DSC of valence-mixing in YBaFeIIFeIIIO5+w with minimum w and varied thermal history.
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Mössbauer accounting of AFM Fe states upon 3 spin- and charge-ordering transitions.
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Mössbauer accounting of the Fe minority-spin electron (mse) d-orbital occupancies.
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Thermally induced valence mixing as two OD steps for the mse: dxz to dx2-y2 to .
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Paramagnetic iron states identified as domain walls and anti-phase boundaries.
Abstract
Thermal evolution of the – 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 , and either (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 orbitals of , 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 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 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.