Elsevier

Journal of Solid State Chemistry

Volume 252, August 2017, Pages 119-128
Journal of Solid State Chemistry

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

https://doi.org/10.1016/j.jssc.2017.04.036Get rights and content
Under a Creative Commons license
open access

Highlights

  • 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.

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 dx2y2 (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 dx2y2/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.

Keywords

Charge ordering
Valence mixing
Planar defects
Mixed valence
Mössbauer spectroscopy
Hyperfine parameters

Cited by (0)