Phonon focusing in the $\alpha$- and $\beta$-quartz phases

The directions of the phonon focusing in the $\alpha$- and $\beta$-quartz phases in the 20—600°C temperature range comprising a second-order transition at Tc = 573°C have been determined. Temperature-dependent variation of focusing both on a symmetry plane and in space has been studied. It is shown that, as the temperature is increased, the phonon focusing becomes weaker in the $\alpha$-phase and disappears in the $\beta$-phase: all waves become virtually ordinary.