Raman and Synchrotron X-ray Diffraction Studies of Cd₂(C₂H₆N₆)₄(NO₃)·4H₂O under High Pressures

Abstract

Cd 2 (C 2 H 6 N 6 ) 4 (NO 3 ) 4 •H 2 O (CdHATr), a triazole-based energetic compound, was selected for high-pressure research. Employing in situ Raman scattering and synchrotron angle-dispersive X-ray diffraction (ADXRD) technologies, this study investigated CdHATr up to ∼16.3 GPa at room temperature. The vibrational modes of CdHATr at ambient pressure were comprehensively resolved based on the experimental results. Detailed spectral analyses revealed that CdHATr underwent three pressure-induced phase transitions at 0.5, 2.2, and 5.2 GPa. ADXRD experiments confirmed the presence of these phase transitions, as observed in Raman spectral analyses. By analyzing the changes of the vibrational spectra and the lattice parameters under pressure, it is suggested that the first phase transition arises from the deformation of the 3-hydrazino-4-amino-1,2,4-triazole (HATr) ligand, the second phase transition results from the rearrangement of hydrogen bonds, and the third phase transition is caused by the conformational change of the triazole ring. ADXRD results show that CdHATr may experience an abnormal expansion at 0.5 GPa, which is probably caused by the deformation of the HATr ligand. This work contributes to the understanding of the structures of triazole-based energetic compounds under pressure, which will help in the synthesis of new compounds in the future.