Ionic and electronic transport in the garnet-type vanadate Ca${}_{2.5}$Mg${}_2$V${}_3$O${}_{12}$

The Ca${}_{2.5}$Mg${}_2$V${}_3$O${}_{12}$ vanadate with cubic structure (space group Ia$\overline{3}$d) has been prepared by solid-state reaction in air. The crystal structure has been determined by Rietveld refinement of powder X-ray diffraction data. In the garnet-type structure, the Ca${}^{2+}$ ions and cationic vacancies occupy the 24c sites. Conductivity has been measured using AC impedance spectroscopy at 400–950 °С in air and in the range of oxygen partial pressure (p${}_{O_2}$) between 10${}^{-10}$ and 0.5 atm. Ca${}_{2.5}$Mg${}_2$V${}_3$O${}_{12}$ exhibits mixed ionic and electronic conductivity. The n-type electronic conductivity is proportional to p${}_{O_2}^{-1/4}$. The ionic and electronic components of conductivity are characterized by the activation energy of about 1.2 and 2.0 eV, respectively. Ion transference number in the air has been shown to increase from 0.6 to almost 1.0 upon temperature decrease from 950 to 600 °С. Analysis of the garnet-like crystal structure allows proposing possible mechanisms of ionic conductivity in which a Ca-ion migration pathway from occupied 24c sites to vacant 24c′ position is carried out through empty 16b sites.