Lattice dynamics of SrFeO2.5 studied by
Ch.
1
Urban ,
1
Janson ,
1,2
Ponkratz ,
57Fe-ME
1
Kasdorf ,
S.
U.
O.
K.
1,
3
3
G. Wortmann T. Berthier , W. Paulus
and
57Fe-NIS
1
Rupprecht ,
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
1. Introduction / Aims
2. Crystal Structure
SrFeO2.5+x
Different physical and chemical properties in
dependence of oxygen content (x = 0 to 0.5).
Ordering of oxygen dislocations
pressure 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 meV
attributed 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].
4. Mössbauer Spectroscopy
Fig.4: Debye temperatures of SrFeO2.5
Fig.3: Partial phonon DOS of SrFeO2.5
at various temperatures.
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.
Isomer shift as function of temperature
Attribution of the
subspectra:
Fe
3+
T
Fe
57Fe-Mössbauerspectra
S(FeO) > S(FeT)
Larger covalency of the
FeT3+ - oxygen bonding
3+
O
• 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.
Fig.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)
Calculation of the
Debye temperatures by:
3 θ /T
D


 T 
9 kB
y3
S = Sc 

 θ D +8T 
   y dy 
18 Mc 
e -1

 θD 
0


θD (FeT ) = 469 K
Fig.6: Temperature dependence of the
isomer shifts of the different Fe sites.
θD (FeO ) = 362 K
The average of the Debye temperature for both Fe sites,
D(FeO+FeT) = 416 K) agrees very well with the high
temperature Debye temperature D,HT calculated from
the Fe partial phonon DOS.
of SrFeO2.5 at various temperatures.
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.
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)