Infrared study of the electronic instabilities in tantalum disulfide and tantalum diselenide

Reflectivity spectra of the 1T form of TaS2 and the 2H form of TaSe2 have been measured over the spectral range 0.04 to 5.0 eV. Spectra were taken at temperatures above and below the phase transitions which have been associated with charge-density waves. Kramers-Kronig analysis shows a metallic-type conductivity at high temperatures in TaS2. This conductivity is removed below 0.5 eV by the phase transitions but no sharp gap structure appears. In TaSe2 an absorption peak appears at 0.3 eV. The spectra are related to the charge-density-wave picture of the transitions