Electronic bonding analyses and mechanical strengths of incompressible tetragonal transition metal dinitrides TMN2 (TM = Ti, Zr, and Hf).

Meiguang Zhang,Baobing Zheng,Qun Wei,Haiyan Yan,Ke Cheng

doi:10.1038/srep36911

Abstract

Motivated by recent successful synthesis of transition metal dinitride TiN2, the electronic structure and mechanical properties of the discovered TiN2 and other two family members (ZrN2 and HfN2) have been thus fully investigated by using first-principles calculations to explore the possibilities and provide guidance for future experimental efforts. The incompressible nature of these tetragonal TMN2 (TM = Ti, Zr, and Hf) compounds has been demonstrated by the calculated elastic moduli, originating from the strong N-N covalent bonds that connect the TMN8 units. However, as compared with traditional fcc transition metal mononitride (TMN), the TMN2 possess a larger elastic anisotropy may impose certain limitations on possible applications. Further mechanical strength calculations show that tetragonal TMN2 exhibits a strong resistance against (100)[010] shear deformation prevents the indenter from making a deep imprint, whereas the peak stress values (below 12 GPa) of TMN2 along shear directions are much lower than those of TMN, showing their lower shear resistances than these known hard wear-resistant materials. The shear deformation of TMN2 at the atomic level during shear deformation can be attributed to the collapse of TMN8 units with breaking of TM-N bonds through the bonding evolution and electronic localization analyses.

Highlights

Motivated by recent successful synthesis of transition metal dinitride TiN2, the electronic structure and mechanical properties of the discovered TiN2 and other two family members (ZrN2 and HfN2) have been fully investigated by using first-principles calculations to explore the possibilities and provide guidance for future experimental efforts
Polyhedral view of this tetragonal structure (Fig. 1(b)) reveals that TiN2 consists of the TiN8 face-sharing tetragonal antiprisms connected by N-N bonds and stacked along the c-axis, in contrast to the TMN6 octahedrons in the previous synthesized noble metals pernitrides7–12
Through the full relaxations of both lattice constants and internal atomic coordination, the obtained equilibrium structure parameters for three TMN2 compounds are listed in Table 1, among which the calculated results for TiN2 compare well with the available experimental data15

Summary

Introduction

Motivated by recent successful synthesis of transition metal dinitride TiN2, the electronic structure and mechanical properties of the discovered TiN2 and other two family members (ZrN2 and HfN2) have been fully investigated by using first-principles calculations to explore the possibilities and provide guidance for future experimental efforts. The experiment found that this phase is recoverable to ambient conditions and possesses a high bulk modulus of 385(7) GPa comparable to those of PtN2 (372 GPa) and ReB2 (360 GPa), much larger than that of TiN (288 GPa)18 This new tetragonal TiN2, the first synthesized high-nitride phase in early transition metal nitrides, is expected to be a candidate as a potential superhard solid for wear- and scratch-resistant materials. This concept for the search of novel superhard materials failed in materials such as PtN219, and ReB220, and others, because plastic deformation occurs in shear at large strain at the atomic level, where electronic instabilities may occur upon bond breaking in the practical measurement of hardness. We hope that the present findings will encourage further theoretical and experimental works on this class of material

Methods

Results

Conclusion