Journal of Radioanalytical and Nuclear Chemistry, Vol. 266, No.3 (2005) 543–551 0236–5731/USD 20.00 Akadémiai Kiadó, Budapest © 2005 Akadémiai Kiadó, Budapest Springer, Dordrecht Mössbauer study on the CMR double perovskite AFe 0.5 Mo 0.5 O 3 with A=(Ba, Sr) or (Sr, Ca): Chemical pressure effect K. Nomura, 1 A. Rykov, 1 T. Yamakoshi, 1 M. Katada, 2 T. Mitsui, 3 Z. Homonnay, 4 A. Vértes 4 1 School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8659, Japan 2 School of Science, Tokyo Metropolitan University, 1-1 Minamiohsawa, Hachioji, Tokyo, Japan 3 Japan Atomic Energy Research Institute,1-1-1 Kouto, Mikazuki, Sayo, Hyogo 679-5198, Japan 4 Department of Nuclear Chemistry, Eötvös Loránd University, Pázmány P. s. 1/A, Budapest, Hungary (Received January 27, 2005) The double perovskites, AFe 0.5 Mo 0.5 O 3 with A = (Ba,Sr) or (Sr,Ca), were prepared by a sol-gel method, and the substitution effect at site A was studied by Mössbauer spectrometry. In the Mössbauer spectra of the double perovskite (Ba, Sr)Fe 0.5 Mo 0.5 O 3 , the isomer shifts decreased from δ = 0.72 mm/s to δ = 0.4 mm/s and the internal magnetic fields increased with the increase of the Sr content. The Ba-rich samples were shown to contain superparamagnetic components under the same preparation conditions. Better crystallinity and larger hyperfine fields were obtained when 5% of the Sr-content of SrFe 0.5 Mo 0.5 O 3 was substituted by Ca as compared with substitution by Ba. Phonon density of states (DOS) of SrFe 0.5 Mo 0.5 O 3 substituted with Ca or Ba were obtained by nuclear inelastic scattering. The peaks of phonon DOS were shifted, depending on chemical compression/expansion of the lattice. The chemical pressure effect could be observed in the Mössbauer spectra and the phonon DOS spectra. Introduction The double perovskite SrFe 0.5 Mo 0.5 O 3 shows negative colossal magnetoresistance (CMR) in low fields even at room temperature, 1 and has recently attracted considerable attention as compared to the perovskite-like manganites. The CMR effect increases by substituting 5% of the Sr sites in SrFe 0.5 Mo 0.5 O 3 by Ca 2+ and decreases abruptly if the substitution level exceeds 5%. 2 An increase of the CMR effect was also found when 20% of the Ba 2+ sites was substituted by Sr 2+ in BaFe 0.5 Mo 0.5 O 3 . 2 The transport properties were found to be determined by spin-polarized tunneling through insulating grain boundaries. 3 From the transport properties of Sr 2 FeMoO 6 , it was concluded that SrMoO 4 impurity at the grain boundaries plays a key role in determining the magnetoresistance of Sr 2 FeMoO 6 . 4 The pure Ca(Fe 0.5 Mo 0.5 )O 3 compound is monoclinic, while the crystal structures of Ca 1–x Sr x (Fe 0.5 Mo 0.5 )O 3 ranges from orthorhombic (0<x≤0.3) to tetragonal (x>0.3). The saturation magnetization observed for Ca 2 FeMoO 6 was 3.1 μ B , and values below 4 μ B are expected on the basis of spin-only contributions of Fe 3+ (high spin 3d 5 ; S = 5/2) and Mo 5+ (4d 1 ; S = 1/2) in antiferromagnetic alignment. These experimentally observed magnetiz- ation values are low due to the effect of order degree and valence fluctuation. 5 A 2 FeMoO 6 consists of alternating FeO 6 and MoO 6 octahedra. In the localized-spin model, the high T c is attributed to a large super-exchange coupling between Fe and Mo ions. The localized-spin model, however, contradicts the metallic nature of single crystalline Sr 2 FeMoO 6 . 6 Thus the origin of high T c is not yet clear. Neither Jahn-Teller lattice polaron formation nor the double exchange mechanism is held in the actual conductivity picture for the (Fe,Mo)-based double perovskites. 7 The physical pressure effect on transport and magnetic properties of A 2 FeMoO 6 (A = Ba, Sr) has also been investigated. 8 By applying pressure, the magnetic transition temperature is slightly enhanced. The saturation magnetization does not exhibit the pressure dependence. Mössbauer spectroscopy is a unique and powerful tool for the investigation of local electronic density and magnetic fields. Several Mössbauer studies were carried out on the double perovskite AFeMoO 3 (A = Ca, Sr, or Ba), 9–13 however, the problem of the chemical pressure effect was not addressed. The cross-substitution of alkaline earth ions does not affect the robust charge/valence state at the various metal sites of the double perovskite lattice, but the magnetic properties of these perovskites may be altered considerably. In our study, the double perovskite oxides, AFe 0.5 Mo 0.5 O 3 , with A = (Ba,Sr) and (Sr,Ca), were prepared by a sol-gel method, and the chemical pressure effect of (Ba,Sr)(Fe 0.5 Mo 0.5 )O 3 and SrFe 0.5 Mo 0.5 O 3 , substituted by a small amount of Ba and Ca, were investigated in particular, using mainly Mössbauer spectrometry and nuclear inelastic scattering techniques. 14 Experimental For the preparation of AFe 0.5 Mo 0.5 O 3 with different compositions of A = (Ba, Sr) and (Sr, Ca,), SrCO 3 , CaCO 3 , BaCO 3 and Fe metal were dissolved in nitric acid and combined with (NH 4 ) 6 Mo 7 O 24 . 4H 2 O, with appropriate stoichiometry. After addition of ethylene glycol and citric acid to this solution, it was gradually heated, the organic reagents were let decompose at 500 °C in air and the system was further annealed at