We have measured the optical conductivity of single crystal LuMnO3 from 10 to 45000 cm(-1) at temperatures between 4 and 300 K. A symmetry allowed on-site Mn d-d transition near 1.7 eV is observed to blueshift ( approximately 0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions. Similar anomalies are observed in the temperature dependence of the TO phonon frequencies which arise from spin-phonon interaction. We find that the known anomaly in the temperature dependence of the quasistatic dielectric constant epsilon(0) below T(N) approximately 90 K is overwhelmingly dominated by the phonon contributions.

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We have measured optical reflectivity of Li and Ti doped NiO (LTNO) in the infrared range at various temperatures. A Drude-like absorption is found at low energy, ω < 100 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 εo. 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 εo.