Dielectric properties of doping-free NaMn7O12: Origin of the observed colossal dielectric constant

The semiconducting NaMn7O12 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 Hz to 1 MHz. Standard measurements on metallized samples exhibit an apparent colossal dielectric constant (CDC) with an epsilon(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-CO=176 K and shows a net bulk dielectric constant value epsilon(R)similar or equal to 68 at room temperature.