Pressure-induced phase transitions and templating effect in three-dimensional organic-inorganic hybrid perovskites

Pressure-induced structural changes of conducting halide perovskites (CH₃NH₃)SnI₃, (CH₃NH₃)_0.5(NH₂CH=NH₂)_0.5SnI₃, and (NH₂CH=NH₂)SnI₃, have been investigated using synchrotron x-ray powder diffraction. In contrast to low-temperature structural changes, no evidence of an increased ordering of the organic cations was observed under pressure. Instead, increase in pressure results first in a ReO₃-type doubling of the primitive cubic unit cell, followed by a symmetry distortion, and a subsequent amorphization above 4 GPa. This process is reversible and points towards a pressure-induced templating role of the organic cation. Bulk compressions are continuous across the phase boundaries. The compressibilities identify these hybrids as the most compressible perovskite system ever reported. However, the Sn-I bond compressibility in (CH₃NH₃)SnI₃ shows a discontinuity within the supercell phase. This is possibly due to an electronic localization.