Dielectric properties of doping-freeNaMn7O12: Origin of the observed colossal dielectric constant

Abstract

The semiconducting $\mathrm{Na}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ is a doping-free compound with several coexistent properties such as orbital ordering, charge ordering, and magnetic orderings of different types. We investigated its dielectric response by means of frequency impedance measurements in the range from $20\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. Standard measurements on metallized samples exhibit an apparent colossal dielectric constant (CDC) with an ${ϵ}_{R}$ value of several thousands at low frequencies, but a careful equivalent circuit analysis allows one to ascribe the observed CDC to the effect of a depletion layer on the metal-semiconductor junctions. We bypass this effect by means of a nonstandard technique employing mica linings: the resulting dielectric behavior exhibits the presence of the charge ordering transition at ${T}_{\mathit{CO}}=176\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and shows a net bulk dielectric constant value ${ϵ}_{R}\ensuremath{\simeq}68$ at room temperature.