Infrared study of superlattice formation in Ti1+xSe2

Infrared reflectivity measurements have been made on two Ti_1+xSe₂ crystals of different stoichiometry over the range 200 cm^?1 to 4000 cm^?1 before and after the formation of a 2a_o × 2c_o superlattice. In both crystals at room temperature a heavily damped Drude edge is observed at about 1000 cm^?1. At liquid helium temperatures, below the phase transition, the Drude minimum of a non-stoichiometric crystal ($\text{x ? 0.02, T}{}_{\mathrm{c}}\text{= 155 K}$) is at 750 cm^?1 whereas the low temperature minimum of a more stoichiometric sample (T_c = 198 K) is below 350 cm^?1. Also, new absorption peaks are seen to develop below T_c at about 3700 cm^?1 and 3300 cm^?1 for the non- stoichiometric and stoichiometric-crystals respectively. The observed phenomena are related to the energy gaps which open up in the band structure below the phase transition temperature.