Observation of the silent soft phonon in β-quartz by means of hyper-Raman scattering

By means of hyper-Raman scattering, a silent soft phonon was observed in {β}-quartz for the first time. The soft phonon was found to become underdamped at high temperatures. Both the square of the frequency ω₀² and the integrated intensity I₀ of the soft phonon were found to obey Landau-Cochran's soft-mode theory. The {α}-{β} phase transition of {β}-quartz is found to be well described by a simple displacive-type phase transition with the silent soft phonon, in contrast to many studies which suggest an order-disorder phase transition.