An incommensurate phase is observed in a Cs 3 Bi 2 I 9 crystal, which belongs to the family of layered A 3 B 2 X 9 structures. Data from 127 I NQR and x-ray powder diffraction patterns show that the second-order normal phase—incommensurate phase transition at T i =220 K is accompanied by a P 6 3 / mmc ↔ P 2 1 / m symmetry change and takes place at the center of the Brillouin zone. The nature of the instability leading to the appearance of this little-studied form of incommensurability is discussed.
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Aleksandrov et al. (1997) studied this question.
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