Exotic high-pressure behavior of double nitride CuPN2

Abstract

On the basis of a combination of the particle-swarm optimization technique and first-principles calculations, we explore the structural and electronic properties of CuPN2 at high pressures up to 160 GPa. The CuPN2 crystallizes in a tI16 structure featuring a three-dimensional tetrahedral network at ambient pressure. Under compression, two structural phase transitions were predicted at 34 and 120 GPa, respectively, with a phase sequence of tI16 → hR4/oC16 → hR4′. The hR4 and oC16 structures are nearly energetically degenerate and both consist of two-dimensional edge-sharing PN6 octahedra network connected by twofold coordinated Cu atoms, whereas the hR4′ structure is built up from edge-sharing PN6 and CuN6 octahedra. All four structures are indirect semiconductors. Unexpectedly, the energy gap collapses by 73% in the first structural transition, and increases abnormally by ∼149% in the second, which is clearly correlated to the evolution trend of the CuN distance, indicating that the CuN bonding environment plays an important role in determining the high-pressure behavior of CuPN2.