Sagittally polarized, mixed polarized, and leaky interface acoustic waves at a 180° ferroelectric domain boundary in tetragonal barium titanate

The possibility of the existence of a number of new interface acoustic modes at a 180° ferroelectric domain boundary in tetragonal single-crystal barium titanate is shown by the use of computational and novel analytical techniques. The paper contains further study in more detail of sagittally and quasisagittally polarized interface waves described recently by the present authors (1995 IEEE Ultrason. Symp., p.649) and the prediction of a family of leaky interface waves with symmetric, antisymmetric, and quite unexpectable nonsymmetric distribution of the wave field with respect to the domain-boundary plane. A secular equation for sagittally polarized interface modes is derived by direct integrating equations of motion. The important condition for the existence of this mode is a concavity of the slowness surface for the quasishear bulk wave in the sagittal plane. It is found that the piezoelectric effect in BaTiO₃ gives rise to the concavity of the slowness surface for the fastest (quasilongitudinal) bulk wave that is impossible in nonpiezoelectric case. An unusual high-velocity sagittally polarized leaky wave is described. All its partial waves decay with distance from the boundary that is connected with the inverse direction of the power flow of radiated wave component, as shown. A fast interface wave without attenuation along the propagation direction is found for one of the leaky wave branches in the form of an isolated solution. The directions of strong phonon focusing for the slow interface mode are calculated