First-principles study on the structure and stability of (H₂dabco)[K(ClO₄)₃] under hydrostatic pressure

Abstract

The crystal structure, molecular structure, electronic structure and mechanical properties of molecular perovskite high-energetic material (H₂dabco)[K(ClO₄)₃] (DAP-2) under hydrostatic pressure ranging from 0 to 50 GPa are calculated and studied based on density functional theory. And the influences of pressure on its stability and impact sensitivity of DAP-2 are investigated. As the external pressure gradually increases, both the lattice parameters and the volume of DAP-2 crystal exhibit a monotonic decreasing trend. In the entire pressure range, the unit cell volume shrinks by up to 40.20%. By using the Birch Munnaghan equation of state to fit <i>P</i>-<i>V</i> relation, the bulk modulus <i>B</i>₀ and its first-order derivative <i>B</i>₀’ with respect to pressure are obtained to be 23.4 GPa and 4.9 GPa, respectively. The observations of the characteristic bond length and bond angle within the crystal indicate that the cage-like structure of organic cation H₂dabco²⁺ undergoes distortion at 25 GPa. Further analysis of the average fractional coordinates of the center-of-mass and Euler angles for H₂dabco²⁺ and KO₁₂ polyhedron shows that within a pressure range from 0 to 50 GPa, both the average fractional coordinates of the center-of-mass and the Euler angles exhibit fluctuations at 25 GPa, but the overall amplitude of these fluctuations is very small. Based on this finding, it is speculated that the space group symmetry of the crystal may remain unchanged in the entire pressure range. In terms of electronic structure, with the increase of pressure, the band gap value increases rapidly and reaches a maximum value at about 20 GPa, followed by a slow decreasing trend. Based on the first-principles band gap criterion and the variation of the band gap under different pressures, it is demonstrated that below 20 GPa, the impact sensitivity of DAP-2 gradually decreases with pressure increasing; however, when the pressure exceeds 20 GPa, the impact sensitivity exhibits a slow increasing trend. In addition, the elastic constants <i>C</i>_<i>ij</i>, Young’s modulus (<i>E</i>), bulk modulus (<i>B</i>), shear modulus (<i>G</i>), and Cauchy pressure (<i>C</i>₁₂ – <i>C</i>₄₄) all increase with pressure rising, indicating that the rigidity and ductility of the crystal under pressure are significantly strengthened. According to the mechanical stability criterion, the crystal maintains the mechanical stability throughout the pressure range.