College Park, Maryland      June 6 - 10 , 2004

TP7: Electronic, Magnetic and Structural Properties of SrAMnMO6 Double Perovskites (A = Sr, La; M = Mo, Ru)

P. M. Woodward, J. Goldberger, P. N. Santhosh (Department of Chemistry, Ohio State University), P. Karen (Department of Chemistry, University of Oslo), A. R. Moodenbaugh (Department of Materials and Chemical Science, Brookhaven National Laboratory)

The electronic, magnetic and structural properties of Sr2MnMoO6, SrLaMnMoO6 Sr2MnRuO6 and SrLaMnRuO6 have been investigated. Powder diffraction measurements show that both Sr2MnMoO6 and SrLaMnMoO6 adopt long-range, rock salt ordering of the transition metal ions, while transition electron microscopy reveals short-range cation order in Sr2MnRuO6 and SrLaMnRuO6. All indicators suggest that Sr2MnMoO6 and SrLaMnMoO6 are Mott-insulators. They exhibit relatively low conductivities (1 x 10-2 Ω-1-cm-1 at 290 K), Curie–Weiss magnetic behavior, and well defined transition–metal valence and spin states: Sr2Mn2+Mo6+O6 adopts a G-type antiferromagnetic structure (TN ~ 10 K), whereas SrLaMn2+Mo5+O6 is a spin glass (TG ~ 85 K). The ruthenates are more conductive by roughly four orders of magnitude at room temperature. Sr2MnRuO6 has a room temperature conductivity of 1 x 102 Ω-1-cm-1, with a temperature dependence characteristic of Mott variable-range hopping behavior. It orders antiferromagnetically (C-type) near 180 K, and exhibits a cooperative Jahn-Teller distortion (orbital ordering). The room-temperature electrical conductivity of SrLaMnRuO6 is similar to that of Sr2MnRuO6 (4 x 101 Ω-1-cm-1) and bond distances suggest that both compounds contain Mn3+. However, the substitution of La3+ for Sr2+ leads to a disappearance of the cooperative Jahn-Teller distortion, a change in the conductivity mechanism (diffusion-assisted small-polaron hopping), and stabilizes a ferromagnetic ground state (TC ~ 220 K).

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