Infrared study of giant dielectric constant in Li- and Ti-doped NiO

We have measured optical reflectivity of Li- and Ti-doped NiO (LTNO) in the infrared range at various temperatures. A Drude-type absorption is found at low energy, $\omega <100{\mathrm{cm}}^{-1}$ and its spectral weight increases substantially as temperature decreases. This observation and dc-resistivity result show that LTNO has a conductive grain and resistive boundary. Such composite structure provides evidence of the Maxwell-Wagner (MW) mechanism as the origin of the high dielectric constant ${\epsilon }_o$. We propose a three-phase granular structure and show that this extended MW model explains the observed frequency- and temperature dependence of the dielectric constant as well as the giant value of ${\epsilon }_o$.