The phases of CS3Bi2I9 between RT and 190 K

The structure of Cs3Bi2I9 has been refined to R = 0.0165 on the basis of X- ray diffraction measurements at T = 290 K (STOE IPDS diffractometer; MoKalp ha-radiation; 296 nonequivalent reflection with I > 2 sigma(I)). The absorp tion correction was accounted for by the Gaussian method. The parameters of the hexagonal unit cell are: a = 8.409(1) Angstrom and c = 21.243(5) Angst rom; space group P6(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 BiI6 octahedra. The analysis of the intensities in the temperature range 288 K to 190 K indicates a reversible phase transiti on into a monoclinic phase at T = 218 K which is accompanied by twinning. N o satellite or superstructure reflections were observed with the current re solution of the diffractometer. The Xray results are compared wih the studi es of the temperature dependence of the linear expansion coefficients, opti cal characteristics, and the nuclear quadrupole spectra of Cs3Bi2I9.