  We have studied the formation of yttrium nitride in which the Y-atoms were
arranged in two common close-packed stacking: The AB-stacking (hcp-symmetry) is
the ground state of metallic yttrium; while the ABC-stacking (fcc-symmetry) is
the ground state of yttrium nitride. Given the different symmetries between YN
and Y, there must be a phase transition (hcp->fcc) as N is incorporated in the
Y-lattice. By means of first principles calculations we have made a
systematical investigation where N was gradually incorporated in octahedral
interstices in AB and ABC stacking of Y. We report the heat of formation, bulk
modulus, lattice parameter and electronic structure of the resultant nitrides.
We found that both metal arrangements are physically achievable. Furthermore,
for low nitrogen incorporation the two phases may coexist. However for high
nitrogen concentration the cubic phases are favored by a 30 kJ mol-1 margin.
These results are important since fcc-YN is semiconducting and could be
utilized as active layer in electronic devices.
