  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.
