The symmetry groups of nonrigid crystals

Abstract

S~nmetry-group construction for nonrigid crystals is described, i.e., crystals in which the nuclei may perform motions different from small vibrations (motion between isoenergetic potential-energy minima separated by potential barriers: rotation around a certain center, inversion motion, etc). The symmetry group of a nonrigid crystal is constructed on the basis of the permutation-inversion symmetry, The construction of such a group is discussed for a system with equivalent groups of nuclei (crystals and molecules with equivalent methyl groups). Examples are given of symmetry-group construction for nonrigid crystals of strontium titanate and sodium nitrate in the region of the phase transitions, Some aspects of nonrigid-system dynamics are considered with reference to a one-dimensional chain of homonuclear diatomic molecules.