We report on phonon properties and electronic transitions in $\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{Ti}}_4{\mathrm{O}}_{12}$, a material which reveals a colossal dielectric constant at room temperature without any ferroelectric transition. The results of far- and midinfrared measurements are compared to those obtained by broadband dielectric and millimeter-wave spectroscopy on the same single crystal. The unusual temperature dependence of phonon eigenfrequencies, dampings, and ionic plasma frequencies of low-lying phonon modes is analyzed and discussed in detail. Electronic excitations below $4\mathrm{eV}$ are identified as transitions between full and empty hybridized oxygen-copper bands and between oxygen-copper and unoccupied $\mathrm{Ti}3d$ bands. The unusually small band gap determined from the dc conductivity $(\sim 200\mathrm{meV})$ compares well with the optical results.

• Received 7 September 2007

DOI:https://doi.org/10.1103/PhysRevB.77.045131