Electrochemical properties of double molybdate LiSm(MoO4)2 ceramics with ultra-low sintering temperature

Abstract

A novel double molybdate LiSm(MoO4)2 composition was prepared through a conventional solid-state ceramic route for its potential application as a solid-state Li+-ion conductor for low-loss dielectric ceramics. Ceramic disks were prepared by uniaxial pressing and sintered at the low temperature of 620 °C. Phase formation was confirmed through powder X-ray diffraction (XRD), which was indexed to the tetragonal crystal system with a Scheelite-type structure. Laser Raman spectroscopy showed that the MoO42− tetrahedra in the LiSm(MoO4)2 material are more regular, suggesting high symmetry for the compound. Scanning Electron Microscopy (SEM) revealed the formation of polygonal grains with an average grain size of less than 1 μm. The electrochemical properties of the ceramic system were investigated by Electrochemical Impedance Spectroscopy (EIS) in O2, air, and Ar (250 °C – 500 °C). Data were systematically analyzed using an Equivalent Circuit Model (EQM) combined with a Distribution Function of Relaxation Times (DFRT) analysis. The results revealed the coexistence of both a delocalized (long-range conductivity) and a localized (dipole relaxation) phenomenon, mainly distinguishable at lower temperatures (i.e., for T ≤ 350 °C) and in a pure O2 atmosphere. This localized phenomenon was tentatively attributed to the presence of trapped hole-vacancy pairs with reduced mobility. Conversely, at higher temperatures in O2 (i.e., for T ≥ 350 °C) and in the whole temperature range in less oxidizing conditions (air and Ar), such a localized phenomenon was no longer observed, as a likely result of a decrease in hole concentration with decreasing oxygen partial pressure (pO2). The current work, thus, underscores a different perspective for the development and characterization of ceramic-based solid-state Li+-ion conductors for their application as low-loss dielectric ceramics.