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The development of super-hard materials has recently focused on systemscontaining a heavy transition metal and light main group elements. Niobiumcarbides and nitrides have previously been identified as potential candidates,however, the volatility of carbon and nitrogen during synthesis makes themprone to the formation of anionic vacancies, which have the ability to changethe electronic structure, dynamical stability and adversely affecting themechanical properties. Here, we present ab initio Density Functional Theorycalculations that probe the occurrence of anionic vacancies as a function ofconcentration, thereafter, pertinent mechanical properties are investigated.Our results showed that the presence of anionic vacancies in NbC and NbN tendsto deteriorate the mechanical properties and ultimately the mechanical hardnessdue to vacancy softening that can be attributed to defect induced covalent tometallic bond transition. Further, it was observed that anionic vacancies inNbC tend to modify its toughness, in particular, NbC in ZB becomes brittlewhile NbC in WZ becomes ductile in the presences of C vacancies of up to 6%. Onthe other hand, the toughness of NbN was found to be insensitive to defectconcentration of even up to 8%. Consequently, stringent control of anionicdefects during the synthesis of NbC and NbN is critical for the realization ofthe desired mechanical response that can make these materials ideal forsuper-hard and related applications.

The Impact of Anionic Vacancies on the Mechanical Properties of NbC and NbN: An ab initio Study