Anisotropy of Optical Activity in Langasite Family Crystals

The rotation of the plane of polarization of light transmitted in the directions parallel (ρ||) and perpendicular (ρ⊥) to the optical axis has been calculated for langasite family crystals. It is shown that the ρ|| and ρ⊥ values are, respectively, negative and positive for all the right-handed crystals under study. An analysis of the interatomic potential constructed based on X-ray diffraction data has suggested that this anisotropy of optical activity is structurally based of electron density regions shaped as helices. The atomic helix with an axis parallel to the optical axis is right-handed and three-start, whereas in the direction perpendicular to the optical axis there are three left-handed two-start helices, which intersect in the origin of the unit-cell coordinate system.