Recent scientific advances on organic-inorganic hybrid perovskites are mainly focused on the improvement of power conversion efficiency. So far, how compression tunes their electronic and structural properties remains less understood. By combining in situ photocurrent, impedance spectroscopy, and X-ray diffraction (XRD) measurements, we have studied the electrical transport and structural properties of compressed CH3NH3PbI3 (MAPbI3) nanorods. The visible light response of MAPbI3 remains robust below 3 GPa while it is suppressed when it becomes amorphous. Pressure-induced electrical transport properties of MAPbI3 including resistance, relaxation frequency, and relative permittivity have been investigated under pressure up to 8.5 GPa by in situ impedance spectroscopy measurements. These results indicate that the discontinuous changes of these physical parameters occur around the structural phase transition pressure. The XRD studies of MAPbI3 under high pressure up to 20.9 GPa show that a phase transformation below 0.7 GPa, could be attributed to the tilting and distortion of PbI6 octahedra. And pressure-induced amorphization is reversible at a low density amorphous state but irreversible at a relatively higher density state. Furthermore, the MAPbI3 nanorods crush into nanopieces around 0.9 GPa which helps us to explain why the mixed phase of tetragonal and orthorhombic was observed at 0.5 GPa. The pressure modulated changes of electrical transport and visible light response properties open up a new approach for exploring CH3NH3PbI3-based photo-electronic applications.

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In situ X-ray diffraction measurements of cesium azide (CsN 3) were performed at high pressures up to 55.4 GPa at room temperature. Three phase transitions were revealed as follows: tetragonal (I4/mcm, Phase II) → monoclinic (C2/m, Phase III) → monoclinic (P2 1 /m or P2 1 , Phase IV) → triclinic (P1 or P 1 , Phase V), at 0.5 GPa, 4.4 GPa, and 15.4 GPa, respectively. During the II-III phase transition, CsN 3 keeps its layered structure; and the azide anions rotate obviously. The compressibility of Phase II is dominated by the repulsions between azide anions. The deformation of unit cell is isotropic in Phase II and IV, and anisotropic in Phase III. With increasing pressures, the monoclinic angle increases in Phase III and then becomes stable in Phase IV. The bulk moduli of Phase II, III, IV, and V are determined to be 18 ± 4 GPa, 20 ± 1 GPa, 27 ± 1 GPa and 34 ± 1 GPa, respectively. The ionic character of alkali azides is found to play a key role in their pressure-induced phase transitions.

The phase analysis of sodium azide (NaN3) has been investigated by in situ synchrotron X-ray diffraction measurements in a diamond anvil cell up to 52.0 GPa at room temperature. Three pressure-induced phase transitions were observed. The phase transition pressures were determined to be 0.3, 17.3, and 28.7 GPa verified by three different pressure transmitting media. The first high pressure phase, α-NaN3 (0.3 ∼ 17.3 GPa), was identified to be monoclinic with a C2/m space group. The β-NaN3 to α-NaN3 transition is a second-order phase transition, accompanied by the shearing of the Na-layers and the tilting of the azide chains. The second high pressure phase, γ-NaN3 (18.4 ∼ 28.7 GPa), has a lower symmetry than the α-NaN3. A further phase transition of γ-NaN3 to δ-NaN3 at 28.7 GPa was observed.