Oxygen Nonstoichiometry of Various Functional Layered Oxides
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Oxygen Nonstoichiometry of Various Functional Layered Oxides Jun-ichi. Shimoyama1,2, Yuui Yokota1, Masahiro Shiraki1, Yuuki Sugiura1, Shigeru Horii1 and Kohji Kishio1 1 Department of Applied Chemistry, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 2 PRESTO, Japan Science and Technology Corporation (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
ABSTRACT The mixed metal oxides having layered crystal structure have been of much interest because of their various new and excellent functions. Precise phase diagrams on the oxygen content have not been clarified for most of these layered oxides, while oxygen nonstoichiometry often plays a crucial role for determining functions of oxide materials. In this paper, we report oxygen nonstoichiometry and related physical properties mainly on the manganese-based and cobalt-based layered oxides. In the La2-2xSr1+2xMn2Oy (x = 0.3, 0.4), dramatic change in magnetization properties with oxygen content were observed in spite of its small oxygen nonstoichiometry. For (Ca2CoO3-δ)0.62CoO2 systems, high valence metal substitution was found to be effective for suppression of oxygen nonstoichiometry, increase of decomposition temperature and thermoelectric properties.
INTRODUCTION In the past two decades, particularly after the discovery of high-Tc superconductivity in layered cuprates in 1986[1], various layered oxides have been eagerly developed for new functional materials, such as superconductors, colossal magnetoresistance (CMR) devices, ferroelectric materials, thermoelectric materials and so on. All these functional layered materials are consisted of transition metal oxide layer showing characteristic physical properties and blocking oxide layers which control valence state of the transition metal ions. In other words, there are several cation and anion sites in the layered oxides. These layered materials usually contain several metal elements and each of them desirably occupies designated site. In order to create new functions or improve physical properties, a large number of layered oxides with various combinations of metal elements including dopants have been designed and synthesized. Numerous studies on the synthesis of new layered oxides give us good hints to the new material design, such as selection of constituent metal elements, their compositions and suitable doping, for achieving ideal physical properties. In the layered oxides, however,
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disorders in the site occupancies of cations, such as unexpected site exchange between cations and partial substitution for other site, sometimes happens and these accompanies degradation of functional properties. Therefore, selection of suitable elements and applying appropriate synthesis conditions should be carefully considered in the design and synthesis of new functional layered oxides On the other hand, oxygen nonstoichiometry often plays an important role to determine physical properties of transition metal oxides. For recent layered oxides, however, there are a few studies on the
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