Semiconducting Mixed Valence Sulfido (Selenido) Ferrates: Optical, Dielectric, and Electrochemical Impedance Properties

The ternary potassium sulfido and selenido ferrates K9[Fe2S(e)7] were synthesized as pure phases through a facile and straightforward solid-state method. The compounds crystalize isotypic as has been reported previously. UV-visible measurements indicate semiconductivity, showing direct optical band gaps with energies of 1.91 eV for the sulfido and 1.72 eV for the selenido ferrate. Investigations of dielectric and impedance properties provide their dielectric constants in the range of 60 to 74 at room temperature at a frequency of 1 kHz, as well as ionic conductivity values in the range of 2.08 ⋅ 10−5 and 2.61 ⋅ 10−5 mS cm−1. Both, the dielectric constants and the calculated ionic conductivity of the selenido ferrate are higher compared to the sulfido ferrate, corresponding to the larger unit cell volume and increased bond lengths in the selenido ferrates. The dielectric constants are comparably higher than the reference material of SiO2, which might introduce them for dielectric applications.