Polarization and energy flow of transverse waves propagating along the acoustic axes of crystals when external static fields are present

Using an expansion of the thermodynamic potential and the quasi-static approximation conditions, a system of equations is obtained determining the velocity of elastic waves in piezoelectric crystals acted upon by static fields. For certain classes of crystals, a nonlinear parameter is calculated which represents the elliptic polarization of the transverse waves propagating along the acoustic axes when a constant mechanical stress or an electric field acts on the crystal. An expression is obtained for the energy flow of acoustic waves in crystals in external fields.