Theoretical investigation on vanadium dinitrides from first-principles calculations

Abstract

Motivated by successful synthesis of the first early transition metal dinitride TiN₂ under high pressure, the possible crystal structures of another family member vanadium dinitride (VN₂) have been systematically explored up to 300 GPa by using a well-developed CALYPSO (Crystal structure Analysis by Particle Swarm Optimization) method for crystal structural prediction. Three theoretical phases with P6₃/mmc, Cmc2₁, and I4/mcm space group have been determined at different pressures. And their dynamical and mechanical stabilities have been confirmed by the phonon dispersion and elastic constants calculations. Among three predicted phase, the tetragonal I4/mcm-VN₂ exhibits highly improved Young's, bulk and shear moduli relative to the traditional fcc-VN, indicating its potential mechanical applications. In addition, the elastic anisotropy of each phase was fully investigated by the dependence of Young's modulus on the crystal orientation. To further study the mechanical property, the ideal tensile, shear, and compressive strengths are calculated. Our results imply the tetragonal phase of VN₂ is not an intrinsic superhard material. The electronic structures are analyzed to study the chemical bonding nature so as to confirm the driving force of the enhancement of the mechanical property.

Introduction

Compounds formed with transition metal and light elements (B, C, N, O) are attracting increasing interest due to their importance in both fundamental science and technological applications [1], [2], [3], [4]. The excellent properties of high-melting point, high hardness, good high-temperature strength, wear and friction properties, and good thermal conductivity have made them exhibit extreme usefulness in a wide variety of industrial applications, such as abrasives, cutting tools, coatings, etc [5], [6], [7], [8]. Especially, the transition metal nitrides have been proved to have potential incompressibility and super hardness [9], which originated from the mixture of covalent, metallic and ionic bonding character between the transition metal and the light element [10], [11], [12]. It is known that crystal structures and bonding nature (including the bond direction, bond length, etc.) are the key for the understanding of mechanical properties of superhard materials [13], [14]. The intercalation of the light element N into transition metal lattice which possesses high valence-electron density is believed to enhance the strength of the materials due to the forming of strong covalent networks [15]. It has been accepted that the early transition metal nitrides always tend to form low nitrogen contents nitrides with excellent mechanical and thermal properties, such as TiN, VN, HfN, ZrN, CrN, ScN, Ta₂N, Ta₃N₅, V₂N, etc [16], [17]. Furthermore, most of the early transition metal mononitrides such as TiN, VN, NbN, MoN, HfN, etc. with hard and refractory nature [18] have been widely used in high strength low alloy (HSLA) steels applications [19], since that they can form microstructural constituents. Recently, different theoretical and experimental researches have focused on changing the nitrogen concentration in this class material to enhance their elastic property [20]. Terao [21] have demonstrated that tantalum nitrides present multiple structures with different nitrogen contents in the tantalum nitrogen system, such as Ta₂N, TaN, Ta₅N₆, Ta₄N₅, Ta₃N₅, etc. Zhao et al., suggesting the most nitrogen rich phases of transition metal nitrides possess the remarkably improved mechanical property [22]. In the recent years, progressive developments of the high-pressure technology have significantly facilitated the syntheses of novel nitrogen-rich transition metal compounds, and unexpected reactivity has been observed in the transition metal metals when heated in nitrogen using the advanced laser-heated diamond anvil cell high-pressure techniques.

More recently, the successful synthesis of transition metal dinitride TiN₂ by fcc-TiN and dense N₂ at high pressure (73 GPa) and high temperature (2400 K) has been reported in 2016 [23]. The new transition metal dinitride adopts a CuAl₂-type structure and can be quenched from high pressure. Further theoretical investigation demonstrates the structure of TiN₂ is tetragonal with the space group of I4/mcm [24] and the elastic property and hardness are greatly improved compared to the fcc-TiN [25]. The neighboring element next to the titanium in the same group is vanadium (V) which possesses the similar valence electron structure. Compared to TiN, the VN adopts the same cubic structure which is also a well-known material with a hardness value of about 15 GPa [26]. However, VN₂ hasn't been synthesized yet and the exploration of structures and related mechanical properties of VN₂ under high pressure is greatly demanded. Inspired by this pioneering experimental work, the structures of vanadium dinitride under different pressures are systematically explored in the present work, using a well-developed CALYPSO (Crystal structure Analysis by Particle Swarm Optimization) method in crystal structure predictions. Indeed, three unexpected phases with P6₃/mmc, Cmc2₁, and I4/mcm space group for VN₂ have been determined at different pressures. They are all dynamical stable in the pressure range from 0 GPa to 100 GPa. At low pressure, the hexagonal phase is favorable and at high pressure it tends to adopt the I4/mcm structure. First-principles calculations are thus employed to investigate the structural property, elastic anisotropy, mechanical strength, electronic structure, and the chemical bonding nature. We hope that the present work will provide useful information and inspiration for the future theoretical and experimental works.