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Abstract
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.
Highlights
Phase transitions from charge-ordered to valence-mixed states are behind electronic properties such as the colossal magnetoresistance in manganites [1,2] or spin-polarized conduction in magnetite [3,4,5]
The pellets of ∼20% porosity were used for quenching [11,22] from high-temperature equilibrium at 1000 °C in flowing atmospheres of O2 partial pressures set in the interval −15.66 < log(pO2 /bar) < −14.10 to obtain YBaFe2O5+w of low nonstoichiometry w or a product reduced to metallic Fe
As inferred in Ref. [11] from the crystal structure of chargeordered YBaFe2O5, the minority-spin electron at low temperatures occupies the dxz orbitals which stack along the strings of Fe2+ that form a checkerboard with strings of Fe3+ (Fig. 4) in a lattice contracted along the string direction
Summary
Phase transitions from charge-ordered to valence-mixed states are behind electronic properties such as the colossal magnetoresistance in manganites [1,2] or spin-polarized conduction in magnetite [3,4,5]. Its valence-mixed phase crystallizes as a double-cell perovskite of Pmmm symmetry, with Y and Ba atoms ordered into layers [11,12]. The Y layer, free of oxygen atoms, leaves the two Fe2.5+ in squarepyramidal coordinations facing each other as a mirror image. These two high-spin iron atoms couple ferromagnetically (FM) in an overall antiferromagnetic (AFM) order of spins along a slightly contracted b (as opposed to a) of the perovskite cell [11,13]. In the charge-ordered YBaFe2O5, these two Fe atoms couple AFM in a magnetic structure of the Wollan–Koehler [14] G-type. Powder-diffraction methods “see” that the two phases coexist as they convert to each other, and this defines their Verwey-type phase transition as discontinuous
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