ON THE DISPLACIVE CHARACTER OF THE PHASE-TRANSITION IN QUARTZ - A HARD-MODE SPECTROSCOPY STUDY

The temperature evolution of the frequencies and absorption cross section of the infrared-active phonons near 795 cm-1 and 695 cm-1 and the Raman-active mode near 355 cm-1 follow that of the symmetry-breaking order parameter of the alpha-beta-phase transition in quartz. The IR signal at 695 cm-1 and the Raman signal at 355 cm-1 are symmetry forbidden in the beta-phase. The experimental results show that these signals do, indeed, disappear at T > T(c) + 10 K. No signal related to microdomains of the alpha-phase was found at these temperatures. Transition models based on the beta-phase being a time-averaged structure with local alpha-states and lifetimes of clusters being much longer than phonon times are incompatible with these observations. The experimental results affirm the displaciveness of the phase transition although the existence of small amounts of alpha or INC phase in a matrix of beta-structure cannot be ruled out.