Perovskite stability

These data confirm that the perovskite stabilizes palladium and suggest that the activity in catalytic combustion can be more efficiently recovered, through a regeneration oxidizing process, for these systems than for the usual Pd/y-Al203 supported catalyst, due to the palladium moiety re-incorporated in the perovskite structure at a high oxidation state (Pd" ). 

The data obtained allowed us to verily that the Goldschmidt tolerance factor t, used to predict the oxide-based perovskites stability [11] could also be utilized to correlate the stability of fluoiinated ternary compounds. The t value is a lunction of the anionic and cationic radii, the closer the t factor is to unity, the greater is the perovskite stability. 

Tsai et a/. also approached the problem of increasing O2 flux and stability. Their approach was to balance the substitutions on the A and B sites of the ABO3 perovskite. Stability is strongly influenced by a stable BO3 skeletal sublattice, and the choice of Fe with a mild Co substitution gives stability with reasonable oxygen anion conductivity. Then different amounts and types of aliovalent cations (Ca, Sr ", Ba " ) were partially substituted for La in a LaFeo.8Coo.203 s perovskite framework to attain higher electron conductivity. 

G. Serghiou, A. Zerr, and R. Boehler, (MgFe)Si03-perovskite stability under lower mantle conditions. Science, 1998, 280, 2093-2095. 

Localized vs. Delocalized Electrons Oxygen Desorption and Perovskite Stability Equations for Oxygen Transport Electronic Conductivity... 

Figure 7.3 Phase diagrams for the systems Mg0-Zr02 and Ca0-Zr02, showing the lower temperature stability of the Ca0-Zr02 system, which also includes the phases CaZr4 Og and the perovskite CaZrOg...

Another application is in tire oxidation of vapour mixtures in a chemical vapour transport reaction, the attempt being to coat materials with a tlrin layer of solid electrolyte. For example, a gas phase mixture consisting of the iodides of zirconium and yttrium is oxidized to form a thin layer of ytnia-stabilized zirconia on the surface of an electrode such as one of the lanthanum-snontium doped transition metal perovskites Lai j.Srj.M03 7, which can transmit oxygen as ions and electrons from an isolated volume of oxygen gas. 

Australia, is the doyen of materials scientists who study the elastic and plastic properties of minerals under hydrostatic pressure and also phase stability under large shear stresses (Paterson 1973). J.-P. Poirier, in Paris, a professor of geophysics, was trained as a metallurgist one of his special skills is the use of analogue materials to help understand the behaviour of inaccessible high-pressure polymorphs, e.g., CaTi03 perovskite to stand in for (Mg, FelSiOi in the earth s mantle (Poirier 1988, Besson el al. 1996). 

From a detailed analysis of the stability regions in the alloy series MR Rh3Bi y and Er(Rh, yM,)3B y the formation of the perovskite structure was discussed in terms of a stabilizing charge transfer (B as electron donor) and a corresponding characteristic VEC, 31 < VEC < 34. 

With respect to CO oxidation an activity order similar to that described above for CH4 combustion has been obtained. A specific activity enhancement is observed for Lai Co 1-973 that has provided a 10% conversion of CO already at 393 K, 60 K below the temperature required by LalMnl-973. This behavior is in line with literature reports on CO oxidation over lanthanum metallates with perovskite structures [17] indicating LaCoOs as the most active system. As in the case of CH4 combustion, calcination at 1373 K of LalMnl has resulted in a significant decrease of the catalytic activity. Indeed the activity of LalMnl-1373 is similar to those of Mn-substituted hexaaluminates calcined at 1573 K. Dififerently from the results of CH4 combustion tests no stability problems have been evidenced under reaction conditions for LalMnl-1373 possibly due to the low temperature range of CO oxidation experiments. Similar apparent activation energies have been calculated for all the investigated systems, ranging from 13 to 15 Kcal/mole, i.e almost 10 Kcal/mole lower than those calculated for CH4 oxidation. 

Further ternary compounds for which the relative sizes of the ions are an important factor for their stability are the perovskites and the spinels, which are discussed in Sections 17.4 and 17.6. 

Hodjati, S., Petit, C., Pitchon, V. et al. (2000) Absorption/Desorption of NOx Process on Perovskites Nature and Stability of the Species formed on BaSn03, Appl. Catal. B Environ., 27, 117. 

Our recent our works show that even higher activity and stability can be demonstrated by the three-layer electrodes with nickel layer, active in the oxygen evolution, middle layer with catalyst, active in the oxygen reduction (Mn02, pyropolymer or a perovskite), and a diffusion (waterproof) layer,... 

The disordered structure can be stabilized to room temperature by inclusion of substitutional impurities on the In sites. Thus the oxide formed when Ga is substituted for In, Ba2(ln1 xGaJt-)205+s to form Galn defects has a disordered cubic perovskite structure even at room temperature for values of x between 0.25 and 0.5, and the similar Ba2iln1 vCox)205+3 with Coin defects has a disordered cubic perovskite structure at room temperature when x lies between 0.2 and 0.8. The defects present in the In sites hinder oxygen ordering during the timescale over which the samples cool from the... 

The enthalpies of formation of selected perovskite-type oxides are given as a function of the tolerance factor in Figure 7.17. Perovskites where the A atom is a Group 2 element and B is a d or / element that readily takes a tetravalent state [19, 20] show a regular variation with the tolerance factor. Empirically, it is suggested that the cations that give t close to 1 have the most exothermic enthalpies of formation. When t is reduced, the crystal structure becomes distorted from cubic symmetry and this also appears to reduce the thermodynamic stability of the... 

Thus garnet, ileminite and perovskite are modifications stabilized by pressure and they are all stable at some specific pressure-temperature conditions. 

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