| Abstract
| - The surfaces of solution-grown β-HMX (β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) crystals were studiedusing vibrational sum-frequency generation spectroscopy (SFG). In earlier work (Kim, H.; Lagutchev, A.;Dlott, D. D. Propellants, Explosives, Pyrotechnics2005, 31, 116), such SFG spectra evidenced considerablevariability as the ∼200-μm diameter laser beam was translated over crystal surfaces. We have now foundthis 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 δ-HMX deposit on aβ-HMX surface could dominate the SFG spectrum. We have found that rapid evaporation from tiny dropletsof HMX solution produces only δ-HMX, presumably because the polar boat conformation of δ-HMX isstabilized in solution. We believe the δ-HMX deposits are created by the rapid drying of droplets of motherliquor clinging to solution-grown crystal surfaces. In this paper, we present SFG spectra of δ-HMX nanocrystalsand β-HMX surfaces in the CH-stretch and NO2-stretch regions. Through an analysis of the β-HMX/δ-HMXSFG intensity ratios and the surface variability, we deduce that the δ-HMX deposits we observe have dimensionsof a few micrometers and a mean surface coverage that, depending on crystal growth conditions, ranges from0.1 to 1 μg/cm2. The implications of these δ-HMX sites for energetic material performance are discussedbriefly.
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