Hydride-Reduced EuSrFeO: A T-to-T' Conversion Enabling Fe in Square-Planar Coordination.

Low-temperature reaction of A-site-ordered layered perovskite EuSrFeO (T structure) with CaH induces a shift in the EuO slabs to form EuSrFeO with a T' structure (4/ space group) in which only the Fe cation is reduced. Contrary to the previously reported T' structures with Jahn-Teller-active d cations (Cu and Ni), stabilization of EuSrFeO with the Fe (d) cation reflects the stability of the FeO square-planar unit. The stability of T'-type EuSrFeO over a T-type polymorph is confirmed by density functional theory calculations, revealing the d occupancy for the T' structure. EuSrFeO has a bilayer magnetic framework with an Fe-O-Fe superexchange and an Fe-Fe direct exchange (where > ), which broadly explains the observed of 390-404 K. Interestingly, the magnetic moments of EuSrFeO lie in the plane, in contrast to the structurally similar SrFeOCl having an out-of-plane spin alignment.