New details on the structure of the 3-q modulated phase of quartz type

The 3‐q incommensurate phase of quartz is investigated both from a theoretical point of view based on symmetry considerations and experimentally by transmission electron microscopy (TEM). It is shown that harmonics of the modulation lead to undulation of discommensurations. Close to the lock‐in transition it is observed that the 6′z symmetry of the 3‐q phase can be locally reduced to 3′z. Furthermore, the structure of block boundaries which appear to be always directed along x‐type axes, is analysed.