Charge transfer in the high dielectric constant materials CaCu3Ti4O12   and CdCu3Ti4O12
  The cubic perovskite-related ceramic CaCu3Ti4O12 has a very high static dielectric constant ~10000 at room temperature (RT), which drops to about 100 below about 100 K. Substituting Cd for Ca reduces the RT value of the dielectric constant by over an order of magnitude. The large the dielectric constant may be due to an internal barrier layer capacitance (IBLC) effect. Infrared optical properties show a low-frequency mode that increases dramatically in strength at low temperature, suggesting a change in the effective charges and a breakdown of the IBLC model due to a semiconductor-to-insulator transition. 
C.C. Homes, T. Vogt, S.M. Shapiro, W. Si (BNL), S. Wakimoto (BNL/MIT),   M.A. Subramanian (Dupont)