Hyperfine Interact
DOI 10.1007/s10751-013-0973-8
Iron orbital occupancies upon valence mixing
of charge-ordered GdBaFe
II
Fe
III
O
5
J. Lind´ en · F. Lindroos · P. Karen
© Springer Science+Business Media Dordrecht 2013
Abstract M¨ ossbauer spectra of charge-ordered/valence-mixed GdBaFe
2
O
5
have been ana-
lyzed in order to determine the orbital occupancies of the minority-spin d electron as a
function of temperature. In the charge-ordered state below the Verwey temperature T
V
, it
occupies the Fe
2+
d
xz
orbital, which is ordered. Above T
V
, the electron partially occurs
also at the trivalent iron and spreads over two additional orbitals, d
x
2
−y
2 and d
z
2 . In the
valence-mixing state (above T
p
) also orbital d
yz
participates. The study illustrates how
the M¨ ossbauer electric-field gradient, center shift and internal field jointly reflect the
distribution of the minority-spin electron culprit behind the Verwey transition.
Keywords
57
Fe M¨ ossbauer spectroscopy · Charge ordering · Valence mixing · Mixed
valence · Double perovskites
1 Introduction
The antiferromagnetic A-site ordered perovskite RBaFe
2
O
5
(R is a rare-earth element)
contains Fe
2+
and Fe
3+
that order at low temperatures and mix at high temperatures [1].
The valence mixing proceeds in two steps [2], where the first one, at the lower transi-
tion temperature T
V
, has all features of the Verwey transition [3] in magnetite. Above the
second transition temperature T
p
, iron is in a single valence state Fe
2.5+
. At low tem-
peratures, a long-range order of alternating chains of di- and trivalent iron coordinations
along b (Fig. 1) is observed by neutron- [2] and synchrotron X-ray powder diffraction [4].
Proceedings of the 32nd International Conference on the Applications of the M ¨ ossbauer Effect
(ICAME 2013) held in Opatija, Croatia, 1–6 September 2013
J. Lind´ en () · F. Lindroos
Physics Department,
˚
Abo Akademi University, 20500 Turku, Finland
e-mail: jlinden@abo.fi
P. Karen
Department of Chemistry, University of Oslo, 0315 Oslo, Norway