An anomalous high-pressure phase and decompression-induced amorphization in dinitrotoluene

We investigate the high-pressure behavior of 2,4-dinitrotoluene (2,4-DNT), an energetic molecular solid, using in situ Raman spectroscopy (up to ∼19 GPa) and synchrotron x-ray diffraction (up to ∼12.3 GPa). Raman spectra reveal conformational rearrangements of the ortho-NO2 group near 1.5 GPa and a structural phase transition between 4 and 8 GPa. X-ray diffraction measurements corroborate this transition, showing the onset at ∼4.5 GPa and sluggish completion by ∼8.4 GPa. The ambient phase shows pronounced anisotropic compression, consistent with its layered crystal structure, while both phases display negative linear compressibility. In the high-pressure phase, only one-dimensional long-range order is preserved, accompanied by the formation of new hydrogen bonds. A larger phonon gap in 2,4-DNT (∼104 cm-1) indicates reduced impact sensitivity compared to trinitrotoluene [trinitrotoluene (TNT), ∼80 cm-1]. Upon decompression, two distinct recovery pathways are observed: samples released from P ≤ 10 GPa revert to the ambient phase with hysteresis, while those released from P ≥ 12.3 GPa undergo decompression-induced amorphization due to the changes in the hydrogen bonding network. The contrasting compression and recovery behavior of 2,4-DNT and TNT are also discussed, highlighting the critical role of functional-group interactions and pressure history in governing phase stability and amorphization in energetic molecular crystals.