Figure 2: raw absorption spectra for all the samples: EXAFS region. In red is the Fourier filtered nearest neighbour signals. XANES features reflects the local structure of the absorbing ions as well its chemical state. XANES data at the Fe K edge were fitted using a linear combination of XANES spectra of the end compounds: Best fit results demonstrate that the Fe is an inhomogeneous phase: in all the doped samples (0<x<1) most of Fe ions (~80%) have local environment/chemical state close to that of Sr 2 FeMoO 6. Only a minority amount of Fe ions present local structure/chemical state similar to that of Sr 2 FeWO 6. Mo k-edge Fe k-edge Sr k-edgeW L III -edge Table (2) Fe-O bond length distribution from EXAFS data analysis. Fe-O distribution is similar in all the x>0 samples, quite different from the Fe-O distribution in x=0 sample. This confirms the primary role of the electronic and local structure of the Fe ions in determining the MI transition. Fitting the Sr, Mo and W EXAFS data confirms the previous qualitative observation: only the local structure around Fe changes significantly as a function of composition. Local Structure of Sr 2 FeMo x W 1-x O 6 double perovskites studied by XAFS F. Bardelli 1,2, C. Meneghini 1,2, S. Mobilio 2,3, D.D. Sarma 4,5 Recently, the double perovskite system, Sr 2 FeMo x W 1-x O 6, have attracted the interest of the solid state scientist because of its remarkable magnetoresistive properties. Sr 2 FeMoO 6 : Sr 2 FeMoO 6 : + The ordered undoped end compound Sr 2 FeMoO 6 ( x = 1 ) has alternating occupancy of Fe 2 and Fe 3+ ions at the B sites of the double perovskite A 2 BO 6 structure, where A and B represents two cations. The interest in this compound derives from the rather high temperature of their ferrimagnetic transition (~ 450 K). In this phase it is believed to be ordered with each B site sublattice of Fe 3+ and Mo 5+ arranged ferromagnetically, while the two sublattices are coupled to each other antiferromagnetically. It has been suggested that high structural order in Fe 3+ and Mo 5+ occupancy leads to a half metallic ferromagnetic state (HMFM), with only minority spins carriers present at the Fermi level. This compound exhibits a large negative magnetoresistance (MR) at 5 K as well at 300 K. Such large magnetoresistance is believed to be driven by the complete spin polarization at the Fermi energy in the HMFM state. Sr 2 FeWO 6 : Sr 2 FeWO 6 : The other end compound, Sr 2 FeWO 6 (x = 1), belongs to the same double perovskite family but shows very different electrical transport and magnetic properties. It is insulating throughout the temperature range with an antiferromagnetic ordering below 37 K. INTRODUCTION The Sr 2 FeMo x W 1-x O 6 series: Sr 2 FeMoO 6 x = 1 Half Metallic Ferromagnetic Sr 2 FeWO 6 x = 0 InsulatingAntiferromagneticx Mo doping SrMoFeO The figure depicts the crystallographic structure of the ordered undoped end compound Sr 2 FeMoO 6. It has alternating occupancy of Fe 2+ and Fe 3+ end compound Sr 2 FeMoO 6. It has alternating occupancy of Fe 2+ and Fe 3+ ions at the B sites of the double perovskite A 2 BO 6 structure, where A ions at the B sites of the double perovskite A 2 BO 6 structure, where A and B represents two cations. The W ions are substitutional to Mo ones. Crystallographic structure of the Sr 2 FeMoO 6 end compound the Sr 2 FeMoO 6 end compound EXAFS signal extracted from the absorption using standard procedures. The Fourier filtered nearest neighbour signals (A-oxygen signals) were analyzed using the standard EXAFS formula [8,9,10]. Figure (4b): values of  and 1-  obtained by XANES fitting XANES results References X-ray absorption spectroscopy (XAS) experiments were performed at the GILDA (BM08) beam line [7] at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. High quality XAS spectra at the Fe (7112 eV), Mo (20000 eV) and Sr (17998 eV) K edges and at W L III edge (10207 eV) have been collected in transmission mode. X-ray absorption spectroscopy (XAS) experiments were performed at the GILDA (BM08) beam line [7] at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. High