NOVEL FUNCTIONAL DOUBLE-PEROVSKITE OXIDES

在我们目前的研究中，我们深入和广泛地研究了双钙钛矿过渡金属氧化物（表示为A'A 'B 'B ' O_<6-ω>）的性质，以寻找“新功能”。本文以三种不同类型的铁钴氧化物双钙钛矿为研究对象，阐明了铁/钴的价态与各种物理性质之间的关系。1999年4月至2003年3月期间的主要研究成果总结如下：(1)为制备BaSmFe_2O_<5+δ>样品，提出了一种“吸氧包封”的新方法。利用该方法，成功制备了各种稀土元素和氧含量的样品。(2)在BaSmFe_2O_<5+δ> (δ< 0.5)中发现了“verwey型相变”（铁价态相变）。通过穆斯堡尔谱分析，发现铁处于混合价态，在T_v以上（= verwey型转变温度）Fel^<II/III>，在T_v以下分裂为Fe^<II> + Fe^<III>。(3)通过不同的取代（即化学压力）技术（如化学压力），对BaREFe_2O_<5+δ>的T_v进行了大范围的控制。sr -换ba和RE -换RE的同价取代，以及ca -换sm的同价取代。通过对样品的同步x射线衍射分析表明，T_v值主要受晶体中Fe-Fe在RE平面上的距离的影响。(4)通过同步x射线衍射和电子衍射首次观测到RE = Dy， Y样品中电荷有序（T_v以下）的超黑子。b位有序双钙钛矿：Sr_2FeMoO_6 (SFMO)(1)成功调整了“吸氧包封”方法，合成了高质量的SFMO样品。(2)从穆斯堡尔光谱结果可知，SFMO中的铁处于混合价态，即Fe^<II/III>。结果表明，Fe^<II/III>态与化合物的“半金属丰度”密切相关。(3)建立了SFMO的合成温度与b -阳离子（Fe/Mo）位有序度的关系。阐明了有序度对各种物理性能的影响。(4)通过Ba/Ca-for-Sr和Ta/W-for-Mo取代，成功地控制了SFMO的磁性能和MR特性。对于ta取代的样品（在5k和7t下），获得了高达200%的MR效应。在正常条件下合成了RE=Nd、Eu、Ho和Y的双钙钛矿型有序（缺氧）钴氧化物BaRECo_2O_<5+δ>样品，并用TCOD法控制了样品的氧含量（5+δ）。系统研究了氧非化学计量对晶体结构、磁性和磁流变特性的影响，以增强磁流变效应。对于RE = Nd, δ= 0.46的样品，在200 K以上出现隧道型MR （TMR）效应，表明该氧化钴呈现半金属态。综上所述，我们在双钙钛矿氧化物的研究工作中取得的主要贡献是：(1)发现了包括verwey型转变在内的新现象；(2)澄清了过渡金属的价态与各种物理性质（如verwey型转变、电荷有序性、半金属丰度等）之间的关系；(3)建立了控制材料功能的有用基础。少

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