Theoretical study of elastic properties and phase stability of M_(0.5)Al_(0.5)N_(1-x)O_x (M=Sc,Ti,V,Cr)

摘要

The influence of oxygen content and transition metal valence electron concentration on the phase stability and elastic properties of cubic M_(0.5)Al_(0.5)N_(1-x)O_x (M = Sc, Ti, V, Cr; x = 0 - 0.5) was studied using ab initio calculations. The negative value of enthalpy of mixing was observed for all phases indicating full miscibility of Mo.5Alo.5N with the hypothetical M_(0.5)Al_(0.5)O. Bulk moduli are decreased as x in M_(0.5)Al_(0.5)N_(1-x)O_x is increased. This can be understood based on the electronic structure. As N is substituted by O, there are no noticeable changes in the chemical bonding nature. However, O is more electronegative than N, giving rise to an increase in the ionic character of the overall bonding. In spite of that, the M - O bond in M_(0.5)Al_(0.5)N_(1-x)O_x is longer than the corresponding M-N bond, which implies that this bond becomes weaker. Hence, we propose that the decrease of bulk moduli upon O incorporation into M_(0.5)Al_(0.5)N_(1-x)O_x is caused by weaker M-O bonds.