The surfaces of solution-grown β-HMX (β-octahydro-1,3,5,7- tetranitro-1,3,5,7-tetrazocine) crystals were studied using vibrational sum-frequency generation spectroscopy (SFG). In earlier work (Kim, H.; Lagutchev, A.; Dlott, D. D. Propellants, Explosives, Pyrotechnics 2005, 31, 116), such SFG spectra evidenced considerable variability as the ∼200-μm diameter laser beam was translated over crystal surfaces. We have now found this variability results from ubiquitous small deposits of δ-HMX on the surface of β-HMX single crystals. SFG is a selective probe of molecules in noncentrosymmetric environments. Since β-HMX is centrosymmetric, only the surface is SFG-active; however, δ-HMX is noncentrosymmetric, so a tiny β-HMX deposit on a β-HMX surface could dominate the SFG spectrum. We have found that rapid evaporation from tiny droplets of HMX solution produces only δ-HMX, presumably because the polar boat conformation of δ-HMX is stabilized in solution. We believe the δ-HMX deposits are created by the rapid drying of droplets of mother liquor clinging to solution-grown crystal surfaces. In this paper, we present SFG spectra of δ-HMX nanocrystals and β-HMX surfaces in the CH-stretch and NO2-stretch regions. Through an analysis of the β-HMX/δ-HMX SFG intensity ratios and the surface variability, we deduce that the δ-HMX deposits we observe have dimensions of a few micrometers and a mean surface coverage that, depending on crystal growth conditions, ranges from 0.1 to 1 μg/cm2. The implications of these δ-HMX sites for energetic material performance are discussed briefly.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Electronic, Optical and Magnetic Materials
- Surfaces, Coatings and Films