Some Optical and Mechanical Properties of Right- and Left-Handed Langasite Family Crystals and Their Structural Basis

— Some optical and mechanical properties of a number of langasite family crystals have been compared. It is shown that electron-density regions in the form of helices, twisted in the opposite directions for the right- and left-handed crystals, make a structural basis of the optical activity. The characteristic surfaces of Young’s modulus have been calculated for some langasite family crystals, and the difference between them is explained from the structural point of view. The fracture anisotropy under Berkovich indentation of the (0001) basal plane has been considered for five crystals. It is shown that crystallographic cracks (straight-line ones and those formed exactly at the indenter angles) are obtained at indenter rotations with a step of 60° from the position in which the indenter angles coincide with the 〈 1̅ 010〉 directions. The structural basis of piezoelectricity of langasites is established, and a difference in the piezoelectric properties of the right- and left-handed crystals (which is important for manufacture of piezoelectric devices) are shown.