Preferential condensation of $\beta$ RDX on In metal surfaces

The energetic compound cyclotrimethylene-trinitramine (RDX) normally crystallizes out of solution at standard temperature and pressure in the $\alpha$ form. This consists of two nitro groups in pseudoaxial positions in relation to the C-N ring, and one nitro group in a pseudoequatorial position in an orthorhombic lattice. A metastable phase, labeled the $\beta$ phase, is difficult to create and rarely observed. It consists of all three nitro groups in pseudoaxial positions, occupying a trigonal lattice. We have observed by means of Raman spectroscopy that RDX crystallized from solution on In metal foil preferentially adopts the $\beta$ phase. We discuss a possible mechanism for this behavior in the context of recently published DFT calculations for RDX on a metal cluster.