Hydride-Reduced Eu 2 SrFe 2 O 6 : A T-to-T' Conversion Enabling Fe 2+ in Square-Planar Coordination.

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 ( I 4/ 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.