Ab initio studies on ammonium iodine under high pressure**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574109, 51632002, 51572108, 91745203, and 11574112), the National Key Research and Development Program of China (Grant Nos. 2016YFB0201204 and 2018YFA0305900), the Program for Changjiang Scholars and Innovative Research Team in University, China (Grant No. IRT 15R23), and the National Fund

Abstract

Ammonium iodine (NH4I) as an important member of hydrogen-rich compounds has attracted a great deal of attention owing to its interesting structural changes triggered by the relative orientations of adjacent ammonium ions. Previous studies of ammonium iodide have remained in the low pressure range experimentally, which we first extended to so high pressure (250 GPa). We have investigated the structures of ammonium iodine under high pressure through ab initio evolutionary algorithm and total energy calculations based on density functional theory. The static enthalpy calculations show that phase V is stable until 85 GPa where a new phase Ibam is identified. Calculations of phonon spectra show that the Ibam phase is stable between 85 GPa and 101 GPa and the Cm phase is stable up to 130 GPa. In addition, ammonium iodine dissociates into NH3, H2, and I2 at 74 GPa. Subsequently, we analyzed phonon spectra and electronic band structures, finding that phonon softening is not the reason of dissociation and NH4I is always a semiconductor within the pressure range.