What is it about?
The trigonal (P-3m1) modification of Cs3Bi2Br9 and Cs3Sb2I9 have been studied using NQR, X-ray single crystal and powder pattern methods. Moreover, the heat capacity was measured in a wide temperature interval: 4–300 K. In Cs3Bi2Br9 a second-order phase transition was found at TC = 96 K. The low-temperature phase is monoclinic (C12/c1), with the unit cell doubled along the [001] direction. Cs3Sb2I9 has a sequence of phase transitions at TC =85 K, Ti =78 K and TL =72.1 K. The monoclinic structure below 85 K is isomorphic with the low-temperature structure of Cs3Bi2Br9. According to calorimetric data the lock-in transition at 72.1 K is discontinuous.
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Why is it important?
The structures of A3B2X9 compounds have been widely investigated at room tempera- ture in connection with a special feature of their atomic arrangement. Crystals with different compositions (A=alkaline metal, B=Fe, As, Sb, Bi, Tl, Cr, W, Mo; X=Cl, Br, I) form the set of polytypes with structures depending on the stacking of AX3-layers: h, hcc or c.
Perspectives
Below 78K, Cs3Sb2I9 presents an interesting case where the commensurate and incommensurate modulations coexist in the structure. The above-mentioned data have stimulated more extensive investigations of the A3B2X9 family in the low-temperature region. In this article we report on NQR, calorimetric and X-ray studies of trigonal Cs3Bi2Br9 and Cs3Sb2I9. The crystals of Cs3Bi2Br9 and Cs3Sb2I9 exhibit second-order phase transitions between the room temperature trigonal P-3 m1 and monoclinic C12/c1 structures. In the crystals under study, a lattice instability takes place at the boundary of the Brillouin zone, as distinct from Cs3Bi2I9, where the transition occurs near the zone center. On cooling below 85K, Cs3Sb2I9 exhibits two subsequent transitions: to an incommensurate phase at 78 K and to a commensurate low-temperature structure at 72 K.
Dr Bagautdin Bagautdinov
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This page is a summary of: Low-Temperature Phase Transitions in the Trigonal Modification of Cs 3 Bi 2 Br 9 and Cs 3 Sb 2 I 9, Phase Transitions, January 2002, Taylor & Francis,
DOI: 10.1080/01411590290029863a.
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