PrBaCo2-xFexO6-δ (x = 0-0.6) double perovskites were shown to have a tetragonal P4/mmm structure in air over the complete temperature range investigated despite the oxygen content reaching a threshold value of 5.5 at temperatures from 850 °C for x = 0 up to 1078 °C for x = 0.6. However the P4/mmm→Pmmm structure transition was found to occur in PrBaCo2O6-δ under reducing conditions at temperatures from 500 °C at pO2 = 10(-3) atm down to 350 °C at pO2 = 10(-4) atm. The lattice chemical expansion along the a-axis and contraction along the c-axis, with decreasing oxygen content, 6 - δ, were found to completely compensate each other in the double perovskites and, therefore, their volume chemical expansion became negligible. As a result the cell volume linearly increases with temperature contrary to simple cubic perovskites.
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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