Propagation of elastic waves near acoustic axes in crystals under the action of extreme external forces: II. Centrosymmetric crystal in an electric field; III. Crystal under mechanical stress

The problem of degeneracy of elastic waves propagating in the vicinity of acoustic axes in centrosymmetric crystals in an external electric field and in crystals with arbitrary symmetry under uniaxial mechanical stress is considered. For the case of acoustic axes coinciding in the absence of external forces with the symmetry axes, equations describing the directions of external forces at which the difference in the velocities of initially degenerate transverse waves or angles of splitting and shift of the initial degeneracies reach maximum values are obtained. By the example of acoustic axes in LiNbO₃, La₃Ga₅SiO₁₄, Bi₁₂SiO₂₀, and SrTiO₃ crystals, extreme directions of uniaxial mechanical stress and the corresponding changes in the characteristics of transverse waves are calculated. The behavior of acoustic axes under uniaxial compression is analyzed.