High-Pressure Stability of Energetic Crystal of Dihydroxylammonium 5,5′-Bistetrazole-1,1′-diolate: Raman Spectroscopy and DFT Calculations Zbigniew A. Dreger,* ,† Yuchuan Tao, † Boris B. Averkiev, † Yogendra M. Gupta, † and Thomas M. Klapö tke ‡ † Institute for Shock Physics and Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164-2816, United States ‡ Department of Chemistry, Ludwig Maximilian University of Munich, Munich, D-81377, Germany ABSTRACT: 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 measure- ments 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. 1. INTRODUCTION There is a continuing need for high-performance, insensitive, and environmentally benign explosives for various military and civil applications. Thus, there have been significant and continuing efforts to obtain high-explosive (HE) crystals that possess these attributes. 1-6 The recently synthesized dihydrox- ylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) is a step forward to meet these needs. 7 TKX-50 is a new azole-based high-performance and low-sensitivity high-explosive crystal that is relatively easy to synthesize from low toxicity components. TKX-50 characteristics are superior to RDX (trimethylene- trinitramine), often used as a benchmark for current high explosives. For example, TKX-50 performance, in terms of detonation velocity, is almost 11% higher, whereas sensitivity to impact is three times lower than RDX. In addition, TKX-50 has a lower toxicity to aquatic life than RDX. 7 The attractive characteristics of TKX-50 have prompted an interest in understanding the underlying processes governing its properties. 7,8 Among these, the low sensitivity or high stability is of significant interest because of the growing need for safer high explosives. Because many HE applications involve shock- wave compression, an understanding of the high-pressure and high-temperature response of TKX-50 is important for understanding its low impact sensitivity. Although several properties of TKX-50 have been inves- tigated experimentally 7 and theoretically, 8 its static high- pressure response is completely unknown. Here, we present the first high-pressure studies on TKX-50. We used Raman spectroscopy because vibrational properties are critical for Received: March 25, 2015 Revised: May 14, 2015 Published: May 18, 2015 Article pubs.acs.org/JPCB © 2015 American Chemical Society 6836 DOI: 10.1021/acs.jpcb.5b02879 J. Phys. Chem. B 2015, 119, 6836-6847