Paradoxical excitation of a non-piezoelectric acoustic bulk wave by electrical fields at the surface

The paradoxical case of efficient excitation of a non-piezoelectric bulk wave in a cubic piezoelectric crystal has been found in the process of verifying the reciprocity of propagation constants of leaky surface acoustic waves in a configuration of two piezoelectric halfspaces separated by a thin air gap. The calculations have been performed for the structure YZ-LiNbO₃/Bi₁₂ GeO₂₀, rotated around the Y-axis. It has been found that the Rayleigh wave propagating in YZ-LiNbO₃ radiates bulk shear waves into Bi₁₂GeO₂₀ even in the case when the propagation direction of the radiated wave coincides with the direction of zero piezoactivity in the bulk of the crystal. The mechanical coupling of the piezoelectrical crystals due to the presence of air in the gap has resulted in jump-like behaviour at the angle of rotation corresponding to the appearance of radiation of the second quasishear bulk wave into Bi₁₂GeO₂₀