Structure and Conductivity in the VIb Group of the Periodic System

In the sequence oxygen, sulfur, selenium, tellurium, and polonium a systematic alteration takes place from diatomic molecules, through ring and chain molecules, to a simple cubic lattice structure formed by atoms. This transition is paralleled by a modification in the electrical behavior from insulator (O,S), to semiconductor (Se,Te), to metal (Po). This paper is concerned with this progressive change in structure and conductivity and with the interrelation of the two phenomena. It discusses first the stability of ring and chain molecules and how the chain lattices of Se and Te may be derived by a simple distortion of the Po structure. Next, it considers schematically the electronic conductivity of selenium from the standpoint of the band picture, and finds that metallic Se is probably an intrinsic P-type conductor. Finally an alternative approach is suggested which connects structure and conductivity by extending qualitatively the concept of quantum-mechanical resonance from molecular structures to lattice structures and makes use of Pauling's resonating bond. The actual structure and conductivity of Se and Te thus appear as the outcome of a resonance between an insulating chain structure held by van der Waals cohesion and a metallic lattice of simple cubic structure, in which for Se the chain structure and for Te the Po lattice makes the stronger contribution.