High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN4. A triclinic phase of beryllium tetranitride tr−BeN4 was synthesized from elements at ∼85 GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN4 layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated π systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN4 layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN4 layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions.
Maxim Bykov, Timofey Fedotenko, Stella Chariton, Dominique Laniel, Konstantin Glazyrin, Michael Hanfland, Jesse S. Smith, Vitali B. Prakapenka, Mohammad F. Mahmood, Alexander F. Goncharov, Alena V. Ponomareva, Ferenc Tasnádi, Alexei I. Abrikosov, Talha Bin Masood, Ingrid Hotz, Alexander N. Rudenko, Mikhail I. Katsnelson, Natalia Dubrovinskaia, Leonid Dubrovinsky, and Igor A. Abrikosov, High-Pressure Synthesis of Dirac Materials: Layered van der Waals Bonded BeN4 Polymorph, Phys. Rev. Lett. 126, 175501, DOI = 10.1103/PhysRevLett.126.175501 abstract
Crystal structure of tr−BeN4 at 84 GPa. (a) The view along the  direction; (b) coordination of a Be atom by one of the polymeric nitrogen chains. (c) BeN6 octahedron with Be-N distances given in Å. (d) Coordination geometry of the nitrogen chain. (e) Scheme showing the transformation of tr−BeN4 upon decompression. (f) Single layer of BeN4 at ambient pressure. (g) Stacking sequence of polymeric nitrogen chains belonging to neighboring layers of tr−BeN4 at ambient pressure; viewing direction is perpendicular to the layer. Beryllium and nitrogen atoms are represented as gray and blue spheres, respectively.