[Paper Review] Physical properties of new MAX phase borides M2SB (M = Zr, Hf and Nb) in comparison with conventional MAX phase carbides M2SC (M = Zr, Hf and Nb): Comprehensive insights

M. A. Ali, M. M. Hossain+4 more|arXiv (Cornell University)|Sep 9, 2020

MXene and MAX Phase Materials87 references9 citations

TL;DR

This study employs first-principles calculations to comprehensively analyze the physical properties of new MAX phase borides M2SB (M = Zr, Hf, Nb), comparing them with conventional M2SC carbides. The Nb2SB compound exhibits the best mechanical properties, with covalent-ionic bonding, dynamic stability, metallic behavior, and potential for solar-reflective coatings confirmed through electronic structure and optical analysis.

ABSTRACT

In this article, a detailed study of the recently synthesized MAX phase borides M2SB (M = Zr, Hf and Nb) has been performed via first principles technique. Investigation of mechanical hardness, elastic anisotropy, optical properties, dynamical stability and thermal properties are considered for the first time. The estimated values of stiffness constants and elastic moduli are found in good agreement with available results. The micro and macro hardness (Hmicro and Hmacro) parameters are calculated. The Vickers hardness is also calculated using Mulliken population analysis. The electronic density of states and charge density mapping are used to explain the variation of stiffness constants, elastic moduli and hardness parameters among the studied ternary borides. The Nb2SB compound is found to show best combination of mechanical properties. Mixture of covalent and ionic bonding within these borides is explained using Mulliken population analysis. The direction dependent values of Youngs modulus, compressibility, shear modulus and Poissons ratio are visualized by 2D and 3D representations and different anisotropic factors are calculated. The important optical constants are calculated and analyzed. The metallic nature of the studied borides is confirmed from the DOS and optical properties. The reflectivity spectra reveal the potential use of Zr2SB as coating materials to diminish solar heating. The studied borides are dynamically stable as confirmed from the phonon dispersion curves. The characteristic thermodynamic properties have also been calculated and analyzed. The physical properties of corresponding 211 MAX phase carbides are also calculated for comparison with those of the titled ternary borides.

Motivation & Objective

• To investigate the mechanical, electronic, optical, and thermal properties of newly synthesized M2SB MAX phase borides (M = Zr, Hf, Nb) using first-principles methods.
• To compare the physical properties of M2SB borides with their conventional M2SC carbide counterparts.
• To evaluate the mechanical performance, including hardness and elastic anisotropy, and identify the most promising candidate for structural applications.
• To analyze bonding character via Mulliken population analysis and confirm dynamical stability through phonon dispersion curves.
• To explore optical properties and assess potential for solar-reflective coating applications.

Proposed method

• First-principles density functional theory (DFT) calculations using the generalized gradient approximation (GGA) for electronic structure and total energy evaluation.
• Calculation of elastic constants, Young’s modulus, shear modulus, Poisson’s ratio, and bulk modulus using the stress-strain method.
• Computation of microhardness (Hmicro) and macrohardness (Hmacro) using the Vickers hardness model based on elastic constants.
• Application of Mulliken population analysis to quantify ionic and covalent contributions to bonding in M2SB compounds.
• Analysis of electronic density of states (DOS) and charge density maps to interpret bonding and electronic behavior.
• Evaluation of optical constants (refractive index, reflectivity, extinction coefficient) and thermodynamic properties from phonon spectra and free energy calculations.

Experimental results

Research questions

• RQ1 How do the mechanical properties of M2SB borides compare with those of M2SC carbides in terms of stiffness, hardness, and elastic anisotropy?
• RQ2 What is the origin of mechanical performance differences among M2SB compounds, and which compound exhibits the best combination of properties?
• RQ3 To what extent do covalent and ionic bonding contributions influence the elastic and mechanical response in M2SB phases?
• RQ4 Are the M2SB compounds dynamically stable, and what does the phonon dispersion reveal about their thermodynamic stability?
• RQ5 What are the optical properties of M2SB compounds, and can they serve as solar-reflective coatings, particularly Zr2SB?

Key findings

• Nb2SB exhibits the highest Vickers hardness (estimated via Mulliken analysis) and best overall mechanical performance among the studied M2SB compounds.
• The calculated elastic constants and moduli are in good agreement with available experimental and theoretical results, validating the computational approach.
• Dynamic stability is confirmed by the absence of imaginary frequencies in the phonon dispersion curves for all M2SB compounds.
• Zr2SB shows high reflectivity in the visible and near-infrared range, indicating strong potential for solar-reflective coating applications.
• Electronic structure analysis confirms metallic character, with significant overlap in the d-band and s-band near the Fermi level.
• Bonding analysis via Mulliken population reveals a mixture of covalent and ionic contributions, with covalent bonding dominating in Nb2SB, correlating with enhanced hardness.