Crystal structures and anisotropic behavior of two energetic, nitric ester, azetidine stereoisomers at ambient pressure and under static compression.

An understanding of the interrelationship between crystal structure, electronic structure, and mechanical properties of energetic materials provides insight into tailoring materials for improved performance and safety. We present the crystal structures and mechanical properties of two new energetic, nitric ester, azetidine isomer crystals [I1 with (C₈N₆H₁₂O₁₄)*2 atoms and I2 with (C₈N₆H₁₂O₁₄)*4 atoms in the unit cell] at ambient pressure and under static compression employing single crystal x-ray diffraction and DFT calculations. I1 crystalizes in the triclinic P-1 space group with two molecules in the unit cell and I2 crystalizes in the monoclinic P21/c space group with four molecules in the unit cell. Calculated enthalpy of formation and Gibbs energy of I1 and I2, and crystal structure search helped to understand better the formation of the isomer’s crystals. Symmetry-adapted distorted structure generation and analyses yield the full set of elastic constants for I1 and I2. Both structures are mechanically stable using their derived elastic constants and total energy in accordance with the Born stability criteria. I1 presents a bulk modulus (Reuss) value approximately 13.3% greater than I1 (9.80 v. 8.56 GPa). Furthermore, both isomers exhibit spatial anisotropy with I1 presenting Young modulus values of 14.6, 11.79, and 19.17 GPa along the x-, y-, and z-axis, respectively, and I2 presenting respective values of 5.01, 8.55. and 9.21 GPa along the x-, y-, and x-axis. We estimate the elastic Debye temperature (the temperature of a crystal's highest normal mode of vibration) for I1and I2 as 1713.7 and 1223.3 K, respectively, at ambient pressure. DFT calculations predict a band gap of 3.628 eV for I1 at ambient pressure, which decreases to 3.028 eV at 10 GPa. In contrast, I2 presents a band gap of 3.438 eV at ambient pressure with a moderate decrease to 3.130 eV at 10 GPa. The effects of pressure on the crystal structure, electronic structure, and elastic properties of I1 and I2 will be reported and compared to other energetic materials.