  Metallic hydrogen is capable of showing room temperature superconductivity,
but its experimental syntheses are extremely hard. Therefore, it is desirable
to reduce the synthesis pressure of metallic hydrogen by adding other chemical
elements. However, for most hydrides, the metallization of hydrogen by
"chemical precompression" to achieve high-temperature superconductivity still
requires relatively high pressure, making experimental synthesis difficult. How
to achieve high-temperature superconductivity in the low-pressure range (0-50
GPa) is a key issue for realizing practical applications of superconducting
materials. Toward this end, this work proposes a strategy for inserting
ammonium ions in the fcc crystal of metals. High-throughput calculations of the
periodic table reveal 12 elements which can form kinetically stable and
superconducting hydrides at low pressures, where the highest superconducting
transition temperatures of AlN2H8, MgN2H8 and GaN2H8 can reach up to 118.40,
105.09 and 104.39 K. Pressure-induced bond length changes and charge transfer
reveal the physical mechanism of high-temperature superconductivity, where the
H atom continuously gains electrons leading to the transition of H+ ions to
atomic H, facilitating metallization of hydrogen under mild pressure. Our
results also reveal two strong linear scalar relationships, one is the H-atom
charge versus superconducting transition temperature and the other is the first
ionization energy versus the highest superconducting transition temperature.
Besides, ZnN2H8, CdN2H8, and HgN2H8 were found to be superconductors at ambient
pressure, and the presence of interstitial electrons suggests that they are
also electrides, whose relatively low work functions (3.03, 2.78, and 3.05 eV)
imply that they can be used as catalysts for nitrogen reduction reactions as
well.
