THE CRYSTAL-STRUCTURE AND OPTICAL-ACTIVITY OF TELLURIUM

The element tellurium has a crystal structure made up of spiral chains of bonded atoms packed in a hexagonal array. Its symmetry leads to th e existence of enantiomorphic forms containing spirals of opposite han dedness, the right-handed one belonging to space group P3(1)21 and the other to P3(2)21, which have opposite optical rotatory powers. The no rmal methods of crystal structure determination cannot distinguish bet ween the enantiomorphs, nor is this feasible using anomalous dispersio n unless there is sufficient asphericity in the tellurium electron den sity due to bonding. Such asphericity also gives rise to small but mea surable differences from unity in the flipping ratios for polarized ne utron scattering due to the polarization dependence of the Schwinger s cattering. This effect is easier to measure than is the intensity diff erence between Bijvoet pairs and it has been used to determine the abs olute structural configuration that corresponds to a particular sense of optical rotation in a tellurium single crystal. The plane of polari zation of the transmitted light rotates in the same sense as the bonde d atoms in the spiral chains. This observation disagrees with a previo us theoretical calculation based on the single polarizable ion model.