Optical activity and crystalline structure of crystals of the langasite family

The dependences of the refraction on the structural parameters are considered for the La3Ga5.5Nb0.5O14, La3Ga5.5Ta0.5O14, La3Ga5SiO14, Ca3Ga2Ge4O14, and Sr3Ga2Ge4O14 crystals of the langasite family. It is shown that the angle of deviation of the 1a octahedron faces that are normal to the optical axis from 60A degrees is one of the main sources of optical activity of these crystals. The interaction of the cations belonging to the 3e dodecahedron and 1a octahedron, cations of the 3e dodecahedron and 2d tetrahedron, and the repulsion of O2- ions are believed to be the basic interactions affecting the angle value. The dependences of the angle on the crystal-chemical characteristics of the considered crystals are analyzed. The role of the relative sizes of the structural polyhedra is demonstrated.