Effects of boron doping on structural, electronic, elastic, and optical properties of energetic crystal 2,6-diamino-3,5-dinitropyrazine-1-oxide: a theoretical study using the first principles calculation and Hirshfeld surface analysis

Boron-contained compounds are one kind of new energetic materials, and have been synthesized successfully lately. However, the effects of introduced boron atoms into the energetic system are unclear. In this work, using the known insensitive energy crystal 2,6-diamino-3,5-dinitropyrazine-l-oxide (LLM-105) as the model compound, boron doping effects on its crystal structure, band gap and structure, intermolecular contacts, sensitivity, elastic property, optical absorption behavior, and dielectric function were studied by the first principles calculations and Hirshfeld surface analysis. One B atom was doped at four different doping sites in the ring (two kinds of nitrogen N1/N2 and carbon atoms C3/C4), respectively, and formed four new crystals LLM-105-B1/B2/B3/B4. The results showed that the B atom and its doping site both make great influence on the structure and properties. The B doping obviously decreased the band gap and weakened the strength of intermolecular contacts, giving rise to higher sensitivity and worse safety. Especially for LLM-105-B4 which has a 0 eV value of band gap, the doped B atom made great contributions to the density of states around the Fermi level, leading to the suddenly move down of lowest unoccupied molecular orbital and directly link of total density of states at the Fermi level. Doping the B atom at the site C3 improved the ductility and plasticity of LLM-105, while LLM-105-B2 was found to be the most brittle and anisotropic crystal. Doping B atoms at sites N2 and C4 increased the absorption to green, orange, and red lights, while the absorption strength to the infrared light was enhanced in most cases. The dielectric constant and polarity were significantly increased by doping boron atoms at sites C3 and C4.