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

Reflectivity spectra of the $1T$ form of Ta${\mathrm{S}}_2$ and the $2H$ form of Ta${\mathrm{Se}}_2$ have been measured over the spectral range 0.04-5.0 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 Ta${\mathrm{S}}_2$. This conductivity is removed below 0.5 eV by the phase transitions but no sharp gap structure appears. In Ta${\mathrm{Se}}_2$ an absorption peak appears at 0.3 eV. The spectra are related to the charge-density-wave picture of the transitions.