Structural stability and elastic properties of WB4 under high pressure

A comparative study on the structure stability and elastic properties for various types of tungsten tetraboride (WB4) has been carried out with the generalized gradient approximation (GGA) and local density approximation (LDA) in the framework of density functional theory (DFT). Five types of WB4 are considered i.e., orthorhombic Immm, Pnnm, Pmmn, monoclinic C2/m and hexagonal P6(3)/mmc structure. Our calculations indicate that the P6(3)/mmc-4u structure of WB4 is unstable at both ambient and pressure conditions, but the other four types of WB4 are stable, in agreement with recent theoretical results. By eliminating mechanical calculations, we find that the four types (C2/m, Immm, Pnnm and Pmmn) of WB4 are potential candidates to be ultra-incompressible and hard materials. Moreover, the WB4 in C2/m type is the most ultra-incompressible among the considered structures due to its superior mechanical properties, and the P6(3)/mmc-2u structure of WB4 is not considered to be hard material because of its low hardness. In addition, the calculated B/G ratio exhibits the positive pressure dependence, and four types show brittle nature within 100 GPa.