Orbital occupancy evolution across spin- and charge-ordering transitions in YBaFe₂O₅

Thermal evolution of the Fe²⁺ – Fe³⁺  valence mixing in YBaFe₂O₅ is investigated using Mössbauer spectroscopy. In this high-spin double-cell perovskite, the d⁶ and d⁵ Fe states differ by the single minority-spin electron which then controls all the spin- and charge-ordering transitions. Orbital occupancies can be extracted from the spectra in terms of the d_xz , d_z2 and either d_{x2− y2} (Main Article) or d_xy (Supplement) populations of this electron upon conserving its angular momentum. At low temperatures, the minority-spin electrons fill up the ordered d_xz orbitals of Fe²⁺, in agreement with the considerable orthorhombic distortion of the structure. Heating through the Verwey transition supplies 93% of the mixing entropy, at which point the predominantly mixing electron occupies mainly the d_{x2− y2}/d_xy orbitals weakly bonding the two Fe atoms that face each other across the bases of their coordination pyramids. This might stabilize a weak coulombic checkerboard order suggested by McQueeney et alii in Phys. Rev. B 87(2013)045127. When the remaining 7% of entropy is supplied at a subsequent transition, the mixing electron couples the two Fe atoms predominantly via their d_z2 orbitals. The valence mixing concerns more than 95% of the Fe atoms present in the crystalline solid; the rest is semi-quantitatively interpreted as domain walls and antiphase boundaries formed upon cooling through the Néel andVerwey-transition temperatures, respectively.