1. Ch. Urban 1, S. Janson 1, U. Ponkratz 1,2, O. Kasdorf 1, K. Rupprecht 1, G. Wortmann 1, T. Berthier 3, W. Paulus 3 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 Lattice dynamics of SrFeO 2.5 studied by 57 Fe-ME and 57 Fe-NIS SrFeO 2.5 Crystallizes in tetragonal Brownmillerite structure with two different iron sites: FeO 4 tetrahedrons and FeO 6 octahedrons With ratio Fe T : Fe O = 1 : 1 The FeO 4 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]. 3. Nuclear Inelastic Scattering (NIS) of Synchrotron Radiation g(E) Soft Mode at 7 meV attributed to collective motion of the FeO 4 -tetrahedrons. Strong deviations from Deye-like behaviour, i.e. g(E) is not proportional to E 2  strong broadening of spectral features with increasing temperature seemingly connected with increasing disorder. 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) 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 Attribution of the subspectra: 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 T N = 705 K. Isomer shift as function of temperature S(Fe O ) > S(Fe T ) Larger covalency of the Fe T 3+ - oxygen bonding Calculation of the Debye temperatures by: The average of the Debye temperature for both Fe sites,  D (Fe O +Fe T ) = 416 K) agrees very well with the high temperature Debye temperature  D,HT calculated from the Fe partial phonon DOS. 2. Crystal Structure 1. Introduction / Aims SrFeO 2.5+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 57 Fe sites by nuclear Inelastic scattering (NIS) of synchrotron radiation and, complementary, by 57 Fe-Mössbauer effect (ME). Fig.1: (left) Crystal structure of SrFeO 2.5 Fig.2: 57 Fe-NIS spectra of SrFeO 2.5 at various temperatures. Fig.4: Debye temperatures of SrFeO 2.5 Fig.3: Partial phonon DOS of SrFeO 2.5 at various temperatures. Fig.5: 57 Fe-Mössbauerspectra of SrFeO 2.5 at various temperatures. Fig.6: Temperature dependence of the isomer shifts of the different Fe sites. 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 V zz with  O = 82.5° and  T = 77.7° with respect to the magnetic hyperfine field. 5. Conclusion Combined study by NIS and ME on SrFeO 2.5 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 CaFeO 2.5, 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 SrFeO 2.5 to collective motions of the FeO 4 tetrahedrons reflected by the soft mode at 7 meV. [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) References