Elastic anisotropy factors for orthorhombic, tetragonal, and hexagonal crystals

Elastic anisotropy factors are derived for each of the three modes of propagation from the special in-plane phonon-focusing considerations arising when wave vectors are constrained to the symmetry planes of orthorhombic, tetragonal, and hexagonal crystals. Elastic anisotropy factors for the pure transverse and quasi-transverse modes depend upon the symmetry plane, whereas anisotropy factors for the quasilongitudinal mode depend both upon a symmetry plane and a symmetry axis. These anisotropy factors provide a convenient measure of in-plane phonon focusing.