Polarons in low temperature phase of (NH4)3FeF6

The frequency and temperature dependence of the dielectric constant and the ac electrical conductivity in (NH4)3FeF6 was investigated. The temperature dependence of the dielectric constant reveals the phase transition from tetragonal to cubic phase at 270 K. At temperatures below 225 K, the ac conductivity and dielectric constant follow the universal dielectric response (UDR), being typical for hopping or tunneling of localized charge carriers. A detailed analysis of the temperature dependence of the UDR parameter s in terms of the theoretical model for tunneling of small polarons revealed that below 225 K this mechanism governs the charge transport in this fluoride system.