Investigations of topological materials and thermoelectric properties of heavy metal chalcogenides

Abstract

Chalcogenides constitute one of the most important classes in the field of chemistry, exhibiting rich compositional and structural diversity on par with oxides and organic compounds. Chalcogenides are compounds that contain at least one chalcogen Q atom (Q = S/ Se/ Te) in a chemically reduced state compared to its elemental form. Chalcogenides bear few chemical resemblances to oxides, but there are also big dissimilarities in chemical character and physical properties among them which are significant enough to warrant a separate treatment of the chalcogenides as a distinct class of materials. These differences must, of course, originate from the differences between the oxygen atom on the one hand and the atoms of S, Se, Te on the other. Some important differences in the atomic properties of oxygen and the chalcogen are:1 (a) The chalcogen atoms are larger (and also heavier) than oxygen atoms. (b) The chalcogens are less electronegative than oxygen. (c) The chalcogens have d orbitals of accessible energy (3d for S, 4d for Se and 5d for Te), while oxygen has not. These differences in the atomic properties cause differences in the bonding of metals to S, Se, and Te relative to the metal-to-oxygen bond. Some of these differences are: (d) The metal-to-chalcogen bonds are more covalent than metal-to-oxygen bonds (consequence of (b)). (e) The metal-to-chalcogen bonds often involve the d orbitals of the chalcogen, while this is not possible for the bonding to oxygen (consequence of (c)). (f) The chalcogenides are more polarizable than oxide ions (consequence of (a) and (c)).