EVIDENCE OF FOLDED ACOUSTIC PHONON MODES IN THE LIGHT-SCATTERING SPECTRUM OF INCOMMENSURATE QUARTZ

Low-frequency light scattering spectra of the incommensurate phase of quartz show the presence of weak additional peaks in the 6-12 cm-1 range. These peaks are shown to correspond to 'folded' acoustic phonons with a wavevector close to the incommensurate modulation wavevector. A transverse acoustic mode and a longitudinal mode can be identified in the (ZZ) and (X + Y, Z) polarization configurations, respectively. Selection rules for Raman activation of acoustic modes are derived in the special case of the 'triple-q' incommensurate structure, and the results are compared with experimental observations.