Electron diffraction and infrared study of the semimetallic layered compound Ti 1−xV xSe 2

Abstract

The crystals of the system Ti 1−xV xSe 2 (C ≤ x ≤ 0.05) undergo a second order structural phase transition. Electron diffraction studies show that the transition temperature decreases with progressive V-doping. The phase transition considerably affects the infrared reflectivity, measured at 300 and 77 K in the spectral range 40 cm −1 to 4000 cm −1. The presence of free carriers and the existence of optical infrared active E u phonon modes ( E ⊥ c) confer their characteristic appearance to the spectra. At room temperature one phonon structure is measured at 143 cm −1. At 77 K a new series of phonon peaks appears up to a V-concentration of 5 % as a direct consequence of superlattice formation. At room temperature the plasma-edge shifts towards higher frequencies as the vanadium concentration increases. This effect is caused by a large increase of N m ∗ , associated with the mixing of impurities. It is indicative of the small density of states at the Fermi level in semimetallic TiSe 2. Our results suggest a phase transition driven by lattice dynamical effects.