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

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 (TX 2 ). Optical phonon frequencies of infrared active mode ( E ⊥ c ) are determined for the group IV compounds HfSe 2 , ZrSe 2 and TiSe 2 and for the group VI compounds MoS 2 , MoSe 2 , WS 2 and WSe 2 . Using available Raman data the localized effective charges ( e l * ) of these materials are estimated. The nature of the bonding in TX 2 is found to be well described in terms of the estimated e l * rather than by the macroscopic effective charge e T * .