  The infrared reflectance of the transition metal chalcogenide 1T-TaS2 has
been measured at temperatures from 30K to 360K over 30-45,000cm^-1
(4meV-5.5eV). The optical conductivity was obtained by Kramers-Kronig analysis.
At 360K only modest traces of the phonon lines are noticeable. The phonon modes
are followed by a pseudogap-like increase of the optical conductivity, with
direct optical transitions observed at frequencies above 1eV. As the
temperature decreases, the low frequency conductivity also decreases, phonon
modes become more pronounced and pseudogap develops into a gap at 800cm^-1
(100meV). We observe an anomalous frequency dependence of the 208cm^-1
infrared-active phonon mode. This mode demonstrates softening as the
temperature decreases below the 180K metal-to-insulator transition. The same
mode demonstrates strong hysteresis of the frequency and linewidth changes,
similar in its temperature behavior to the hysteresis in the dc-resistivity. We
discuss a possible relation of the observed softening of the mode to the
structural changes associated with the metal-to-insulator transition.
