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We report on phonon properties and electronic transitions in CaCu3Ti4O12, amaterial which reveals a colossal dielectric constant at room temperaturewithout any ferroelectric transition. The results of far- and mid-infraredmeasurements are compared to those obtained by broadband dielectric andmillimeter-wave spectroscopy on the same single crystal. The unusualtemperature dependence of phonon eigenfrequencies, dampings and ionic plasmafrequencies of low lying phonon modes are analyzed and discussed in detail.Electronic excitations below 4 eV are identified as transitions between fulland empty hybridized oxygen-copper bands and between oxygen-copper andunoccupied Ti 3d bands. The unusually small band gap determined from thedc-conductivity (~200 meV) compares well with the optical results.

Broadband dielectric response of CaCu3Ti4O12: From dc to the electronic transition regime