Perovskites thermodynamic stability

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... 

Nesbitt et al. (1981) performed a detailed analysis of the thermodynamic and kinetic stability of perovskite. Thermodynamic calculations and data for natural groundwaters and hydrothermal... 

The general trend observed from the pioneering studies on oxygen permeation through perovskites of the t)q5e Lni xAxCoi yBy03.5 (Ln = La, Pr, Nd, Sm, Gd A = Sr, Ca, Ba B= Mn, Cr, Fe, Co, Ni, Cu) by Teraoka et al. [37-39] is that higher oxygen fluxes are facilitated by increased A-site substitution, and a lower thermodynamic stability of the particular perovskite. 

Reduction of NO with CO or H2 was found to be an interesting example of intrafacial catalytic process (30). If this reaction is conducted over a transition-metal oxide, the reaction rate appears to be related primarily to the thermodynamic stability of the oxygen vacancies adjacent to a transition metal ion. Associative as well as dissociative adsorption of NO have been reported on perovskite oxides (14, 22, 80, 174) (see also Section VI,B) the adsorption on the reduced oxides is stronger than in the oxidic compounds. Dissociative adsorption takes place at moderate temperatures as in NO reduction over Lao.gsBao.isCoOs at 100°C with the subsequent formation of N2 and N20 (73). 

No reoxidation process to RC0O3 under the POM reaction was observed for Gd-Co-O and Sm-Co-O. However, Nd-Co-0 was partially reoxidized to NdCoOj. The authors proposed that the catalyst deactivation by reoxidation of Co0 dispersed on rare-earth oxides depends strongly on the nature of the rare-earth element and on the thermodynamic stability of the parent perovskite structure. 

Best known examples of oxides with pure ionic conductivity are yttria-stabilized zirconia (YSZ), gadolinia-doped ceria (CGO) with fluorite structure, and strontia-doped lanthanum gal-lates (LSG) with perovskite structure. These oxides exhibit high thermodynamic stability because of the absence of transition-metal ions. These ionic conductors differ in their high oxide-ionic conductivity as follows LSG > CGO > YSZ. They show low electronic conductivity and low catalytic activity. For their utilization as oxygen membrane materials, an electronic part of conductivity is required. 

Yokokawa H, Sakai N, Kawada T, Dokiya M (1992) Thermodynamic stabilities of perovskite oxides for electrodes and other electrochemical materials. Solid State Ionics 52 43-56... 

One of them is that the oxidation of organics over perovsldte oxides proceed by two reaction pathways the first suprafacial and the second intrafacial. In the suprafacial process, the reaction rate is correlated with the electronic configurations of the surfeice transition metal ions and takes place between the adsorbed species on the surface at relatively low temperatures. Conversely, the intrafacial mechanism takes over at high temperatures and the reaction rate appears to be correlated primarily with the thermodynamic stability of oxygen vacancies adjacent to a transition metal ion. However, the role of these oxygen species in complete and selective oxidation and their participation in combustion over various ceramic perovskite phases is still an open question. 

The reductive (and oxidative) nonstoichiometry and the stability in reducing oxygen atmospheres of perovskite-type oxides was reviewed by Tejuca et al. Data from temperature-programmed reduction (TPR) measurements indicate that the stability (or reducibility) of the perovskite oxides increases (decreases) with increasing size of the A ion, which would be consistent with the preferred occupancy of the larger Lrf ion in a 12-fold coordination. The trend is just the reverse of that of the stability of the corresponding binary oxides. The ease of reduction increases by partial substitution of the A ion, e.g., La by Sr. Trends in the thermodynamic stabilities of perovskite oxides have been systematized in terms of the stabilization energy from their constituent binary oxides and the valence stability of the transition metal ions by Yokokawa et al. ... 

Strictly speaking, interfaces between perovskite cathode and doped ceria are not thermodynamically stable, and some chemical reactions can take place [21, 25]. In addition, cation diffusion can occur. In particular, Sr diffusion through doped ceria is important. There are some differences between LSF and LSC as far as reactivity and interdiffusion are concerned that is, no products are formed for the diffusion couple between Gd-doped ceria (GDC) and LSC, because GdCo03 exhibits no thermodynamic stability. In other interfaces, there arises a driving force of forming another perovskite phase from the dopant in ceria and the B-site ions (Fe or Co ions) in the perovskite this is accompanied with Sr diffusion. 

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