  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.
