Pressure induced metallization of Cu3N

Motivated by their applications in electronic-optical industry, interest has been grown in fabricating copper nitride (Cu3N) films [1, 2]. The crystal structure of Cu3N is of the anti-ReO3 type (a perovskite missing A site atom) with a simple cubic unit cell of lattice constant 3.817 A as shown in Fig. 1. In this structure, the copper atoms occupy the center of the cubic edges forming collinear bonds with two nearest neighbor anions instead of occupying the face-centered cubic close-packing sites. As a consequence, this crystal structure has many vacant interstitial sites. This open crystal structure is suited for the interposition of other elements or compression under high-pressure conditions. Electronic structure calculation has shown that pure Cu3N is semi-conductor with a small indirect band gap of about 0.23 eV while Cu3N with Pd interposition exhibits semi-metallic behavior [3]. Recent theoretical calculation also confirmed the metallic property of Cu3N with extra Cu interposition [4]. The effect of lattice parameters on the electronic properties of Cu3N were also discussed in Ref [4] with an emphasis on the increase of the energy gap with the increase of the lattice parameters. Compared with bulk ReO3, of which the highpressure properties have been studied extensively both experimentally and theoretically [5–7], little work has been done on bulk Cu3N, especially under high pressure conditions. In order to understand the electronic nature of this material under high pressure, we performed first principles calculations on Cu3N with both ideal antiReO3 type (denoted type I hereafter) and a hypothetic Cu3Au type (denoted type II hereafter) structures. The space group is the same for the two structures (Pm3m), but the Cu and N atoms occupy different wyckoff sites. For the anti-ReO3 structure, N is in 1a sites and Cu in 3d sites; for the Cu3Au structure, N is still in 1a sites, but Cu in 3c sites. The Cu3Au is expected to be favored under high-pressure condition because of the close-packed nature of this structure. We employed accurate full-potential densityfunctional theory (DFT) and the full potential linearized augmented plane wave (FP-LAPW) method as implemented in WIEN2K code to investigate the electronic properties and possible phase transition of Cu3N un-