New ultra-incompressible phases of NbB4 predicted from first principles

Unraveling the specific structures of transition metal polyborides is critical for understanding their underlying physical and electronic properties. Using an ab initio particle swarm optimization algorithm for the crystal structure determination, we have predicted the monoclinic C2/c and Amm2 phases as the stable structures for NbB4. It is firstly found that the C2/c phase transforms to Amm2 phase at about 42 GPa. Intriguingly, both phases are dynamically and mechanically stable at the ambient conditions. Furthermore, both phases of NbB4 have high bulk and shear moduli and a low Poisson's ratio and can be classified as ultra-incompressible materials. Besides, the strong covalent bonding nature of NbB4 polymorphs was confirmed by the ELF analysis. We hope that these findings will stimulate experimental work on synthesis of this technologically important material.