Emergence of New Nitrogen-Rich Compounds in Lead–Nitrogen Phase Diagram Under Pressure

In this study, we employed a variable-composition evolutionary algorithm in conjunction with first-principles calculations to perform a comprehensive structural search for the Pb–N binary system under pressure and determine a hyperfine pressure–composition phase diagram. The findings revealed the existence of 11 new pressure-stabilized structures in the pressure range of 0–100 GPa and at 0 K. Six nitrogen-rich compositions, PbNz (z = 2–4, 6, 8, and 10), and seven thermodynamically stable phases were identified. The most prevalent nitrogen motif observed in the predicted PbNz phases is the N2 dumbbell (z = 2–4, 6, and 8). As z increases, the lowest N–N vibrational frequencies decrease from 2076 cm–1 in Im3̅ Pb(N2)4 to 1302 cm–1 in I4/mcm Pb(N2) at 25 GPa. This finding is related to a decrease in the charge transfer from Pb to N2 dimers. The νN–N frequencies act as a clear signature of the existence of high-pressure PbNz phases containing N2 dumbbells. In addition to N2 dimers, the aromatic pentazolate unit N5– is stabilized in the I4/m Pb(cyclo-N5)2 phase, which becomes thermodynamically stable at 25 GPa. This nitrogen-rich phase can be recoverable to ambient pressure and exhibits a high detonation velocity of 10.2 km/s. Additionally, one- and two-dimensional covalent nitrogen nets are also observed in PbNz (z = 6 and 8). Infinite polymeric chains are found in PbN6 and a two-dimensional nitrogen covalent net is encountered in Cc Pb@2D-N8, which is composed of fused aromatic 18-rings.