Optical activity in a non-enantiomorphous crystal: AgGaS2

Abstract

There are no symmetry properties in the theory of optical activity that forbid optical activity in the four non-enantiomorphous crystal classes m, mm2, {\bar 4} and {\bar 4}2m, but the existence of such a phenomenon has not, until now, been experimentally verified. This work describes the first positive observation of optical activity in a non-enantiomorphous crystal, silver thiogallate (AgGaS2). Measurements of the refractive indices of this crystal show that it is accidentally optically isotropic at 4974 Å, although it is of class {\bar 4}2m. At this wavelength the rotation of the plane of polarization has been measured for propagation along both diad axes. The optical rotatory power is 522 deg.mm−1, the sense of rotation being opposite for the two diad axes. There was no rotation along the c axis. These observations substantiate the theory of optical activity and show that there are no unsuspected conditions forbidding optical activity in these classes; the previous absence of experimental verification of this phenomenon has been due to the lack of crystals with suitable optical properties.