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Double perovskites RBaB2O6-δ and Sr2 BMoO6, where R=rare-earth element and B=3d-transition metal, with A-site and B-site, respectively, cation ordering are very promising materials for a variety of different devices for moderate high temperature applications such as solid oxide fuel cells (SOFCs) and mixed ionic and electronic conducting (MIEC) membranes. The unique feature of the oxides is their ability to undergo both thermal strain and that induced by the defects of oxygen nonstoichiometry in the oxide lattice. The latter is called as chemical or defect-induced strain. This property was shown recently to be anisotropic for tetragonal double perovskites unlike that for perovskites with pseudo-cubic structure. The crystal lattice of a double perovskite expands along a-axis and simultaneously contracts along c -axis with the decreasing lattice oxygen content. Expansion along a-axis was found to obey the model for pseudo-cubic oxides proposed by us earlier and based on relative change of mean ionic radius. The possible reasons for lattice contraction along c axis are discussed as well.

Standard enthalpies of formation of CsPbX3 (X = Cl, Br, I) perovskites from halides and from elements at 298 K were measured using solution calorimetry. Intrinsic and extrinsic stabilities of CsPbX3 halides were analyzed and compared with those of CH3NH3PbX3. The main difference between the stabilities of CsPbX3 and CH3NH3PbX3 halides was found to stem from the different chemical natures of cesium and methylammonium cations. Indeed, the enthalpies of formation of CsPbX3 from binary constituent halides, Δf H°hal, are only slightly more negative than those of CH3NH3PbX3. Small values of Δf H°hal imply that the entropic contribution to the Gibbs free energy of the formation of CsPbX3 and CH3NH3PbX3 is significant and, hence, of utmost importance for understanding the intrinsic stability of these compounds and their analogues. Regarding the extrinsic stability, the presence of gaseous O2, H2O, and CO2 was shown to be crucial for the stability of the iodide, CsPbI3, for which several decomposition reactions, exergonic at 298 K, were identified. At the same time, chloride, CsPbCl3, and bromide, CsPbBr3, are much less sensitive to these chemical agents. However, liquid water should degrade all of the CsPbX3 halides.

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The oxygen nonstoichiometry, δ, and oxidation enthalpy, ΔH ox , of double perovskites RBaCo 2 O 6−δ (R = Sm or Eu) were simultaneously measured depending on the temperature and oxygen partial pressure, p O 2 . Theoretical equations for ΔH ox (T, δ) and p O 2 (T, δ) were derived from the defect structure model based on the oxygen exchange and cobalt disproportionation reactions. These equations were fitted independently to each of the experimental ΔH ox (T, δ) and p O 2 (T, δ) data sets. The resulting enthalpies of defect reactions were found to be almost the same irrespective of the calculation method. In other words, the models, describing satisfactorily the data, can be used to calculate both compositional dependences and redox thermodynamics of RBaCo 2 O 6−δ (R = Sm or Eu). In addition, from the previously published data and the data presented here, trends were determined in the defect reaction thermodynamics of RBaCo 2 O 6−δ (R = La, Pr, Nd, Sm, Eu, Gd, or Y). Drop calorimetric measurements were performed in air to obtain enthalpy increments for RBaCo 2 O 6−δ (R = Sm or Eu) with variable oxygen content because the samples lost oxygen upon being heated in the calorimetric cell. As-obtained data were used to calculate the functional dependences of enthalpy increments of EuBaCo 2 O 5.56 and SmBaCo 2 O 5.6 with a constant oxygen content. In addition, as an example of practical application-oriented calculations for solar energy conversion and oxygen storage, the performances at equilibrium of RBaCo 2 O 6−δ (R = Pr, Sm, Eu, or Gd) were evaluated and compared to those of SrFeO 3−δ as a reference material.

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