Infrared-reflectivity spectra have been obtained for a large number of group-IV (Ti, Zr, Hf) and group-VI (Mo, W) transition-metal dichalcogenides. Three distinct types of spectra have been identified: In ZrS₂, HfS₂, and HfSe₂, very strong reststrahlen with effective charges eT*~4e; in MoS₂, MoSe₂, WS₂, and WSe₂, weaker reststrahlen with eT*~0.5e; and in TiS₂, ZrTe₂, and HfTe₂, plasmon-type reflection bands corresponding to free-carrier densities in excess of 10²⁰/cm³. These observations, coupled with considerations of ion size, have led to a new description of the chemical bonding in these layered materials that is not consistent with a simple rigid-band scheme.
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Lucovsky et al. (1973) studied this question.
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