Abstract
The present work aims to study the structural, elastic, mechanical and thermodynamic properties of the newly discovered orthorhombic Cmcm structure HfB4 (denoted as Cmcm-HfB4 hereafter) under pressure by the first-principles calculations. The obtained equilibrium structure parameters and ground-state mechanical properties were in excellent agreement with the other theoretical results. The calculated elastic constants and phonon dispersion spectra show that Cmcm-HfB4 is mechanically and dynamically stable up to 100 GPa and no phase transition was observed. An analysis of the elastic modulus indicates that Cmcm-HfB4 possesses a large bulk modulus, shear modulus and Young's modulus. The superior mechanical properties identify this compound as a possible candidate for a superhard material. Further hardness calculation confirmed that this compound is a superhard material with high hardness (45.5 GPa for GGA); and the relatively strong B–B covalent bonds’ interaction and the planar six-membered ring boron network in Cmcm-HfB4 are crucial for the high hardness. Additionally, the pressure-induced elastic anisotropy behaviour has been analysed by several different anisotropic indexes. By calculating the B/G and Poisson's ratio, it is predicted that Cmcm-HfB4 possesses brittle behaviour in the range of pressure from 0 to 100 GPa, and higher pressures can reduce its brittleness. Finally, the thermodynamic properties, including enthalpy (ΔH), free energy (ΔG), entropy (ΔS), heat capacity (CV) and Debye temperature (ΘD) are obtained under pressure and temperature, and the results are also interpreted.
Highlights
Ultra-incompressible and hard materials have attracted a great deal of attention owing to their outstanding physical and chemical properties in fundamental science and industry applications [1,2,3,4]
transition metal borides (TMBs) can be synthesized at ambient condition, which reduces the cost of synthesis and is beneficial for practical application
We studied the structural, elastic and mechanical properties of the novel CmcmHfB4 under ambient and high pressures by the first-principles approach based on density functional theory (DFT)
Summary
Ultra-incompressible and hard materials have attracted a great deal of attention owing to their outstanding physical and chemical properties in fundamental science and industry applications [1,2,3,4]. The compounds formed by TMs and light atoms usually possess high valence electron density and directional covalent bonds, which contribute to improve their mechanical properties and hardness Based on this design principle, a series of ultra-incompressible and hard materials were successfully synthesized [6,10,11,12,13,14,15,16]. The newly boron-rich TM compounds, such as WB4 [21,22], CrB4 [23,24], MnB4 [25,26,27], FeB4 [28,29], YB4 [30] and VB4 [31], have been extensively investigated This may be due to boron having a strong ability to form covalent bonds with TMs. A lot of calculated results show that all these materials exhibit excellent mechanical properties and high hardness. The study will open up a new way for seeking new ultra-incompressible and hard materials
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