  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.
