Chemical Bonds and Electronic States in Monoclinic As₂Se₃

Abstract

Materials with highly anisotropic bonds offer an opportunity to study the influence of the chemical bonds on the electronic and vibrational states in solids. Anisotropic bonds lead to smaller units within the solid: chains like in Se, sheets of atoms in the layered crystals or molecules as in sulfur. These materials have strong bonds within and weak bonds between those subunits, henceforth considered as molecules. For most organic molecular crystals the states of the individual molecules are not much affected by their weak mutual interaction but for inorganic solids the situation becomes more complicated because the intermolecular bonds may be strong enough to mix the states of the molecular subunits considerably. Se and Te are the most simple crystals with anisotropic bonds. Only one molecule, the chain belongs to one unit cell but this simplicity is not always an advantage. Splitting of vibrational or electronic states of the molecule by the intermolecular coupling which provides direct access to the weak coupling requires at least two molecules in the unit cell. More complex structures therefore are better suited to study the weak bonds. As₂Se₃ appears as an appropriate material to start with because the bonds are as simple as in Se but its unit cell has sufficient complexity to separate properties due to weak bonds from those due to the strong covalent bonds.