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

Abstract

By means of hyper-Raman scattering, a silent soft phonon was observed in \ensuremath{\beta}-quartz for the first time. The soft phonon was found to become underdamped at high temperatures. Both the square of the frequency ${\mathrm{\ensuremath{\omega}}}_{0}^{2}$ and the integrated intensity ${\mathit{I}}_{0}$ of the soft phonon were found to obey Landau-Cochran's soft-mode theory. The \ensuremath{\alpha}-\ensuremath{\beta} phase transition of \ensuremath{\beta}-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.