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It is shown that the incommensurate (IC) phase near the α ↔ β transition point in quartz (SiO 2 ) is an improper ferroelastic one and should be split into domains. The birefringence induced by such a ferroelastic phase is estimated, and it is shown that it can be as strong as is observed in experiments only if one accepts a new model for the IC transition in quartz.

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We show that a ferroelectric phase transition takes place in the incommensurate phase of the K 2 SeO 4 crystal. The ferroelectric character of the incommensurate phase explains the second-harmonic generation observed in the corresponding temperature range.

The Debye-Waller factors for displacive-type incommensurate (IC) structures are calculated taking into account the correlation of the phases of the modulation. It is shown that and that IC satellites should not be observed in the immediate vicinities of transitions.

The ferroelasticity effect of the incommensurate phase of quartz is discussed. It is pointed out that the effect discussed in the paper by Saint-Grégoire et al (Saint-Grégoire P, Snoeck E, Roucau C, Luk'yanchuk I and Janovec V 1996 JETP Lett. 64 410) is negligibly small, and cannot explain the anomalous observations near the transition in quartz.

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