Electron Localization Induces Large Hardness Enhancement In CrN

Early 3d transition-metal mononitrides are a class of extensively exploited hard materials for fabricating a wide variety of cutting and machining tools.Origin of the hardness in such nitrides has previously been well explored to link to the electron filling of a particular ? bonding state formed between metal d:eg and nitrogen p orbitals.Over filling of the ? bonds with valence-electron concentrations(VEC)above 8.4 would lead to a substantial reduction in hardness.However,chromium nitride(CrN),one of most frequently studied nitrides,possesses a large VEC value of 11,its hardness has an anomalously high values,which cannot be explained in terms of the scenario of bond filling.In this work,in conjunction of experimental observations,we present ab initio calculations for CrN to investigate the unexpectedly enhanced mechanical properties at a fundamental level.We find that the 3d electrons in half-filled Cr:t2g orbitals have a higher density than nonmagnetic calculations.Therefore they are tightly localized for producing strong t2g-t2g magnetic exchanges in the metal sublattice,resulting in a suitably occupied ? bonding state with a VEC of 8.Based on the electron localization model,the determined shear modulus for magnetic CrN is around 160 GPa,nearly triple that of non-magnetic calculation with completely delocalized 3d electrons(i.e.,55 GPa).This suggests the electron localization can profoundly strengthen the shear modulus in CrN to strongly resist the shearing deformation that prevails under indentation,as manifested with high hardness These findings provide a new avenue for design novel refractory transition-metal compounds with extraordinarily enhanced mechanical properties by the control of valence-electron concentration.