Jahn-Teller distortion in Sr2FeO4: group-theoretical analysis and hybrid DFT calculations

We present theoretical justification for distorted Ruddlesden-Popper (RP) phases of the first-order by using hybrid density functional theory (DFT) calculations and group-theoretical analysis. We, thus, demonstrate the existence of the Jahn-Teller effect around an Fe4+ ion in Sr2FeO4. On the calculation side, we have established a combination of Wu-Cohen (WC) exchange and Perdew-Wang (PW) correlation in a three-parameter functional WC3PW, giving the most accurate description of Sr2FeO4 from the comparison of three hybrid DFT functionals. Self-consistently obtained Hartree- Fock exact exchange of 0.16 demonstrates consistent results with the experimental literature data. Importantly, we explain conditions for co-existing proper and pseudo-Jahn-Teller effects from the crystalline orbitals, symmetry-mode analysis and irreps products. Moreover, phonon frequency calculations support and confirm the results of symmetry-mode analysis. In particular, the symmetry mode analysis identifies a dominating irreducible representation of the Jahn-Teller mode (X2+) and corresponding space group (SG) of ground state structure (SG Cmce model). Therefore, the usually suggested high-symmetry tetragonal crystal structure (SG I4/mmm model) is higher in energy by 121 meV/f.u. (equivalent to the Jahn-Teller stabilization energy) compared with the distorted low-symmetry structure (SG Cmce model). We also present diffraction patterns for the two crystal symmetries to discuss the differences. Therefore, our results shed light on the existence of low symmetry RP phases and make possible direct comparisons with future experiments.

Automated summary

We present a theoretical justification for the distorted Ruddlesden-Popper (RP) phases through hybrid density functional theory (DFT) calculations and group-theoretical analysis. We demonstrate the existence of the Jahn-Teller effect in Sr2FeO4. Through comparison of three hybrid DFT functionals, we have established a three-parameter functional WC3PW, which accurately describes Sr2FeO4. We explain the conditions for co-existing proper and pseudo-Jahn-Teller effects using crystalline orbitals, symmetry-mode analysis, and irreps products. Phonon frequency calculations support and confirm the results of symmetry-mode analysis. Additionally, we provide diffraction patterns to discuss the differences between the two crystal symmetries.