Compressive behavior and electronic properties of ammonia ice: a first-principles study

Abstract

Understanding the compressive behavior of ammonia ice is an enduring topic due to its salient implications in planetology and the origin of life as well as its applications in agriculture and industry. Currently, the most stable crystal structures of ammonia ice with increasing pressure have been determined to be P2₁3, P2₁2₁2₁, Pma2, Pca2₁, P2₁/m and Pnma, respectively. Taking these six crystal structures for consideration, the pressure-induced structural and electronic behavior of ammonia ice was systematically investigated using density functional theory calculations. According to our calculations, the transition from molecular phase P2₁2₁2₁ to ionic phase Pma2 can be ascribed to the bonds between H atoms and N atoms on adjacent NH₃ molecules. Analysis of the Mulliken population and electron density of states implies decreased charge transfer between the N and H atoms and enhanced bonds with increasing pressure. In addition, charge overlap between NH₃ molecules was found at high pressure in the molecular phases of ammonia ice, which is also observed between NH₂⁻ and NH₄⁺ groups in ionic phases. With increasing pressure, the band gap of ammonia ice increases rapidly and then decreases gradually, which is a consequence of the subtle competition between the strong coupling in the H 1s and N 2p states and the charge overlap. These simulations help us understand the characteristics of ammonia ice under high pressure and further provide valuable insights into the evolution of planets.