Finding merit in dividing neighbors

The IV-VI binary semiconductors and and column V elements crystalize in one of the three varieties derived from the cubic rocksalt structure. The rocksalt structure can be seen as a network of interpenetrating octahedra with an atom at the center of each octahedron and its six nearest neighbors at the vertices. What drives lower symmetry is the problem of placing ten electrons along three perpendicular axes with six equivalent neighbors. Rhombohedral and orthorhombic distortions divide the six immediate neighbors in two distinct sets of first neighbors and second neighbors. This structural competition is at the origin of the the remarkable thermoelectric figure of merit in SnSe and PbTe.