The infrared reflectance of the transition metal chalcogenide 1T -TaS2 has been measured at temperatures from 30 K to 360 K over 30-45,000 cm −1 (4 meV-5.5 eV). 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 800 cm −1 (100 meV). We observe an anomalous frequency dependence of the 208 cm −1 infrared-active phonon mode. This mode demonstrates softening as the temperature decreases below the 180 K 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.

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