Raman and infrared studies of superlattice formation in TiSe2

Raman and far-infrared reflectivity spectra have been obtained for both stoichiometric and nonstoichiometric Ti${\mathrm{Se}}_2$ above and below the transition temperatures corresponding to formation of the $2a_0\times 2c_0$ superlattice. In the normal phase above $T_c$ Raman-active lines are observed at 134 ${\mathrm{cm}}^{-1\mathrm{}}$ ($E_g$) and at 195 ${\mathrm{cm}}^{-1\mathrm{}}$ ($A_{1g}$). Normal-incidence reflectivity shows a single $E_u$ mode at 137 ${\mathrm{cm}}^{-1\mathrm{}}$ superimposed upon a highly damped Drude background. Below $T_c$, strong new $E_g$ and $A_{1g}$ Raman lines appear together with several weaker lines and bands. Likewise, a Kramers-Kronig analysis of the low-temperature infrared data shows many new optically active lattice modes. We have predicted the number and symmetry of all even and odd modes which are folded into the center of the original Brillouin zone from the points $A$, $L$, and $M$ at the zone surface due to the periodic lattice distortion. Reasonable agreement is found for the suggested superlattice, although some weak lines in both the low-temperature Raman and infrared data are unexplained.