Perovskite Oxides
Perovskite oxides have the general formula ABO3, with a large A cation (often a rare earth or alkaline earth such as La or Sr) and a smaller transition metal B cation octahedrally coordinated by oxygen. Changing the A cation changes the formal oxidation state of B (for example +3 in LaBO3 and +4 in SrBO3), and changing B changes the d-electron count, so the family offers systematic control over composition. Perovskites are used as solid oxide fuel cell cathodes and in catalysis, sensors and electronic devices, and their oxygen chemistry (O adsorption and oxygen vacancy formation) is central to those uses. See also Oxygen vacancies in oxides, d-Band model and DFT + U.
Bulk d-band structure of cubic 3d perovskites (2011)
In the bulk companion to the surface study, Akhade and Kitchin computed the B-projected d band of cubic LaBO3 and SrBO3 (B = Sc to Cu) with non-spin-polarized PW91 DFT under strains of -5% to +5%. Because the B d states hybridize with O s,p states, they defined the d band by tracking the projected states under very large tensile strain and in the lattice with oxygen removed. The d-band width followed a solid state table model, scaling with the d-orbital radius to the 3/2 power and the B-O distance to the -7/2 power, so larger B atoms such as Ti are more strain-sensitive than Cu. Changing the A cation (and hence the B oxidation state from +3 to +4) acted mainly through the lattice constant (Akhade 2011). Summary: akhade2011.
OER activity trends on LaMO3 and SrMO3 perovskites (2011)
DFT calculations on (100) surfaces of LaMO3 and SrMO3 perovskites (M = Ti to Cu) placed these materials on a universal OER volcano. LaMnO3 binds intermediates too strongly, LaCuO3 too weakly, and SrCoO3 sits near the top with an O*-HO* free energy difference of 1.48 eV. The predicted order SrCoO3 > LaNiO3 > SrNiO3 > SrFeO3 > LaCoO3 > LaFeO3 > LaMnO3 matches trends measured in alkaline media, although the authors note that perovskite-type SrCoO3 is hard to make experimentally (Man et al. 2011). Summary: man2011.
Descriptors for surface reactivity of 3d perovskites (2012)
Akhade and Kitchin used GGA (PW91) DFT on BO2-terminated (001) surfaces of cubic LaBO3 and SrBO3 with B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni and Cu, under strains from -5% to +5%. They found that d-band filling and oxidation state set the overall reactivity, while strain shifts it only modestly. O adsorption and vacancy formation energies were approximately linearly correlated, so they cannot be tuned independently; both scaled roughly linearly with the number of B d electrons with slopes that depend on oxidation state, and both correlated with the B-site d-band center once the d band was separated from hybridized O states. LaScO3 and SrTiO3, with empty d bands, were outliers. O adsorption was stronger on the La series, while vacancies formed more easily on the Sr series (Akhade 2012). Summary: akhade2012; see also Strain and ligand effects.
Mesoporous (La,Sr)MnO3 powders by evaporation-induced self-assembly (2012)
Chao and co-workers made crystalline orthorhombic LaMnO3 and La0.8Sr0.2MnO3 (LSM) powders by surfactant-templated evaporation-induced self-assembly with a 700 C calcination, reaching BET surface areas up to 50 m2/g (LaMnO3) and 46 m2/g (LSM) with CTAB, far above the roughly 2 m2/g of solid state LSM. Area increased with precursor loading and with humidity during aging, and decreased with surfactant molecular weight. After 100 h at 750 C, LSM templated with CTAB lost 35% of its area, while P-123 and F-127 powders lost only 11% and 6%, leaving mesoporous powders of 17 to 24 m2/g (Chao et al. 2012). Summary: chao2012.
Outer-electron grids for perovskite adsorption (2013)
Calle-Vallejo and co-workers included La/Y perovskites (M at +3) and Ca/Sr/Ba perovskites (M at +4) with 3d M sites in an RPBE DFT study of *O, *OH and *OOH adsorption. Adsorption weakened with the number of outer electrons on M, and perovskites sharing the same M and the same A-cation oxidation state (for example LaNiO3 and YNiO3, or CaCrO3, SrCrO3 and BaCrO3) adsorbed similarly, suggesting that lattice strain and A-site identity are secondary to the M electron count. Closed-shell perovskites such as LaScO3, YScO3, SrTiO3, CaTiO3 and BaTiO3 were inert outliers (Calle-Vallejo et al. 2013). Summary: callevallejo2013.
Which modeling simplifications change bulk perovskite trends (2014)
Curnan and Kitchin tested the shortcuts common in perovskite screening (small cubic cells, no magnetism, plain GGA) on bulk vacancy formation energies of ABO3 with A = La, Sr, Ba, K and B = Sc to Cu, using PBE in VASP. Perovskites whose A cations share a valence gave very similar trends, so the A cation acts mainly through its oxidation state. Rhombohedral (R-3c) and orthorhombic (Pbnm) distortions lowered the energy of the stoichiometric compounds relative to cubic but raised vacancy energies almost uniformly, leaving the ordering intact. Ordering did change with ferromagnetism in some cases (SrMnO3 versus SrFeO3) and with the Hubbard U on the B 3d states, so these factors cannot be dropped as safely (Curnan 2014). Summary: curnan2014; see also Oxide polymorphs.
Perovskites as substrates for epitaxial TiO2 (2017)
Cubic SrTiO3 and BaTiO3 were used as substrates in a DFT study of epitaxial anatase and rutile TiO2 on their (001), (011) and (111) facets. The cubic-stacked AO3 close-packed {111} planes of the perovskite match the stacking of anatase, allowing coherent anatase interfaces on every facet, while rutile matched coherently only on (111). Interface energies for anatase were systematically lower on BaTiO3 than on SrTiO3 (by 2 to 11% on the (001) and (011) facets and 10 to 51% on (111)), suggesting a modest role for substrate chemistry beyond geometry (Xu et al. 2017). Summary: xu2017.
DFT check of machine-learning-proposed double perovskites (2023)
The WhereWulff workflow was applied to BaSrCo2O6 and BaSnTi2O6, two double perovskites suggested by an earlier machine-learning screen for OER activity (Sanspeur et al. 2023). The (001), (101) and (110) facets dominated the Wulff shape for both. Under alkaline conditions, BaSrCo2O6 surfaces were clean and BaSnTi2O6 surfaces were OH*-covered; BaSrCo2O6-(001) had the lowest theoretical overpotential, 0.63 V, consistent with the 0.5 to 0.75 V previously reported, while BaSnTi2O6 facets gave 0.73 to 0.86 V. Summary: sanspeur2023.