Hydride-Reduced Eu₂SrFe₂O₆: A T-to-T′ Conversion Enabling Fe²⁺ in Square-Planar Coordination

Abstract

Low-temperature reaction of A-site-ordered layered perovskite Eu 2 SrFe 2 O 7 (T structure) with CaH 2 induces a shift in the Eu 2 O 2 slabs to form Eu 2 SrFe 2 O 6 with a T′ structure (I4/mmm space group) in which only the Fe cation is reduced. Contrary to the previously reported T′ structures with Jahn−Teller-active d 9 cations (Cu 2+ and Ni + ), stabilization of Eu 2 SrFe 2 O 6 with the Fe 2+ (d 6 ) cation reflects the stability of the FeO 4 square-planar unit. The stability of T′-type Eu 2 SrFe 2 O 6 over a T-type polymorph is confirmed by density functional theory calculations, revealing the d z 2 occupancy for the T′ structure. Eu 2 SrFe 2 O 6 has a bilayer magnetic framework with an Fe−O−Fe superexchange J ∥ and an Fe−Fe direct exchange J ⊥ (where J ∥ > J ⊥ ), which broadly explains the observed T N of 390− 404 K. Interestingly, the magnetic moments of Eu 2 SrFe 2 O 6 lie in the ab plane, in contrast to the structurally similar Sr 3 Fe 2 O 4 Cl 2 having an out-of-plane spin alignment.

Spin–Orbital States and Strong Antiferromagnetism of Layered Eu2SrFe2O6 and Sr3Fe2O4Cl2

“…The bilayered Sr 3 B 2 O 4 Cl 2 ( B = Fe, Co) − compounds were also prepared recently. Recently, another bilayered Eu 2 SrFe 2 O 6 with ab planar magnetism was synthesized from the A -site-ordered layered perovskite Eu 2 SrFe 2 O 7 . The temperature dependence of the Mossbauer spectrum and neutron powder diffraction data indicated the onset of G-AF (both intra- and interplane AF couplings) magnetic order at a Néel temperature T N ≈ 390–404 K for Eu 2 SrFe 2 O 6 with a HS Fe 2+ configuration.…”

“…From the above GGA+SOC +U result of the easy in-plane magnetization with the anisotropy energy of 2.8 meV/Fe, D = −0.70 meV can be derived. Using the J ab , J c , and D parameters, our Monte Carlo simulations of the magnetic specific heat give T N = 420 K for Eu 2 SrFe 2 O 6 , see Figure 6, and it is close to the experimental magnetic specific heat of about 400 K. 9 Moreover, to imitate the magnetization (the spontaneous magnetic response in FM materials) for the AF Eu 2 SrFe 2 O 6 , we calculate the AF order parameter 18 (the staggered magnetization) 6 The elongated FeO 4 Cl pyramid gives a level sequence similar to the case in Eu 2 SrFe 2 O 6 , see Figures 1 and 2. Indeed, we verify this picture by the spin-polarized GGA calculations, see Figures 3 and 7 for a comparison.…”

“…Density functional theory (DFT) calculations were performed by the full-potential augmented plane waves plus local orbital code (Wien2k) . The optimized lattice parameters of Eu 2 SrFe 2 O 6 (Sr 3 Fe 2 O 4 Cl 2 ) are a = b = 3.89 Å and c = 18.80 Å ( a = b = 3.96 Å and c = 22.48 Å), which are close to the experimental values of a = b = 3.96 Å and c = 19.01 Å ( a = b = 4.01 Å and c = 22.65 Å). , A a × b × c supercell is used to treat the intralayer AF configuration. The muffin-tin sphere radii are chosen to be 2.5, 2.0, 1.8, and 1.5 bohr for the Eu/Sr, Fe, Cl, and O atoms, respectively.…”

“…This inplane orbital moment favors the in-plane magnetization with a magnetic anisotropy energy of 2.8 meV/Fe. This in-plane magnetization is in good agreement with both the experiment results 9 and the prediction by the selection rule based on the SOC-induced HOMO−LUMO interaction. 17 In contrast, we also obtain the d 5↑ L z+ ↓ state as seen in Table 1 and Figure 4b.…”

“…Recently, another bilayered Eu 2 SrFe 2 O 6 with ab planar magnetism was synthesized from the A-site-ordered layered perovskite Eu 2 SrFe 2 O 7 . 9 The temperature dependence of the Mossbauer spectrum and neutron powder diffraction data indicated the onset of G-AF (both intra-and interplane AF couplings) magnetic order at a Neél temperature T N ≈ 390−404 K for Eu 2 SrFe 2 O 6 with a HS Fe 2+ configuration. However, with the same HS Fe 2+ configuration and a G-AF magnetic order below T N = 378 K, Sr 3 Fe 2 O 4 Cl 2 was reported to have c axis magnetization.…”