High-Pressure Stability of Energetic Crystal of Dihydroxylammonium 5,5'-Bistetrazole-1,1'-diolate: Raman Spectroscopy and DFT Calculations.

The vibrational and structural behavior of a novel, energetic crystal, dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50), was examined over a broad pressure range to elucidate its structural and chemical stability at high pressures. Raman measurements were performed on single crystals compressed to 50 GPa in a diamond anvil cell, and data were obtained over the entire frequency range of TKX-50 Raman activity. The Raman spectroscopy results were complemented by density functional theory (DFT) calculations to provide vibrational mode assignments and to gain insight into pressure effects on the vibrational and crystal response of TKX-50. Several features were observed in Raman spectra measured in the ranges 4-10, 10-13, and 32-36 GPa. We suggest that the changes between 32 and 36 GPa may be associated with a phase transformation. In addition, a number of vibrational modes showed intensity exchange and avoided crossing of vibrational frequency at various pressures, characteristic of the coupling of modes. Despite all these pressure effects, the compression of TKX-50 to 50 GPa and the subsequent release of pressure did not result in any irreversible spectral changes, demonstrating its remarkable chemical stability. DFT calculations, using the PBE functional with an empirical dispersion correction by the Grimme, PBE-D method, were used to calculate pressure effects on Raman frequencies and unit cell parameters. The calculated Raman shifts to 20 GPa are in good overall agreement with the measured shifts over a broad range of frequencies. The calculations also show that TKX-50 exhibits anisotropic compressibility, with a highly incompressible response along the a axis. The calculated bulk modulus, a measure of average stiffness, of TKX-50 is significantly higher than the calculated or measured bulk moduli of other energetic crystals. We suggest that the strong intermolecular interactions and the coupling of vibrational modes may potentially contribute to the shock insensitivity of TKX-50. This work demonstrates the robust high-pressure response of TKX-50, making this crystal attractive for practical applications.