Leaky SAW branches coupled with oblique acoustic axes in trigonal crystals

The relationship between leaky surface acoustic wave (SAW) branches and acoustic axes for bulk waves is studied in the case of trigonal crystals. Three types of inclinations of acoustic axes are considered: 1) axes lying in the mirror planes of acoustic properties, oblique to the Z axis, 2) axes oblique to the mirror planes, 3) inclinated axes with respect to the boundary plane of the crystals. A new leaky SAW branch is found on the X cut of quartz. This branch is coupled with an acoustic axis lying in the boundary plane at an angle of about 66.3° from the Z axis. The other border of its existence region is determined by an acoustic axis oblique to the mirror planes. The theory by Khatkevich (1962) of acoustic axes of general position is revised and the results for trigonal crystals are corrected. It is shown that the relationship under study still holds when small deviations from the boundary plane are introduced in the acoustic axes. However, the permissible angles of deviation are smaller, as a rule, than the angles expected when the degeneracy solution is interpreted in terms of a bulk-wave-reflection problem.