Novel structures and mechanical properties of Zr₂N: Ab initio description under high pressures*

Abstract

With the formation of structural vacancies, zirconium nitrides (key materials for cutting coatings, super wear-resistance, and thermal barrier coatings) display a variety of compositions and phases featuring both cation and nitrogen enrichment. This study presents a systematic exploration of the stable crystal structures of zirconium heminitride combining the evolutionary algorithm method and ab initio density functional theory calculations at pressures of 0 GPa, 30 GPa, 60 GPa, 90 GPa, 120 GPa, 150 GPa, and 200 GPa. In addition to the previously proposed phases P42/mnm-, Pnnm-, and Cmcm-Zr2N, five new high-pressure Zr2N phases of P4/nmm, I4/mcm, P21/m, P 3 ¯ m 1 , and C2/m are discovered. An enthalpy study of these candidate configurations reveals various structural phase transformations of Zr2N under pressure. By calculating the elastic constants and phonon dispersion, the mechanical and dynamical stabilities of all predicted structures are examined at ambient and high pressures. To understand the structure–property relationships, the mechanical properties of all Zr2N compounds are investigated, including the elastic moduli, Vickers hardness, and directional dependence of Young’s modulus. The Cmcm-Zr2N phase is found to belong to the brittle materials and has the highest Vickers hardness (12.9 GPa) among all candidate phases, while the I4/mcm-Zr2N phase is the most ductile and has the lowest Vickers hardness (2.1 GPa). Furthermore, the electronic mechanism underlying the diverse mechanical behaviors of Zr2N structures is discussed by analyzing the partial density of states.

Boron at tera-Pascal pressures

“…Here, we used the ab initio evolutionary algorithm USPEX, [18][19][20][21][22][23][24] which can simultaneously find crystal structures of all stable elemental crystals in multi-pressures. In our searches at 500 GPa, 1500 GPa, 2000 GPa, and 5000 GPa pressure points, the initial population included 120 structures with up to 28 atoms per primitive cell, with all subsequent generations consisting of 50 structures produced by heredity (30%), transmutation (20%), soft mutation (20%), and a random symmetric generator (30%).…”

Semiconductors with a chiral crystal structure in group IVB transition metal pernitrides

“…Group IVB transition metal (TM = Ti, Zr, and Hf) nitrides have been widely applied in the coatings on cutting tools, 1,2 wear and oxidation resistant coatings, 3,4 and hot carrier solar cells 5,6 owing to their high hardness, 7 excellent wear and oxidation resistance, 8,9 and a large phononic bandgap. 5,6 The metal-rich nitrides of this group generally exhibit metallic properties 10,11 because of the free-electrons generated by the TM atoms. Furthermore, a residual electron 12 per atom in the metal d orbital also leads to metallic TMN compounds (i.e., Fm% 3m-TMN 13 and Pm% 3m-TMN 13 ).…”