Interplay of structural polytypes and electronic correlations in A MnO 3 manganites ( A = La, Y, Ca, Sr, Ba): limitations of screened exchange

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Abstract
The generic perovskite-like A BO 3 stoichiometry allows for tuning electronic correlations in condensed matter physics, for example A MnO 3 manganites. Manganites are particularly exotic due to partial occupancies of Mn(3 d ) orbitals, typically composed of Mn 3+ (3 d 4 ) or Mn 4+ (3 d 3 ) magnetic cations and size dependence of non-magnetic cations, A = Ca, Sr, La, Y, and Ba. The Coulomb repulsion of 3 d -electrons, is perturbed via chemical strain imposed by varying A -site radii and deformations of Mn–O polyhedrons in different crystal symmetries—orthorhombic vs hexagonal. Here, we disentangle the role of Mn-site’s electronic correlations from structural symmetry, spin-orbital ordering, [MnO n ] m − polyhedral deformations, and on-site Coulomb correction (Hubbard on-site U ) using density functional theory (DFT) calculations. The ad hoc U ( U eff ) results in identical structural descriptors before and after U correction vs the A -site radii. Overall, relative magnetic ground state stability Δ E is monotonously increasing with the A -site radii in the orthorhombic Mn 3+ manganites (towards ferro-magnetic), but decreasing (towards G-type anti-ferro order) in the less distorted class Mn 4+ manganites. The unexplained origin of the ad hoc but correct relative magnetic ground state of these manganites in experimental structures with intrinsic lattice distortions is found to be associated with cancellations of self-interaction errors and underestimations of orbital localization. We note that the trend of electronic band gaps in the Mn 3+ and Mn 4+ manganites are random in the DFT + U eff approach. An optimal on-site Hubbard U that is fast and reasonably accurate for predicting the structure and properties for manganites is proposed. The on-site U correction on the Mn(3 d ) and/or O(2 p ) orbitals against the screened hybrid functional HSE06 by comparing with the self-consistent Hubbard U i.e. U self is established, showing the limitations of screened exchange. The Γ -point Raman mode analysis (i.e. spin-phonon coupling) across various magnetic and non-magnetic ground states, we finally rationalize the accuracy of U self in A MnO 3 oxides.
Funding Information
  • Anusandhan National Research Foundation (ANRF), Science and Engineering Research Board (SERB), Govt. of India (RJF/2021/000120)

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