  Transition metal nitrides have attracted much interest of the scientific
community for their intriguing properties and technological applications. Here
we focus on yttrium dinitride (YN$_{2}$) and its formation and structural
transition under pressure. We employed a fixed composition USPEX search to find
the most stable polymorphs. We choose yttrium as a proxy for the lanthanide
series because it has only $+3$ oxidation state, contrary to most transition
metals. We then computed thermodynamic and dynamical stability of these
structures compared to the decomposition reactions and we found that the
compound undergoes two structural transitions, the latter showing the formation
N$_{4}$ chains. A closer look into the nature of the nitrogen bonding showed
that in the first two structures, where nitrogen forms dimers, the bond length
is intermediate between that of a single bond and that of a double bond, making
it hard to rationalize the proper oxidation state configuration for YN$_{2}$.
In the latter structure where there is the formation of N$_{4}$ chains, the
bond lengths increase significantly, up to a value that can be justified as a
single bond. Finally, we also studied the electronic structure and the
dynamical stability of the structures we found.
