Displacive phase transition, structural stability, and mechanical properties of the ultra-incompressible and hard MoN by first principles

The structural stability, electronic, and mechanical properties of MoN were investigated by use of the density functional theory. Nine structures were considered, i.e., hexagonal delta(1)-MoN, delta(2)-MoN, delta(2)'-MoN, delta(3)-MoN, delta(4)-MoN, and wurtzite ZnS structures, cubic NaCl, zincblende, and CsCl structures. The calculated results indicated that delta(3)-MoN is the ground state among the considered structures. The second-order displacive phase transition has been found from delta(3)-MoN to delta(2)'-MoN with increase in pressure, in agreement with the experimental observation. delta(2)'-MoN has the largest calculated bulk and shear moduli among the considered structures, followed by delta(3)-MoN. The estimated hardness of delta(2)'-MoN and delta(3)-MoN is 34 and 29 GPa, respectively. Both of them are thermodynamically and mechanically stable. The compressibility along the c-axis for both compounds is smaller than that for diamond and cubic boron nitride (c-BN). The ideal strengths of delta(2)'-MoN were discussed. [GRAPHICS] The stress strain relationship for delta(2)'-MoN shows that the ideal tensile strength of 107.0 GPa along the direction is very large, which might have potential technological and industrial applications. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim