From charge- to orbital-ordered metal-insulator transition in alkaline-earth ferrites

Abstract

While CaFeO3 exhibits upon cooling a metal-insulator transition linked to charge ordering, SrFeO3 and BaFeO3 keep metallic behaviors down to very low temperatures. Moreover, alkaline-earth ferrites do not seem prone to orbital ordering in spite of the d 4 formal occupancy of Fe 4+ . Here, from first-principles simulations, we show that the metal-insulator transition of CaFeO3 is structurally triggered by oxygen rotation motions as in rare-earth nickelates. This not only further clarifies why SrFeO3 and BaFeO3 remain metallic but allows us to predict that an insulating charge-ordered phase can be induced in SrFeO3 from appropriate engineering of oxygen rotation motions. Going further, we unveil the possibility to switch from the usual charge-ordered to an orbital-ordered insulating ground state under moderate tensile strain in CaFeO3 thin films. We rationalize the competition between charge and orbital orderings, highlighting alternative possible strategies to produce such a change of ground state, also relevant to manganite and nickelate compounds.

Origin of band gaps in 3d perovskite oxides

“…Figure 2 summarizes our results for the lowest energy phase considering spin-ordered phases (energy differences between all tested magnetic configurations and initial symmetries are given in Supplementary Table 6). Consistent with experiments and previous DFT theoretical literature 27–30,32,40,41,56 , for all explored compounds, we find (i) the correct low T crystal structure— orthorhombic for CaVO 3 , CaMnO 3 , LaMnO 3 , and LaFeO 3 ; monoclinic for LaVO 3 , CaFeO 3 , and YNiO 3 ; cubic for SrVO 3 ; (ii) the correct low T spin-ordered phase, including AFM (for YTiO 3 , LaVO 3 , CaMnO 3 , LaMnO 3 , LaFeO 3 , and YNiO 3 ) or FM (for YTiO 3 ) (except for CaFeO 3 that exhibits an incommensurate antiferromagnetic spin spiral order at low temperature 9 not included in our modeling); (iii) all compounds adopting a spin-ordered ground state (YTiO 3 , LaVO 3 , CaMnO 3 , LaMnO 3 , CaFeO 3 , LaFeO 3 , and YNiO 3 ) are predicted insulating. Furthermore, (iv) the key cell-internal lattice distortions (O 6 group rotations, Jahn–Teller distortions, and bond disproportionation) observed experimentally, are reproduced by theory with mode amplitudes in excellent agreement with experiments (see Supplementary Table 7).…”

“…A number of calculations have used this “DFT + U ” approach, where DFT is amended by an on-site potential that removes part of the spurious self-interaction error and thereby creates a distinction between occupied and empty states producing at times gapped states 25,27–30,32,40–42 . DFT + U successfully obtained gapping in simple binary oxides such as MnO, FeO, CoO, or NiO 25 , dioxides such as UO 2 26 , and in the spin-ordered phases of the much more complex 3 d transition metal ABO 3 compounds 27–32,41,43,44 .…”

Superlattice-induced ferroelectricity in charge-ordered La 1/3 Sr 2/3 FeO 3

“…By comparing the total energies of CS-APAF, CS-AF, and CS-Fi phases and assuming S = 5/2 for Fe 3+ and S = 3/2 for Fe 5+ , we calculate the nearest-neighbor exchange coupling constants for Fe 3+ -Fe 5+ and Fe 3+ -Fe 3+ pairs to be −8.3 meV and 9.5 meV, respectively, showing strong magnetic interactions consistent with inelastic neutron scattering measurements (25). For ferromagnetic (F) ordering, we find a metallic P3c1 phase with no charge ordering or breathing distortions at an energy of 24 meV per Fe above the CS-APAF phase, in contrast to pure CaFeO3, in which a ferromagnetic Fe 3+ /Fe 5+ insulating charge-ordered state is found as the ground state (36,37).…”

Unraveling the Suppression of Oxygen Octahedra Rotations in A3B2O7 Ruddlesden-Popper Compounds: Engineering Multiferroicity and Beyond

“…By tuning tolerance factor through substitution of [58]. Recent works have revealed the strong coupling between Jahn-Teller distortions and rotations [59][60][61][62]. Similar to the ME coupling in RP systems, notable coupling between polarization and Jahn-Teller distortions can be expected since they are controlled by the same lattice distortions, which makes SrTb 2 Fe 2 O 7 F 2 an attractive candidate to realize electric field tuning of electronic property at room temperature [61].…”