High-pressure studies of Mn2(C2H6N6)4(NO3)4·2H2O by Raman scattering, infrared absorption, and synchrotron X-ray diffraction

Mn₂(C₂H₆N₆)₄(NO₃)₄·2H₂O (Mn) as one of the energetic coordination complexes was chosen for high-pressure research. In this work, Mn was analyzed by in situ Raman scattering, infrared absorption, and synchrotron angle-dispersive X-ray diffraction (ADXRD) technologies up to ~20 GPa at room temperature. The vibrational modes of Mn at ambient pressure were comprehensively resolved based on the experimental results. Detailed spectral analyses revealed that Mn underwent three pressure-induced phase transitions at 0.5, 2.5, and 5.7 GPa, respectively. ADXRD experiments confirmed the existence of these three phase transitions in Raman and infrared spectra analyses. Based on the analysis of the vibrational spectra and the changes of lattice parameters under pressure, it can be considered that the deformation of the 3-hydrazino-4-amino-1, 2, 4-triazole (HATr) ligand led to the first phase transition, and the distortion of the triazole ring induced the second phase transition, and the rearrangement of the hydrogen bonds resulted in the third phase transition. In addition, it can be inferred from Raman spectra and ADXRD data that Mn may have experienced the abnormal expansion during the first phase transition. This work may lay the foundation for further investigating the structure and properties of energetic coordination complexes under pressure.