Low-temperature phase transitions in the trigonal modification of Cs3Bi2Br9 and Cs3Sb2I9

The trigonal (P-3 m1) 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 T-C= 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 T-C = 85 K, T-i = 78 K and T-L = 72.1 K. The mononclinic 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.