I. Introduction:
During the last decade, considerable effort has been directed towards understanding the chemistry and physics of transition metal chalcogenides. As a result of these investigations, a variety of new materials with interesting properties have been discovered. Unlike the transition metal fluorides which contain localized 3d electrons, sulfides and oxides of transition metals contain either localized or itinerant 3d electrons or both simultaneously. Studies of transition metal oxides have been instructive in understanding the behavior of d electrons in solids; ~'-3~ however, properties of transition metal chalcogenides are quite different from those of oxides since d electrons in'chalcogenides participate in covalent bonding. Such covalent bonding in chalcogenides reduces the formal charge on transition metals and favors formation of metal-metal bonds. Metal-metal interactions indeed play a significant role in determining the properties of many of the transition metal chalcogenides. The influence of such interactions on the electronic structure of these solids can be studied by means of their transport properties and these interactions become particularly important in transition elements sulfides of the second and third series.
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