FERROELASTIC PHASE TRANSITION IN Cs3Bi2I9: A STUDY BY NEUTRON DIFFRACTION

A. Jorio, R.Currat, I.P.Aleksandrova, G.McIntyre, P.Saint-Gregore

Cs₃Bi₂I₉ crystal, which belongs to the family of layered A₃B₂X₉ structure presents an incommensurate phase below approximately 220K as evidenced by NQR experiments (I.P.Aleksandrov et. el., Phys. Solid State 39(6) pp.991 (1997)). We present here the results of a study by means of neutron diffraction between 50K and room temperature, where the crystal phase is hexagonal with the space group P63/mmc. The phase transition at T = 220 K has a ferroelastic character, as exhibited by the splitting of several Bragg reflections, or the increase of width. Below the phase transition temperature, the appearence of small peaks around (006) and (200) Bragg reflections has been observed. The profile of Bragg reflections is discussed and the thermal change of the ferroelastic distortion is analyzed.