We present evidence for the existence of a 1q incommensurate phase at zero stress in quartz, in a temperature interval of about 0.05 K, between the high-temperature P phase and the usual 3q incommensurate phase. White-beam synchrotron-radiation topography gives a direct picture of the three possible orthorhombic 1q domains. The coexistence of the two phases at the first-order 1q-3q transition has also been observed. Further information was obtained by light-scattering experiments, giving a peak of the scattered light intensity at the P-lq transition, and a step increase of the intensity at the lq-3q transition, and by light-depolarization measurements.These results on the observation of the zero-stress 1q phase are discussed in relation with predictions from mean-field theory and from considerations of fluctuation and impurity effects.

High-resolution diffraction experiments using white-beam synchrotron radiation have shown that discontinuous variations of the modulation wave vector q are observed in the incommensurate phase of quartz for a slow rate of temperature variation. Spatial discontinuities of q are also observed under a stable temperature gradient. These observations are discussed in relation with various models, and a mechanism for the temperature variation of q is suggested.

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