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 to determine a hyperfine pressure composition phase diagram. The findings revealed the existence of eleven new pressurestabilized 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 Im-3 Pb(N2)4 to 1302 cm−1 in I 4/mcm Pb(N2) at 25 GPa. This finding is related to the lowering of 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 N−5 is stabilized in the I 4/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.