Optical Phonon Modes and Localized Effective Charges of Transition-Metal Dichalcogenides

Abstract

Reflection spectra have been measured in the far infrared region (10 cm⁻¹–500 cm⁻¹) for several kinds of crystals of transition-metal dichalcogenides (TX₂). Optical phonon frequencies of infrared active mode (E⊥c) are determined for the group IV compounds HfSe₂, ZrSe₂ and TiSe₂ and for the group VI compounds MoS₂, MoSe₂, WS₂ and WSe₂. Using available Raman data the localized effective charges (e_l^*) of these materials are estimated. The nature of the bonding in TX₂ is found to be well described in terms of the estimated e_l^* rather than by the macroscopic effective charge e_T^*.