Mixed Valence in YBaFe₂O₅

YBaFe2O5 has been synthesized by heating a nanoscale citrate precursor in a carefully controlled reducing environment. Successful synthesis of a single-phase sample can only be achieved in a narrow window of oxygen partial pressures and temperatures. YBaFe2O5 adopts an oxygen-deficient perovskite-type structure, which contains double layers of corner sharing FeO5 square pyramids separated by Y3+ ions. At TN ≅ 430 K, tetragonal (P4/mmm) and paramagnetic YBaFe2O5 orders antiferromagnetically (AFM) experiencing a slight orthorhombic distortion (Pmmm). Around this temperature, it can be characterized as a class-III mixed valence (MV) compound, where all iron atoms exist as equivalent MV Fe2.5+ ions. The magnetic structure is characterized by AFM Fe−O−Fe superexchange coupling within the double layers and a ferromagnetic Fe−Fe direct-exchange coupling between neighboring double layers. Upon cooling below ∼335 K, a premonitory charge ordering (2Fe2.5+ → Fe2.5+δ + Fe2.5-δ) into a class-II MV phase takes place. This transition is detected by differential scanning calorimetry, but powder diffraction techniques fail to detect any volume change or a long-range structural order. At ∼308 K, a complete charge ordering (2Fe2.5+ → Fe2+ + Fe3+) into a class-I MV compound takes place. This charge localization triggers a number of changes in the crystal, magnetic, and electronic structure of YBaFe2O5. The magnetic structure rearranges to a G-type AFM structure, where both the Fe−O−Fe superexchange and the Fe−Fe direct-exchange couplings are antiferromagnetic. The crystal structure rearranges (Pmma) to accommodate alternating chains of Fe2+ and Fe3+ running along b and an unexpectedly large cooperative Jahn−Teller distortion about the high-spin Fe2+ ions. This order of charges does not fulfill the Anderson condition, and it rather corresponds to an ordering of doubly occupied Fe2+ dxz orbitals. Comparisons with YBaMn2O5 and YBaCo2O5 are made to highlight the impact of changing the d-electron count.