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

• Published in: Journal of solid state chemistry Vol. 252; pp. 119 - 128
• Main Authors: Lindén, J., Lindroos, F., Karen, P.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.08.2017; Elsevier
• Subjects: Charge ordering; Hyperfine parameters; Mixed valence; Mössbauer spectroscopy; Planar defects; Valence mixing; Hyperfine parameters; Mixed valence; Charge ordering; Valence mixing; Planar defects; Mössbauer spectroscopy
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2017.04.036

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 dxz, dz2 and either dx2−y2 (Main Article) or dxy (Supplement) populations of this electron upon conserving its angular momentum. At low temperatures, the minority-spin electrons fill up the ordered dxz 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 dx2−y2/dxy 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 dz2 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 and Verwey-transition temperatures, respectively. [Display omitted] •DSC of valence-mixing in YBaFeIIFeIIIO5+w with minimum w and varied thermal history.•Mössbauer accounting of AFM Fe states upon 3 spin- and charge-ordering transitions.•Mössbauer accounting of the Fe minority-spin electron (mse) d-orbital occupancies.•Thermally induced valence mixing as two OD steps for the mse: dxz to dx2-y2 to dz2.•Paramagnetic iron states identified as domain walls and anti-phase boundaries.