Junru Jiang

Plasma-assisted synthesis and pressure-induced structural transition of single-crystalline SnSe nanosheets

Two-dimensional tin selenide (SnSe) nanosheets were synthesized using a plasma-assisted direct current arc discharge method. The structural characterization indicates that the nanosheets are single-crystalline with an average thickness of ~25 nm and a lateral dimension of 500 nm. The high pressure behaviors of the as-synthesized SnSe nanosheets were investigated by in situ high-pressure synchrotron angle-dispersive X-ray diffraction and Raman scattering up to ~30 GPa in diamond anvil cells at room temperature. A second-order isostructural continuous phase transition (Pnma → Cmcm) was observed at ~7 GPa, which is considerably lower than the transition pressure of bulk SnSe. The reduction of transition pressure is induced by the volumetric expansion with softening of the Poisson ratio and shear modulus. Moreover, the measured zero-pressure bulk modulus of the SnSe nanosheets coincides with bulk SnSe. This abnormal phenomenon is attributed to the unique intrinsic geometry in the nanosheets. The high-pressure bulk modulus is considerably higher than the theoretical value. The pressure-induced morphology change should be responsible for the improved bulk modulus.

Pressure-induced Phase Transitions in Rubidium Azide: Studied by In-situ X-ray Diffraction

We present the in-situ X-ray diffraction studies of RbN3 up to 42.0 GPa at room temperature to supplement the high pressure exploration of alkali azides. Two pressure-induced phase transitions of α-RbN3 → γ-RbN3 → δ-RbN3 were revealed at 6.5 and 16.0 GPa, respectively. During the phase transition of α-RbN3 → γ-RbN3, lattice symmetry decreases from a fourfold to a twofold axis accompanied by a rearrangement of azide anions. The γ-RbN3 was identified to be a monoclinic structure with C2/m space group. Upon further compression, an orthogonal arrangement of azide anions becomes energetically favorable for δ-RbN3. The compressibility of α-RbN3 is anisotropic due to the orientation of azide anions. The bulk modulus of α-RbN3 is 18.4 GPa, quite close to those of KN3 and CsN3. By comparing the phase transition pressures of alkali azides, their ionic character is found to play a key role in pressure-induced phase transitions.

High Pressure Raman Scattering and Synchrotron X-ray Diffraction Studies of Benzyl Azide

Benzyl azide was investigated by high-pressure Raman scattering spectroscopy and X-ray diffraction technologies. A complete vibrational analysis of benzyl azide was performed by combining the experimental measurements and theoretical calculations using DFT-based scaled quantum chemical approach. The high-pressure Raman spectra and calculation results indicate that benzyl azide underwent a conformational change at 0.67 GPa accompanied by rotation of methylene group and azide group. The frequency of the CH2 bending mode decreases with increasing pressure due to the increase of the C-H···π interactions, which is similar to the role of the hydrogen bond. A liquid to solid phase transition occurred at 2.7 GPa, which was confirmed by the X-ray diffraction measurements. As the pressure reached 25.6 GPa, all the azide group vibrations vanished, indicating that the decomposition pressure of the molecular azide groups in organic azides is lower than that of the azide ions in inorganic azides.

High pressure studies of trimethyltin azide by Raman scattering, IR absorption, and synchrotron X-ray diffraction

We present the high-pressure behavior of energetic material trimethyltin azide ((CH3)3SnN3, TMSnA) by in situ Raman scattering, IR absorption, and synchrotron angle-dispersive X-ray diffraction techniques with pressure up to 35 GPa at room temperature. Raman and IR spectra analyses provide evidence for two reversible phase transitions at 1.4 and 6.6 GPa, respectively, which were confirmed by XRD results. The first phase transition at 1.4 GPa is attributed to the rotation of CH3 group, resulting in the symmetry transformation from Pnna to P222. Upon compression, the distortion of the unit cell induced a significant modification of the CH3 group leading to the second phase transition at 6.6 GPa. Further analyses of the Raman and IR spectra reveal that the azide group becomes increasingly asymmetric with increasing pressure. At 35.2 GPa, all the Raman vibration modes of the azide group vanish, which means the molecular azide group transforming into a nonmolecular state. The previous studies indicate that the polymerization of nitrogen undergoes an amorphization process. We thus anticipate the high-pressure study of TMSnA which may offer a promising strategy for exploiting and improving in the formation of polymeric nitrogen of energetic azides.

High pressure studies of Ni3[(C2H5N5)6(H2O)6](NO3)6·1.5H2O by Raman scattering, IR absorption, and synchrotron X-ray diffraction

Herein, we report the high-pressure studies of Ni3[(C2H5N5)6(H2O)6](NO3)6·1.5H2O (1) by in situ Raman scattering, infrared absorption, and synchrotron angle-dispersive X-ray diffraction techniques up to ∼22 GPa at room temperature. We assigned all the vibration modes of 1 at ambient conditions. Detailed spectroscopy analyses revealed a chemical transformation at ∼0.75 GPa and a phase transition at ∼4.7 GPa, which are related to the behaviors of energetic ligands and flexible structures. Upon compression, the distortion of the energetic ligand induced the disconnection of NH2 and the triazole ring at 0.75 GPa. Further analyses of the N–H vibration modes indicated the phase transition at 4.7 GPa accompanied with the rearrangement of hydrogen bonds. In addition, the lattice structure abnormally expanded above 8.6 GPa due to the deformation of nitrate ions and the extension of the triazole ring. This study helps to understand the properties and the behavior of energetic coordination complexes under high pressure.

High-pressure spectroscopic study of silver azide

The pressure-induced structural phase transition in AgN3 was investigated using Raman and infrared spectroscopy. At ambient pressure, the Raman-active bending and asymmetric stretch modes of NNN indicate the N3− is nonlinear and/or asymmetrical. Upon compression, a reversible orthorhombic-to-tetragonal phase transition was observed at ∼2.7 GPa, while the degeneration of the lattice modes was present due to the increasing symmetrical element. The softening of the v2(B2u) mode and hardening of the v2(B3u) mode demonstrate the rotation of the N3− during the phase transition, which reveals the essential source of the a-axis expansion and the structural phase transition. The perpendicular arrangement of N3− in the tetragonal structure is energetically favorable and beneficial for structural stability.

Anomalous phase transition of InN nanowires under high pressure

Uniform InN nanowires were studied under pressures up to 35.5 GPa by using in situ synchrotron radiation x-ray diffraction technique at room temperature. An anomalous phase transition behavior has been discovered. Contrary to the results in the literature, which indicated that InN undergoes a fully reversible phase transition from the wurtzite structure to the rocksalt type structure, the InN nanowires in this study unusually showed a partially irreversible phase transition. The released sample contained the metastable rocksalt phase as well as the starting wurtzite one. The experimental findings of this study also reveal the potentiality of high pressure techniques to synthesize InN nanomaterials with the metastable rocksalt type structure, in addition to the generally obtained zincblende type one.