MICROSCOPIC THEORY OF SECOND-, THIRD-, AND FOURTH-ORDER ELASTIC CONSTANTS. EXAMPLE OF ALKALI-HALIDE COMPOUNDS WITH CsCl STRUCTURE

摘要

The correlation between symmetrically independent elastic constants of the second, third, and fourth orders and interaction energies depending on relative positions of both pairs and triplets of atoms was detected for O_h crystals. Calculation was carried out irrelatively of the perturbation theory and without concrete definition of the interaction energy type depending on relative positions of three particles. The nature of violation of the Cauchy relations for elastic constants of any order due to one of the symmetry-allowed types of three-particle interactions was determined. The obtained relations were used to calculate the third- and fourth-order elastic constants for CsBr, CsCl, and CsI crystals.