NOVEL FUNCTIONAL DOUBLE-PEROVSKITE OXIDES

本论文对双钙钛矿过渡金属氧化物（A 'A“B' B“O_<6-ω>）的性质进行了深入而广泛的研究，以寻找“新功能”。我们专注于三种不同类型的铁和钴氧化物双钙钛矿，以阐明铁/钴的价态和各种物理性能之间的关系。本研究在1999年4月至2003年3月期间取得的主要研究成果如下：1. A位有序（缺氧型）双钙钛矿型铁氧化物BaREFe_2O_<5+δ>（1）提出了一种新的制备BaSmFe_2O_<5+δ>样品的方法--“吸氧剂包埋法”。利用这种方法，成功地制备了不同Res和氧含量的样品。(2)发现BaSmFe_2O_（5+δ）（δ< 0.5）存在Verwey型相变（铁的价态相变）。通过穆斯堡尔谱学研究，发现铁以混合价态存在， ...更多信息 在高于T_v（= Verwey型转变温度）时，Fel^&lt;II/III&gt;被分解，在低于T_v时，Fel^&lt;II/III&gt;被分解为Fe^<II>+ Fe^<III>。(3)BaREFe_2O_（5+δ）通过不同的置换（即化学压）技术实现了对T_v的宽范围控制。Sr取代Ba和RE ′取代RE，Ca取代Sm。从样品的同步辐射X射线衍射分析结果可以看出，晶体中的T_v值主要受晶体中穿过RE面的Fe-Fe距离的影响。(4)对于RE = Dy，Y样品，通过同步辐射X射线衍射和电子衍射首次观察到了由于电荷有序（低于T_v）而产生的超点. B位有序双钙钛矿结构：Sr_2FeMoO_6（SFMO）（1）成功地调整了“样品包封与吸氧剂”方法，合成了高质量的SFMO样品。(2)从穆斯堡尔谱的结果，铁在SFMO被发现是在一个混合价态，即Fe^&lt;II/III&gt;。结果表明，Fe^&lt;II/III&gt;状态与化合物的“半金属性”密切相关。(3)建立了SFMO合成温度与B阳离子（Fe/Mo）位有序度之间的关系。阐明了有序度对各种物理性质的影响。(4)通过Ba/Ca替代Sr和Ta/W替代Mo，成功地实现了对SFMO磁性能和MR特性的控制。Ta取代样品在5 K和7 T下获得了高达200%的大MR效应.在常规条件下合成了A位有序（缺氧型）双钙钛矿钴氧化物BaRECo_2O_（5+δ）样品，其中RE=Nd，Eu，Ho，Y。系统研究了氧的非化学计量比对晶体结构、磁性和MR特性的影响，以增强MR效应。对于RE = Nd和δ= 0.46的样品，在200 K以上检测到隧道型MR（TMR）效应，表明这种钴氧化物中出现了半金属态。综上所述，我们通过目前对双钙钛矿氧化物的研究工作所取得的主要贡献是：（i）发现了包括Verwey型转变在内的新现象，（ii）过渡金属的价态与各种物理性质之间的关系（如Verwey型转变、电荷有序、半金属度等）已经澄清，和（iii）控制物质功能的有用的基础已经建立。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