Exposed Facets and Surface Properties of Highly Explosive RDX Studied by the Crystal-Face-Indexing Method and Theoretical Calculations

The crystal morphology of highly explosive cyclotrimethylenetrinitramine (hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)) has been characterized by the crystal-face-indexing method in combination with single-crystal X-ray diffraction and rotation imaging techniques. As a result, the morphology importance of the facets in RDX crystals grown in acetone and dimethyl sulfoxide (DMSO) is on the order of (111) ≈ (210) > (021) ≈ (102) > (001) ≈ (010) ≈ (100) and (111) > (210) > (010) > (001) ≈ (021), respectively. Crystal sizes have also been measured along the a-, b-, and c-axes, which show a less-preferred orientation of RDX crystals than octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystals. In addition, the {111} and {210} facets show good thermodynamic stability, while the appearance of the other facets is dynamically affected by solution conditions during the growth of RDX crystals from the hundred-micrometer scale to the millimeter scale. After crystal face indexing, surface models were built for the dominant {111} and {210} facets as well as the special {010} facet. On the surface of the {111} facet, the outstanding H and O atoms lead to a regular arrangement of positive and negative charges. For the {210} facet, twisted grooves are observed on the surface in the Connelly surface model. More interestingly, large hollows with dimensions of 11.6 × 11.8 Å2 are generated on the surface of the {010} facet. The high adsorption ability of the hollows to DMSO molecules plays a key role in the enhanced morphology importance of the {010} facet in DMSO, as evidenced by molecular dynamics simulations. All of these results can be applied to the morphology control of energetic crystals and will be a good reference to the interfacial interaction of the RDX crystal in both polymer-bonded explosives (PBXs) and propellants.