Defect Structure and Oxide Ion Conduction of Potassium Ion Substituted CaWO₄

Materials (Basel). 2018 Jun 27;11(7):1092. doi: 10.3390/ma11071092.

Abstract

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.

Keywords: CaWO4; high-temperature XRD; oxide ion conductor; oxide ion vacancy.