NOVEL FUNCTIONAL DOUBLE-PEROVSKITE OXIDES

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In our present research, the properties of double-perovskite transition-metal oxides (expressed as A'A"B'B"O_<6-ω>) were intensively and extensively investigated to search for "novel functions". We focused on three different kinds of iron- and cobalt-oxide double-perovskites to elucidate the relationships between the valence state of iron/cobalt and various physical properties. Major results of our research during the period of April 1999-March 2003 are summarized in the following :1. A-site ordered (oxygen-deficient) double-perovskite iron oxides : BaREFe_2O_<5+δ>(1) For synthesizing BaSmFe_2O_<5+δ> samples, a novel technique named "sample encapsulation with an oxygen getter" method was developed. Utilizing this method, samples with various Res and oxygen contents were successfully prepared.(2) A "Verwey-type transition" (a phase transition in the iron valence state) was discovered for BaSmFe_2O_<5+δ> (δ< 0.5). By Mossbauer spectroscopy, it was found that iron is in a mixed-valence st … More ate, Fel^<II/III> above T_v( = the Verwey-type transition temperature) and gets split into Fe^<II> plus Fe^<III> below T_v.(3) A wide-range control on T_v was attained for BaREFe_2O_<5+δ> through different kinds of substitution (i.e. chemical pressure) techniques, i,e. Sr-for-Ba and RE'-for-RE isovalent substitutions, and Ca-for-Sm aliovalent substitution. From the results of synchrotron x-ray diffraction analysis for the samples, it was clarified that the T_v value is influenced mainly by the Fe-Fe distance across the RE plane in the crystals.(4) For RE = Dy, Y samples, superspots due to a charge ordering (below T_v) were for the first time observed by synchrotron x-ray diffraction and electron diffraction.2. B-site ordered double-perovskites : Sr_2FeMoO_6 (SFMO)(1) The "sample encapsulation with an oxygen getter" method was successfully tuned to synthesize high-quality SFMO samples.(2) From the results of Mossbauer spectroscopy, iron in SFMO was found to be in a mixed-valence state, i.e. Fe^<II/III>. It was revealed that the Fe^<II/III> state is strongly correlated with the "halfmetallicity" of the compound.(3) The relationship between the synthesis temperature and the degree of order at the B-cation (Fe/Mo) site was established for SFMO. The influence of the degree of order on the various physical properties was clarified.(4) The control on the magnetic properties and the MR characteristics of SFMO was successfully made through the Ba/Ca-for-Sr and Ta/W-for-Mo substitutions. Large MR effect of up to 200 % was attained for a Ta-substituted sample (at 5 K and 7 T).3. A-site ordered (oxygen-deficient) double-perovskite cobalt oxides : BaRECo_2O_<5+δ>Samples with RE=Nd, Eu, Ho and Y were synthesized under normal condition and the oxygen content, 5 +δ, of the sample was controlled by the TCOD method. The effects of oxygen nonstoichiometry on the crystal structure, magnetism, and MR characteristics were systematically studied to enhance the MR effect. For a sample with RE = Nd and δ= 0.46, tunneling-type MR (TMR) effect was detected above 200 K, suggesting the appearance of half metallic state in this cobalt oxide.In summary, the key contributions we have achieved through the present research work on double-perovskite oxides are : (i) novel phenomena including a Verwey-type transition were discovered, (ii) the relationships between the valence state of the transition metal and various physical properties (such as Verwey-type transition, charge ordering, half metallicity, etc.) have been clarified, and (iii) useful bases for controlling the material functions have been established. Less