Giant Dielectric Permittivity in Ca₃Co₄O₉ Ceramics and Thin Films

Ca₃Co₄O₉ is a thermoelectric material representing the family of the so-called misfit layered oxides. It consists of a Ca₂CoO₃ layer, an insulating subsystem that maintains the charge supply for the conducting CoO2 layer, where Co³⁺ and Co⁴⁺ ions coexist. Recently, a giant dielectric permittivity was measured in Ca₃Co₄O₉ ceramic at room temperature.<br/>In this work, we investigated and compared the structural and electrical properties of Ca₃Co₄O₉ (CCO) ceramics and thin films. The CCO thin films were produced from a stoichiometric, in-house made CCO ceramic by pulsed laser deposition (PLD) onto (100) oriented lanthanum aluminate (LaAlO₃) substrates. Both the CCO ceramics and thin films were subsequently characterised by X-ray diffraction (XRD). To evaluate the samples’ dielectric properties, capacitor structures were formed. Their capacitance was measured and the corresponding permittivity was evaluated in a temperature range RT – 450 K.<br/>For the CCO ceramics, the dielectric permittivity measured at room temperature was ~5,300, which corresponds well with the previously reported values. It was increasing monotonically with no maximum observed at temperatures up to 450 K, where the value was several tens of thousands. Similar behaviour of the capacitance as a function of the temperature with a maximum at 430K was measured for the thin film sample. Due to the small thickness of the layer, the error associated with the dielectric permittivity evaluation procedure is very high. Nevertheless, one could speculate that it is in a similar order of magnitude as those of the CCO ceramic.<br/>The observed giant dielectric behaviour makes Ca₃Co₄O₉ an attractive material for applications like high-K dielectric layers, capacitors and thermoelectric converter devices.