Conspicuous interatomic bonding in chalcogenide crystals and implications on electronic, optical, and elastic properties

Chalcogenide crystals are a unique class of materials very different from semiconductors or metallic alloys. They also have many practical applications, especially in relation to their optical properties. However, the fundamental understanding of their electronic structure and physical properties is rather scattered and incomplete. We present a detailed study using first-principles calculations on the electronic structure, interatomic bonding, and optical and mechanical properties for 32 chalcogenide crystals. They consist of 22 binary (AnBm) and 10 ternary (AnA′Bm) crystals with A = Ag, As, Cu, Ge, Sb, Sn, Cd, Zn; A′ = In, Ge, Sn; and B = S, Se, Te with n = 1, 2, 4 and m = 1, 2, 3, 4, 9. We use the novel concept of total bond order density as a single quantum mechanical metric to characterize the internal cohesion of these crystals and correlate them with the calculated properties, especially the mechanical properties. Based on this large database, we are able to identify some new and conspicuous observations to reach some useful conclusions related to the chemical composition of the chalcogenide compounds and their complex interatomic interactions. Specific examples from crystals and their unique properties in relation to the elements A and A′ and chalcogenide element B (S, Se, and Te) are discussed and highlighted. Finally, generalization of the observed trends and empirical rules can be extended to much larger classes of ternary and quaternary chalcogenide crystals and glasses so far unexplored.