Perovskite structure types

When 0.4 < x < 0.53, an orthorhombic phase is observed in the AgxNb02+xFi.x system. This phase undergoes a phase transition at 900°C that leads to the formation of a tetragonal phase, which crystallizes in a tetragonal tungsten bronze-type structure with cell parameters a = 12.343 and c = 3.905 A. When 0.82 < x < 1, solid solutions based on AgNb03 were found, which crystallize in a perovskite-type structure. 

Lithium dioxyfluoroniobate (IV), LiNb02F, also has a LiNb03-type crystal structure, while dioxyfluoroniobates of sodium and potassium, NaNb02F and KNb02F, crystallize in a perovskite-type structure [247]. 

In perovskite-type catalysts the formation of the final phase is completed already at 973 K. XRD and skeletal FTIR/FTFIR data for LalCol, LalMnl and LalFel calcined at 973 K evidence that only LalFel-973 is actually monophasic and consists of a perovskite-type phase with orthorombic structure. A perovskite type phase with hexagonal-rombohedral structure represents the main phase of LalCol-973, but traces of C03O4 and La2C05 are also present. In the case of LalMnl-973 two phases have been detected both with perovskite-type structure, one orthorombic and the other rombohedral. The calculated cell parameters of the dominant perovskite-type phase are reported in Table 1 for the three samples. The results compare well with those reported in the literature [JCPDS 37-1493, 32-484, 25-1060] which refer to similar samples prepared via solid state reartion. All the perovskite-type samples are markedly sintered... 

Uenishi et al. [95] investigated the redox behaviour of palladium at start up in the perovskite-type structure LaFePdOx. An interesting behaviour is reported due to their self regenerative function, which provides high catalytic performances under cycling... 

The parent perovskite-type structure (Fig. 4.13A>) is composed of corner-linked BOe octahedra surrounding large A cations and is conveniently idealized to cubic symmetry (Fig. 4.27a). (The real structures have lower symmetry than the idealized structures, mainly due to temperature-sensitive distortions of the BOft octahedra.) In the phases related to Ca2Nb2C>7 the parent structure is broken into slabs parallel to 110 planes. The formula of each slab is A B 03 +2, where n is the number of... 

The oxide Ba2In205 is another well-studied phase that adopts the brownmillerite structure. This material disorders above 930°C to a perovskite-type structure containing oxygen vacancies. Both the Sr-Fe and Ba-In oxides are of interest for electrochemical applications in fuel cells and similar devices (Section 6.10). 

It should be mentioned that oxygen vacancies are often formed in the perovskite-type structure ABO3 in cases where the B atom is a transition metal that readily exists in more than one oxidation state. 

Figure 7.16 The perovskite-type structure. Small black circles represent the B atom, large grey circles represent O atoms and open circles represent the A atom.

The configurational entropy term, given by the degeneracy, gc, is included in AfG but not in AfGc. Let us assume the existence of two compounds with different formal oxidation states for the B atom, ABO3 and ABO2.5. The two compounds have the same (perovskite-type) structure and the non-stoichiometric phase... 

Figure 25. Proton conductivity of various oxides, as calculated from data on proton concentrations and mobilities, according to Norby and Larring (the type of dopant is not indicated see ref 187 for source data). The conductivity of oxides with a perovskite-type structure are shown by bold lines, and the conductivity of the oxide ion conductor YSZ (yttria-stabilized zirconia) is shown for comparison, (reproduced with the kind permission of Annual Reviews, http //www.AnnualReviews.org).

At this point, it may be informative to present a chronological listing of the different discoveries in oxide superconductors reported prior to 1975. In this listing, Table 5, we present the year that the oxide compound was first reported, then the year in which superconductivity was first observed in the system and the group credited for the discovery. Of particular interest is the compound Ba(Pb1 xBix)Os discovered by Sleight at du Pont in 1975. This oxide material adopts the perovskite-type structure and contains no transition metals. 

Alkaline earth-containing ternary nitrides make up the second largest group of ternary phases. Because the alkaline earth metals form stable binary nitrides, most alkaline earth containing ternary nitrides have been synthesized by the reaction of a binary nitride with a metal or by the reaction of two binary nitrides. This synthesis has resulted in a number of new ternary nitrides with a variety of structures. For example, the reaction of calcium nitride with Group 14 or 15 metals or metalloids forms a series of structurally related ternary nitrides with the anti-perovskite type structure. In Ca3MN (M = P, As, Sb, Bi, Ge, Sn, Pb) (Figure 8.5) the... 

Fig. 2.2 MOg octahedra arrangements in (a) perovskite-type structures, (b) Ti02 and (c) hexagonal BaTiOs.

As described in Section 8.2.6, along with YSZ, mixed oxygen-ion, and electron-conducting oxides with a perovskite-type structure, the so-called Aurivillius phase and pyrochlore materials are fundamentally used for the production of a variety of high-temperature electrochemical devices [50-58],... 

Fourgnet, Piet and Pape [238] obtained AgNb sF while investigating phases formed in an NbC F — AgNb03 system. This compound can also be represented by die general formula AgxNbCb+xFi-x- When 0 < x < 0.25, solid solutions based on NbC F are formed. The solid solutions are characterized by perovskite-type structures, and the cell parameter increases linearly from 3.901 to 3.916 A, with the increase in x. 

In half of the relatively short (2x10s steps) runs for the case (n = l,m = 1), the best sub-optima found were perovskite-type structures, while in only 20% of the... 

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