A photosensitive energetic material via pressure-induced phase transition of NH4N3

Abstract

Developing next-generation energetic materials combining ultrahigh energy density with safe, controllable ignition is critical. Using the CALYPSO method and first-principles calculations, we systematically investigated the high-pressure structural evolution of ammonium azide (NH4N3) and discovered a novel energetic phase, Pnma, with exceptional photosensitive potential. Stable between 49.5 and 124.1 GPa, the Pnma phase features polymerized azide anions forming one-dimensional (1D) armchair chains with intact ammonium cations. This configuration yields pronounced optical anisotropy and a massive visible-light absorption coefficient of 9.5 × 105 cm−1. As a narrow-bandgap semiconductor (0.37 eV), Pnma exhibits highly anisotropic carrier mobility for efficient unidirectional electron transport. These properties enable superior photothermal conversion, highlighting its potential as a laser-ignited explosive. Additionally, two other high-pressure phases, Pbcm and P1, were identified. The Pbcm phase (18.9–49.5 GPa) is a wide-bandgap material, while the P1 phase emerges above 124.1 GPa and remains stable up to 200 GPa. In P1, cations completely decompose into infinite 1D NH chains and N4H7 units. Remarkably, the polymeric P1 structure demonstrates extraordinary detonation performance. Its energy density reaches 9.36 kJ/g—more than twice that of 2,4,6-trinitrotoluene (4.4 kJ/g)—and it excels in material density, detonation velocity, and pressure. This study reveals the multidimensional physical properties of high-pressure NH4N3, providing a theoretical foundation for designing advanced energetic materials with both ultrahigh energy density and light-responsive safety.