Trends in Stability of Perovskite Oxides

Abstract

Perovskite oxides with general formula AMO 3 have a large variety of applications as dielectrics and piezoelectrics, [1] ferroelectrics [2] and/or ferromagnetic materials, [3] among others. Rare earth and alkaline earth metal perovskites are useful as catalysts for hydrogen generation, [4] as oxidation catalysts for hydrocarbons, [5] and as effective and inexpensive electrocatalysts for state-of-the-art fuel cells, [6] mainly due to the possibility of tuning their mixed ionic-electronic conductivity through substitution of A and M and subsequent formation of oxygen vacancies. Despite the general interest in perovskites, so far there have been no ab initio studies devoted to their formation energies, and the trends in stability are unknown.Among the available theoretical techniques to investigate perovskites, DFT is an appealing candidate, since it has proved useful for understanding metals and alloys at the atomic scale.[7] Nevertheless, the well-known shortcoming of DFT in describing strongly correlated systems has prevented its use for the estimation of properties such as band gaps and electron localization-delocalization of oxides, and there are numerous corrections. [8] Despite these limitations, Figure 1 a shows the experimental formation energies from elements and O 2 of 20 perovskites at 298 K and the corresponding standard DFT energies using the RPBE-GGA [9] exchange-correlation functional. The simulations are able to reproduce trends in the formation energies, and the calculated energies are shifted by about 0.75 eV compared to experiments. The A component is Y, La, Ca, Sr, or Ba, while M is a 3d metal from Ti to Cu. However, it is possible to combine the formation energies of these compounds with those of their sesquioxides (A 2 O 3 and M 2 O 3 ), rutile dioxides (MO 2 ), monoxides (AO and MO), and O 2 to reproduce the energetics of several reactions (Figure 1 b-d). The reactions are shown in the Supporting Information. The excellent correspondence between experiments and theory shows that DFT very accurately captures the mixing energies between oxides. The chemical reaction depicted in Figure 1 a and the way of representing its Gibbs energy, are given by Equations (1) and (2).In terms of trends the agreement is beyond the expected accuracy of DFT in general, but the shift is about 0.75 eV. We note that imitations in O 2 description by DFT are well known and some alternatives have been proposed, obtaining remarkable agreements with experiments. [8c,10] We obtain the total DFT energy of O 2 indirectly from the tabulated Gibbs energy of formation of water and from the DFT energies of H 2

Accuracy of density functional theory in predicting formation energies of ternary oxides from binary oxides and its implication on phase stability

“…While comparisons between experimental and computed energies of reactions for solids exist in the literature, they tend to focus on very specific chemistries and/or crystal structures (e.g., perovskites 28,29 ). In contrast to the evaluation of molecular systems in the quantum chemistry field, 30,31 there is a lack of statistically broad studies on the accuracy of specific DFT functionals in predicting energies of reactions between solid phases.…”

Thermodynamic modeling of the Sr–Co–Fe–O system

“…Thus, we expect higher uncertainty of first principle calculations compared to experiments. Nonetheless, deviation of assessed thermodynamic data from first principles results [23] was minimal. For Sr 3 Co 2 O 7 , A 0 and B 0 in Eq.5 were adjusted such as to make the phase stable at 1373 K in air in accordance with the findings from Aksenova et al and Cherepanov et al [34,65].…”

“…From the neutron diffraction experiments, the structure of two intermediate phases, SrCoO 2.75 and SrCoO 2.82±0.07 , were characterized. Calle-Vallejo et al [23] reported the Gibbs energy of formation for SrCoO 3 as −163 kJ/mol at 298 K obtained from first-principle calculations.…”

“…2 15000 [13,16], and, with lower weight in the optimization, enthalpies of formation from first-principles analysis [23]. While tremendous progress on accuracy of firstprinciple results has been achieved for metallic phases, the same quality does not apply to complex oxide phases due to many free parameters associated with competition between various interactions on the atomistic level.…”

“…Vallejo et al [23], while B 1 was obtained by assuming that the entropy of SrCoO 3−δ at 298 K is similar to that of Sr 2 Co 2 O 5 , and making further adjustments using phase diagram data. A 2 and B 2 in Eq.9, A 3 and B 3 in Eq.…”