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Defect Structure and Oxide Ion Conduction of Potassium Ion Substituted CaWO4

Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
Graduate School of Engineering, Tottori University, Koyama-cho Minami, Tottori 680-8552, Japan
Laboratory for Materials and Structures, Tokyo Institute of Technology, Nagatsuta-Cho, Midori-ku, Yokohama 226-8503, Japan
Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
National Institute of Technology, Kagawa College, Chokushi-cho, Takamatsu, Kagawa 761-8058, Japan
Author to whom correspondence should be addressed.
Takao Esaka has died in April, 2018.
Materials 2018, 11(7), 1092;
Received: 25 May 2018 / Revised: 16 June 2018 / Accepted: 19 June 2018 / Published: 27 June 2018
(This article belongs to the Section Energy Materials)
We have prepared Ca1−xKxWO4−x/2 solid solutions with the Scheelite-type structure to investigate high-temperature electrochemical properties. Room-temperature X-ray diffraction suggested the solid solution range was x ≤ 0.2, since the second phase presumably of K2WO4 was detected for x = 0.3. For all the substituted samples up to x = 0.4, a large jump in conductivity has been observed around 500 °C. At higher temperatures, oxide ion conduction is found to be predominant even for x = 0.4, exceeding the solution limit estimated from the room-temperature XRD. The conductivity at high temperature is essentially proportional to the amount of substituted potassium ions up to x = 0.4, indicating that oxide ion conduction is associated with the formed oxide ion vacancy. High-temperature X-ray diffraction detected no apparent change in lattice parameters around 500 °C for x = 0.1, and the remaining second phase seems to be incorporated into the Scheelite lattice at high temperatures. View Full-Text
Keywords: CaWO4; oxide ion conductor; oxide ion vacancy; high-temperature XRD CaWO4; oxide ion conductor; oxide ion vacancy; high-temperature XRD
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Takai, S.; Shitaune, S.; Sano, T.; Kawaji, H.; Yabutsuka, T.; Esaka, T.; Yao, T. Defect Structure and Oxide Ion Conduction of Potassium Ion Substituted CaWO4. Materials 2018, 11, 1092.

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