Stability and superionicity of NH4S and NH4S2 at extreme conditions

Stable compounds comprising nitrogen, sulfur, and hydrogen elements hold significant promise in hydride superconductors and astrophysical matter. Herein, we utilize a blend of structure prediction techniques and first-principles calculations to comprehensively explore the N--H--S system. We present the discovery of two ionic compounds, NH₄S and NH₄S₂, characterized by similar NH₄ configurations but distinct arrangements of sulfur atoms. As the sulfur content increases, we observe a transition in the bonding pattern of sulfur-sulfur interactions from S₂ dimers to S₄ units. Furthermore, the electron transfer from NH₄ to each S unit escalates from 1.04 e (NH₄S) to 1.24 e (NH₄S₂). Through ab initio molecular dynamics simulations, we unveil a sequence of phase transitions encompassing solid, superionic, and liquid phases for both compounds. Notably, the high-pressure and high-temperature phase diagrams reveal that only the superionic phase of NH₄S corresponds to the extreme conditions found in the interior of the Earth. Moreover, the liquid regions of both compounds encompass the isentropic lines of Neptune, Uranus, and Jupiter. These results could provide essential information for understanding the N--H--S system at high pressure and the formation models of the planets.