of NaCl, contribute the [Na]p term. Escape of Na+ ions from the polyelectrolyte phase due to their thermal motion, which produces a site vacancy of the polyion, should also be taken into consideration. The fraction of site vacancy of polyelectrolytes is available as a practical osmotic coefficient, c/>p-Na, which can simply be related to the linear charge separation of the PAA polyion. It has been revealed that PiNa is not affected by the change in the polyion concentration nor Cs, which is known as an additivity rule [16,17]. Thus the (aNa)p term can finally be expressed as... 

It seems that the lattice oxygen plays a direct role in the oxidation of CO. Indeed, it was found that the catalytic activity is maximum, if the bond energy of the lattice oxygen is minimum [3], Hence, the oxygen vacancy is also an important factor for the catalytic activity. The partial substitution of the A element can monitor the oxygen vacancy. If Ce(IV) is introduced in the perovskite structure A site vacancies are produced [4]. The partial substitution of B element by R (IV) increases greatly the CO oxidation activity of Lao.TPbo.sMnOs perovskite [5]. 

WO, and ReO, may be regarded as the limiting cases of A-site vacancy perovskites. Both the oxides possess corner-linked framework of the octahedra, but unlike ReO, WO3 is never cubic. It shows several polymorphic transitions starting from the low temperature triclinic structure to more symmetric forms with increasing temperature. The transitions arise from temperature-dependent displacements of the tungsten atom from the centre of the WOg octahedron, ... 

The superconducting double perovskite (NaQjsKg jlBajBi Ojj has a complex ordering of A-site cations. The structure is cubic space group Im3m (229), a=0.8550nm. The A-sites are subdivided so that one set is occupied by (Na V), where V represents an A-site vacancy, while the other (A -sites) are fully occupied by Ba. The Bi cations occupy the B-site octahedra. 

A number of perovskite phases are able to support considerable populations of A-site vacancies provided that charge neutrality is maintained. The best known of these are the (3,4) perovskites La2 3Ti03, which has a vacancy population on 1/3 of... 

Additional La can also be accommodated in the stmcture to partially occupy the remaining A-site vacancies. In this case the substitution may be balanced by the creation of further TP or oxygen ion vacancies. For example, the phase Laj TiO has a nominal formula La5,gTi 53 Ti 02gg2-... 

A-site ionic diffusion enhanced by an A-site vacancy population is found in derivatives of La jTiOj. This phase contains Ti and has 1/3 of the A-sites vacant. Lithium ion conductivity is of interest in materials used in Li batteries, and the doped phases are favoured as electrolytes because of the pre-existing... 

During oxidation we might similarly see a preferential incorporation of A-site vacancies, resulting in a precipitation of an A-rich phase ... 

Gao X, Pisher CAJ, Kimura T et al (2013) Lithium atom and A-site vacancy distributions in lanthanum lithium titanate. Chem Mater 25 1607-1614... 

The ion valence of donor-type impurity atoms is higher than that of the constituent atoms, and A site vacancies are introduced. In perovskite BaTiOj, donor atoms are known to suppress the peak dielectric and piezoelectric properties. This is believed to be due to a compensating valence change of some of the TL to TL since Ba is not volatile. In lead perovskites, such as lead zirconate titanate (PZT) or lead titanate (PT), it is believed that the excess lead created by the vacancies is allowed to leave the structure because of its high... 

Perovskite (La, Li)Ti03 (LLTO) is a promising oxide conduction system. When the molar ratio of La/Li is above 1, A site vacancies appear, which keep the material electrically neutral. This compound is generally expressed... 

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]. 

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