Giant dielectric constant in calcium manganese oxide ceramics

Giant dielectric constant (GDC) materials have been promising for their potential utilization in multifunctional devices, including high-performance energy storage and development of efficient electronic elements. The preliminary charge transport study of ceramics possessing GDC would provide insight for utilization in world-wide applications. GDC of ~${10}^6$ is realized in the temperature range of 133–593 K through perfectly dense calcium manganese oxide (CaMnO₃) ceramics prepared using hot-press technique. The dc-conductivity and dielectric relaxation spectra exhibit Mott-variable range hopping in the temperature range of 133–393 K and small polaron hopping above 493 K. The Nyquist plot provides an equivalent model with series combination of two sets of parallelly connected circuits comprising of resistance and constant phase element, attributed to the contributions from grains and grain boundaries. The ac-conductivity suggests the presence of non-overlapping small polaron tunneling in temperature range of 133–333 K and overlapping large polaron tunneling above 333 K. An electron-polaron band located at 0.27 eV below the conduction band edge is demonstrated with consideration of polaron hopping and band gap energy. Scaling procedure employed to the ac-conductivity universality evidences the corrected Summerfield model and presence of Coulomb interaction between the charge carriers. The in-depth study of the frequency and temperature dependent electric and dielectric properties demonstrates GDC over a wide frequency range of 1 Hz ̶ 10 MHz, and low loss tangent above 10 kHz in the currently prepared perfectly dense CaMnO₃ ceramics and suggests its suitability for engineering charge storage devices in the radio frequency region.