New Tungsten Borides, Their Stability and Outstanding Mechanical Properties.

We predict new tungsten borides, some of which are promising hard materials that are expected to be stable in a wide range of conditions, according to the computed composition-temperature phase diagram. New boron-rich compound WB5 is predicted to be superhard, with a Vickers hardness of 45 GPa, to possess high fracture toughness of ∼4 MPa·m0.5, and to be thermodynamically stable in a wide range of temperatures at ambient pressure. Temperature dependences of the mechanical properties of the boron-richest WB3 and WB5 phases were studied using quasiharmonic and anharmonic approximations. Our results suggest that WB5 remains a high-performance material even at very high temperatures.