Pressure-Induced Phase Transition of Hydrogen Storage Material Hydrazine Bisborane: Evolution of Dihydrogen Bonds

We report the high-pressure behavior of dihydrogen-bonded hydrogen storage material hydrazine bisborane (BH₃N₂H₄BH₃, HBB) via in situ angle-dispersive X-ray diffraction (ADXRD) and Raman spectroscopy in a diamond anvil cell up to 2.0 GPa. A reversible phase transition at 0.4 GPa was confirmed by ADXRD experiments. The Rietveld refinement showed the high-pressure phase was consistent with the crystal structure of α′-phase (low-temperature phase). Through the analysis of structure changes, Raman spectroscopy, and the Hirshfeld surface, we studied the evolution of dihydrogen bonds under high pressure and attributed the pressure-induced phase transition to the distortion and rotation of the NH₂–NH₂ group. This work will further the understanding of the characteristics of dihydrogen bonds and provide some contribution to future hydrogen storage applications of HBB

Pressure-Tailored Band Gap Engineering and Structure Evolution of Cubic Cesium Lead Iodide Perovskite Nanocrystals

Metal halide perovskites (MHPs) have attracted increasing research attention given the ease of solution processability with excellent optical absorption and emission qualities. However, effective strategies for engineering the band gap of MHPs to satisfy the requirements of practical applications are difficult to develop. Cubic cesium lead iodide (α-CsPbI₃), a typical MHP with an ideal band gap of 1.73 eV, is an intriguing optoelectric material owing to the approaching Shockley–Queisser limit. Here, we carried out a combination of in situ photoluminescence, absorption, and angle-dispersive synchrotron X-ray diffraction spectra to investigate the pressure-induced optical and structural changes of α-CsPbI₃ nanocrystals (NCs). The α-CsPbI₃ NCs underwent a phase transition from cubic (α) to orthorhombic phase and subsequent amorphization upon further compression. The structural changes with octahedron distortion to accommodate the Jahn–Teller effect were strongly responsible for the optical variation with the increase of pressure. First-principles calculations reveal that the band-gap engineering is governed by orbital interactions within the inorganic Pb–I frame through the structural modification. Our high-pressure studies not only established structure–property relationships at the atomic scale of α-CsPbI₃ NCs, but also provided significant clues in optimizing photovoltaic performance, thus facilitating the design of novel MHPs with increased stimulus-resistant capability

Pressure Effects on Structure and Optical Properties in Cesium Lead Bromide Perovskite Nanocrystals

Metal halide perovskites (MHPs) are gaining increasing interest because of their extraordinary performance in optoelectronic devices and solar cells. However, developing an effective strategy for achieving the band-gap engineering of MHPs that will satisfy the practical applications remains a great challenge. In this study, high pressure is introduced to tailor the optical and structural properties of MHP-based cesium lead bromide nanocrystals (CsPbBr₃ NCs), which exhibit excellent thermodynamic stability. Both the pressure-dependent steady-state photoluminescence and absorption spectra experience a stark discontinuity at ∼1.2 GPa, where an isostructural phase transformation regarding the <i>Pbnm</i> space group occurs. The physical origin points to the repulsive force impact due to the overlap between the valence electron charge clouds of neighboring layers. Simultaneous band-gap narrowing and carrier-lifetime prolongation of CsPbBr₃ trihalide perovskite NCs were also achieved as expected, which facilitates the broader solar spectrum absorption for photovoltaic applications. Note that the values of the phase change interval and band-gap red-shift of CsPbBr₃ nanowires are between those for CsPbBr₃ nanocubes and the corresponding bulk counterparts, which results from the unique geometrical morphology effect. First-principles calculations unravel that the band-gap engineering is governed by orbital interactions within the inorganic Pb–Br frame through structural modification. Changes of band structures are attributed to the synergistic effect of pressure-induced modulations of the Br–Pb bond length and Pb–Br–Pb bond angle for the PbBr₆ octahedral framework. Furthermore, the significant distortion of the lead–bromide octahedron to accommodate the Jahn–Teller effect at much higher pressure would eventually lead to a direct to indirect band-gap electronic transition. This study enables high pressure as a robust tool to control the structure and band gap of CsPbBr₃ NCs, thus providing insight into the microscopic physiochemical mechanism of these compressed MHP nanosystems