The phases of Cs₃Bi₂I₉ between RT and 190 K

The structure of Cs 3 Bi 2 I 9 has been refined to R = 0.0165 on the basis of X-ray diffraction measurements at T = 290 K (STOE IPDS diffractometer; Mo K α -radiation; 296 nonequivalent reflection with I > 2 σ ( I )). The absorption correction was accounted for by the Gaussian method. The parameters of the hexagonal unit cell are: a = 8.409(1) Å and c = 21.243(5) Å; space group P 6 3 / mmc . The structure consists of identical perovskite-like layer fragments described by the general formula A 3 B 2 X 9 (A = Cs, B = Bi, X = I) with ordered vacancies in the B-sublattice. These fragments alternate along the c -axis and are related by mirror planes which contains the shared faces of the BiI 6 octahedra. The analysis of the intensities in the temperature range 288 K to 190 K indicates a reversible phase transition into a monoclinic phase at T = 218 K which is accompanied by twinning. No satellite or superstructure reflections were observed with the current resolution of the diffractometer. The X-ray results are compared wih the studies of the temperature dependence of the linear expansion coefficients, optical characteristics, and the nuclear quadrupole spectra of Cs 3 Bi 2 I 9 .