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 ∼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.