Incommensurate phase of quartz : III. Study of the coexistence state between the incommensurate and the α-phases by neutron scattering and electron microscopy

A detailed study of the lock-in transition of the incommensurate phase of quartz has been made by two complementary techniques : elastic neutron diffraction and electron microscopy. The neutron experiment was made on bulk samples (a few cm3) with a simultaneous thermal expansion measurement. The transition from the incommensurate phase to the α-phase is 1st order and in general a single interface sweeps the sample. The modulation wavevector was observed to decrease continuously in the vicinity of this interface. Electron microscopy allows to obtain a direct space picture of this coexistence state on very thin samples (a few 100 nm) and to observe how the incommensurate modulation period increases from 10 nm to macroscopic size with the help of « dislocations » in the incommensurate networks of columnar Dauphiné twin domains. (Analogous results obtained on AlPO4 are also presented). The results of these two experimental techniques are well correlated and are discussed in relation to previous results on the quartz transition mainly from light scattering studies.