Across the Structural Re-Entrant Transition in BaFe₂(PO₄)₂: Influence of the Two-Dimensional Ferromagnetism

Abstract

BaFe2(PO4)2 was recently prepared by hydrothermal synthesis and identified as the first two-dimensional (2D) Ising ferromagnetic oxide, in which honeycomb layers made up of edge-sharing FeO6 octahedra containing high-spin Fe(2+) ions (S = 2) are isolated by PO4 groups and Ba(2+) cations. BaFe2(PO4)2 has a trigonal R-3 structure at room temperature but adopts a triclinic P-1 structure below 140 K due to the Jahn-Teller (JT) instability arising from the (t2g)(4)(eg)(2) configuration. The triclinic crystal structure was refined to find significantly distorted Fe(2+)O6 octahedra in the honeycomb layers while the distortion amplitude QJT was estimated to 0.019 Å. The JT stabilization energy is estimated to be ∼7 meV per formula unit by DFT calculations. Below ∼70 K, very close to the ferromagnetic transition temperature Tc = 65.5 K, the structure of BaFe2(PO4)2 returns to a trigonal R-3 structure in the presence of significant ferromagnetic domains. This rare re-entrant structural transition is accompanied by a discontinuous change in the quadrupolar splitting of Fe(2+), as determined by Mössbauer spectroscopy. EPR measurements show the presence of magnetic domains well above Tc , as expected for a ferromagnetic 2D Ising system, and support that the magnetism of BaFe2(PO4)2 is uniaxial (g⊥ = 0).

Polarized Light Microscopy Study on the Reentrant Phase Transition in a (Ba1 – xKx)Fe2As2 Single Crystal with x = 0.24

“…The fact that the interplay between magnetic and lattice structures leads to reentrant phase transition is manifested in other compounds. For example, BaFe2(PO4)2 undergoes a rare reentrant structural transition owing to the competition between uniaxial magnetism and the Jahn-Teller distortion [25,26].…”

Tuning ferromagneticBaFe2(PO4)2through a high Chern number topological phase

“…Due to a substantial interlayer separation of the Fe layers separated by Ba 2+ and (PO 4 ) 3− insulating layers, BFPO is 2D electronically. BFPO was synthesized by Mentré and coworkers [22,23], who identified the rare 2D Ising FM nature with Curie temperature T C = 65 K. The very large calculated magnetocrystalline anisotropy is related to the large orbital moment. Our previous study [21] provided the relaxed atomic positions and revealed large exchange splitting ∆ ex =3 eV of the d 6 ion, enforcing the high spin S = 2 configuration with its filled and inactive majority d orbitals.…”

Reversible Exsolution of Nanometric Fe2O3 Particles in BaFe2–x(PO4)2 (0 ≤ x ≤ 2/3): The Logic of Vacancy Ordering in Novel Metal-Depleted Two-Dimensional Lattices

“…This separation length is such that this compound was identified as the unique "oxide" behaving as a genuine twodimensional Ising ferromagnetic phase with Tc = 65.5 K 19 . In addition, it shows a re-entrant rhombohedral ଵସ ሱۛۛሮ triclinic ሱۛሮ rhombohedral transition, driven by the competition between Jahn-Teller instability and stabilization of the system by the magnetic ordering 13 . At high temperature a reversible Fe-exsolution was recently demonstrated.…”