Emergence of exotic electronic and magnetic phases upon electron filling in Na2BO3 (B = Ta, Ir, Pt, and Tl): a first-principles study

Abstract

Competition between spin–orbit interaction and electron correlations can stabilize a variety of non-trivial electronic and magnetic ground states. Using density functional theory calculations, here we show that different exotic electronic and magnetic ground states can be obtained by electron filling of the B-site cation in the Na₂BO₃ family of compounds (B = Ta, Ir, Pt and Tl). Electron filling leads to a Peierls insulator state with a direct band gap to j = 1/2 spin–orbit assisted Mott-insulator to band insulator and then to negative charge-transfer half-metal transition. The magnetic ground state also undergoes a transition from a non-magnetic state to a zigzag antiferromagnetic state, a re-entrant non-magnetic state and finally to a ferromagnetic state. The electron localization function shows a ladder type dimerization or Peierls instability in Na₂TaO₃. Maximally localized Wannier function calculations reveal delocalization of electrons through the e_g orbitals, which form a π bond, and localization of electrons through the t_2g orbitals, which form a σ bond, between the neighbouring tantalum ions. Na₂TlO₃ shows Stoner or band ferromagnetism due to the localized moments with up-spin on oxygen ligands created by the negative charge-transfer character, interacting via the down-spin itinerant electrons of the Tl 5d–O 2p hybridized band. These findings are significant for practical applications; for instance the direct band gap insulator Na₂TaO₃ shows potential for utilisation in solar cells, while Na₂TlO₃, which exhibits ferromagnetic half metallicity, holds promise for spintronic device applications.