Struktur von SrFeO2.5

1
Lattice dynamics of SrFeO2.5 studied by 57Fe-ME
and 57Fe-NIS
Ch. Urban1, S. Janson1, U. Ponkratz1,2, O.
Kasdorf1, K. Rupprecht1, G. Wortmann1, T.
Berthier3, W. Paulus3
1 Universität Paderborn, Department Physik, 33095
Paderborn, GERMANY 2 ESRF, 6 rue Jules Horowitz,
38043 Grenoble, FRANCE 3 Universitè Rennes,
LCSIM, UMR 6511, F-35042 Rennes, FRANCE
1. Introduction / Aims
2. Crystal Structure
SrFeO2.5x Different physical and chemical
properties in dependence of oxygen content (x
0 to 0.5). Ordering of oxygen dislocations pressur
e and temperature induced phase
transitions Oxygen diffusion already at room
temperature with possible technical application,
e.g. for fuel cells. Here we study the lattice
dynamics at the 57Fe sites by nuclear Inelastic
scattering (NIS) of synchrotron radiation and,
complementary, by 57Fe-Mössbauer effect (ME).
SrFeO2.5 Crystallizes in tetragonal
Brownmillerite structure with two different iron
sites FeO4 tetrahedrons and FeO6
octahedrons With ratio FeT FeO 1 1 The FeO4
tetrahedrons are forming plains containing 1D
channels of oxygen vacancies. Structural disorder
in these plains and chains is attributed to the
high oxygen mobility 1, 2.
Fig.1 (left) Crystal structure of SrFeO2.5
3. Nuclear Inelastic Scattering (NIS) of
Synchrotron Radiation
Soft Mode at 7 meVattributed to collective
motion of the FeO4-tetrahedrons. Strong
deviations from Deye-like behaviour, i.e. g(E) is
not proportional to E2 ? strong broadening of
spectral features with increasing temperature
seemingly connected with increasing disorder.
g(E)
Fig.2 57Fe-NIS spectra of SrFeO2.5 at various
temperatures.
NIS experiments were carried out at beamline ID18
at ESRF (Grenoble) with an energy resolution of
3 meV. Data were recorded for various
temperatures between 15 K and 500 K (see Fig.
2). The derived partial phonon density-of-states
(DOS) at the Fe sites are shown in Fig. 3. The
spectral features of the DOS at 300 K as well as
the derived parameters agree well with Ref. 3.
Fig.4 Debye temperatures of SrFeO2.5
Strong difference between Debye temperatures
determined by the initial slop of the phonon DOS
(low temperature Debye temperature ?D,LT 250
K) and by integrating the whole phonon DOS (high
temperature Debye temperature ?D,HT 425 K)
Fig.3 Partial phonon DOS of SrFeO2.5 at various
temperatures.
4. Mössbauer Spectroscopy
Isomer shift as function of temperature
Mössbauer absorption spectroscopy was carried out
at Paderborn University Absorption spectra were
recorded for various temperatures between 4 K
and 935 K, the magnetic ordering temperature is
TN 705 K.
Attribution of the subspectra
S(FeO) gt S(FeT)
Larger covalency of the FeT3 - oxygen bonding

• Additional structures above 700 K can be
  attributed to oxygen vacancies and formation of
  metallic iron.
• Detailed analysis of combined magnetic dipol /
  electric quadrupole interaction reveals tilting
  angels of Vzz with ?O 82.5 and ?T 77.7 with
  respect to the magnetic hyperfine field.

Calculation of the Debye temperatures by
Fig.6 Temperature dependence of the isomer
shifts of the different Fe sites.
The average of the Debye temperature for both Fe
sites, ?D(FeOFeT) 416 K) agrees very well
with the high temperature Debye temperature
?D,HT calculated from the Fe partial phonon DOS.
Fig.5 57Fe-Mössbauerspectra of SrFeO2.5 at
various temperatures.
References
1 P. Bezdicka et al., Z. anorg. allg. Chem.
619, 1 (1993) F. Girgsdies, R. Schöllhorn,
Solid State Commun. 91, 111 (1994) R.
Le Toquin et al., (University of Rennes),
unpublished 2 A.I. Rykov et al., Physica B 350,
287 (2004) 3 W. Sturhahn and A.I. Chumakov,
Hyp. Interact. 123/13234, 809 (1999) 4
Ch. Urban, Diploma thesis (Paderborn 2005) 5 O.
Kasdorf, Bachelor thesis (Paderborn 2005)
5. Conclusion
Combined study by NIS and ME on SrFeO2.5 delivers
a detailed picture of the lattice dynamics, where
ME provides a site selective analysis. From
similar NIS 2 and ME studies 4, 5 of
CaFeO2.5, which has also the Brownmillerite
structure, but without disorder of
the tetrahedral sites and without a high oxygen
mobility, and which does not show the soft mode
peak at 7 meV 2, we attribute the high oxygen
mobility in SrFeO2.5 to collective motions of the
FeO4 tetrahedrons reflected by the soft mode at 7
meV.