Pressure and temperature effects on the Raman spectra of LLM-105

Currently, only one crystal structure of LLM-105 (2,6-diamino-3,5-dinitropyrazine-1-oxide) (P2₁/n) has been discovered, and there are still debates on its phase transition point and phase diagram. Based on previous work, we performed crystal structure, Raman spectra, and vibrational properties calculations on LLM-105 crystal. Our results indicate that the crystal structure of LLM-105 remains stable until compressed to 49 GPa, beyond which it may undergo two phase transitions at pressure intervals of 49.0-49.1 GPa and 51.4-51.5 GPa, respectively. Analysis of Raman shift results suggests that these two phase transitions may be reversible, with an intermediate phase possibly acting as a transition phase. Additionally, based on the quasi-harmonic approximation, we fitted the experimental data of LLM-105 lattice expansion state, obtaining the volume at zero pressure and using it for Raman spectra calculations. The results demonstrated the accuracy of this quasi-harmonic approximation method in describing the redshift of Raman peaks during the heating process and the excitation ratio of Raman peaks in different wavenumber ranges.