Defect Perovskites under Pressure: Structural Evolution of Cs2SnX6 (X = Cl, Br, I)

The application of pressure on halide perovskite materials provides key insights into their structural properties and the collective motions of their basic structural units. Here, we use synchrotron X-ray diffraction and Raman spectroscopy measurements combined with density functional theory calculations to perform a comprehensive study on the structural and vibrational properties of the so-called defect halide perovskites Cs2SnX6 (X = Cl, Br, I) under high hydrostatic pressures up to 20 GPa. We find that, while Cs2SnCl6 and Cs2SnBr6 retain a face-centered cubic (FCC) structure for all studied pressures, Cs2SnI6 undergoes successive phase transformations initially to a more disordered structure and secondly to a low-symmetry monoclinic I2/m phase. The first transition is only evidenced by certain features emerging in the Raman spectra at similar to 3.3 GPa, whereas the latter happens in a pressure window of around 8-10 GPa and involves tilting and elongation of SnI6 octahedra and hysteretic behavior with pressure release. Overall, the results reveal that pressure can alter significantly the structural characteristics of certain defect perovskites, such as Cs2SnI6, which is also anticipated to affect their optoelectronic properties.