quality XAS spectra at the Fe (7112 eV), Mo (20000 eV) and Sr (17998 eV) K edges and at W L III edge (10207 eV) have been collected in transmission mode. XAFS measurements (1) (3) Figure 3: Fourier transforms of the EXAFS signals. The marked windows are the back Fourier transform limits. Mo k-edgeFe k-edge Sr k-edgeW L III -edge Doping-induced Metal to Insulator Transition (MIT), why? Since Sr 2 FeMoO 6 and Sr 2 FeWO 6 have contrasting transport properties, it is expected that an alloy system of these compounds, such as the Sr 2 FeMo x W 1-x O 6 series, would exhibit a metal insulator transition (MIT) as a function of x. Two models have been proposed to explain the MIT. The aim of this work is mainly to try to discern between those two scenarios. 1. Valence Transition In this scenario [6], Fe is believed to be in the 3 + state in the metallic range (x C ~ 0.25) and Mo and W in the 5 + state; one itinerant electron is provided by each Mo 5+ (4d 1 ) and W 5+ (5d 1 ) sites. For x < x C, Fe transforms into the 2 + state converting Mo and W into the 6+ state (W, 5d 0 ). This give rise to the insulating behavior since, for the 5d 0 electronic configuration of the W 6+ sites, the Fe 2+ ions couple antiferromagnetically with each other via the Fe-O-W-O-Fe type super exchange interaction. In this scenario a valence transition is the driving force for the MIT. In a more recent work [1], it has been proposed that Mo remains in the 5 + state in the entire composition range while W in the 6 + state. This implies that there is no valence transition of Mo and W ions at the critical concentration. The substitution of W 6+ in place of Mo 5+ requires the transformation of Fe 3+ into Fe 2+ for charge neutrality. Thus the system is view as an inhomogeneous distribution of metallic Sr 2 FeMoO 6 and insulating Sr 2 FeWO 6 parts; in this scenario the MIT at the critical composition is driven by the percolation threshold of the system. Above x C the system is in a macroscopically metallic ferromagnetic state of Sr 2 FeMoO 6 with a distribution of small clusters of insulating antiferromagnetic Sr 2 FeWO 6. At higher W doping, the insulating Sr 2 FeWO 6 clusters grow in size, eventually engulfing the metallic Sr 2 FeMoO 6 clusters for x < 0.3, giving rise to the observed MIT. 2. Percolative Transition ? Figure 1: raw absorption spectra for all the samples: XANES region. Sr 2 FeMo x W 1-x O 6, (2) The XAFS data at Fe K edge show large differences among x = 0 sample and x>0.3 compounds. On the contrary XAFS data at Mo, Sr, W edges depict negligible differences as a function of x M EXAFS results Figure (4a): example of AXNES fitting. 1 Dip. Di Fisica Univ. Di “Roma Tre”, Via della Vasca Navale 84, I Roma Italy, 2 INFM-GILDA c/o ESRF Grenoble France, 3 INFN Laboratori Nazionali di Frascati Italy, 4 Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore India, 5 Jawaharlal Nehru Centre for Advanced Research, Bangalore India 0 Fe k-edge Mo k-edge Sr k-edgeW L III -edge Discussion & Conclusions 1) Our findings strongly excludes the valence transition scenario since XAS data of W and Mo does not change as a function of composition. 2) On the other hand to interpret the XAFS results within the percolative scenario requires some care: a) Assuming that the XANES region describes the valence state of absorbing ion, the fitting lead to the following formula unit: Sr 2 Fe  3+ Mo x 5+ Fe (1-  2+ W (1-x) 6+ O 6 2- which shows an excess of positive charge  -x, that, in x = 0.3 sample, reach the ~0.5e per formula unit. This could imply some compositional effects, such as an excess of Oxygen or a cation deficiency, to be considered to restore the charge neutrality. This would suggest some other physics to be included as for vacancy doped manganese perovskite compounds (LaMnO 3+  ). b) If the XANES signal mainly reflects the local structure around Fe, our data suggest that, in the composition range 0.3  x  1, most (>80%) of the Fe ions preserve a local structure similar to that in Sr 2 Fe MoO 6, in good agreement with the percolative hypothesis.