Cation disorder in MgSnN2 and its effects on the electronic properties

Ternary nitride ${\mathrm{MgSnN}}_2$ is a promising candidate to fill the “green gap” of nitride-based light-emitting diodes. The coexistence of two different valence cations offers a unique tunability on its electronic properties through controlling the degree of cation site ordering without a concomitant change in stoichiometry. In this work, the structural and electronic properties of cation-disordered ${\mathrm{MgSnN}}_2$ are studied through a combination of density functional theory calculations, cluster expansion, and Monte Carlo simulations. The order-disorder phase transition is analyzed, and the short-range and long-range order parameters quantifying the degree of disorder are calculated. A strong correlation between the two parameters is observed, indicating the absence of the octet-rule-conserving disorder. Cation disorder has two main effects on the electronic properties of ${\mathrm{MgSnN}}_2$. One is the reduction of the band gap, and the other is the strong localization of valence band edge states. Further analysis showed that the localization is a consequence of the weak interatomic coupling between the N atoms and the disorder-induced fluctuation of the local electrostatic potentials on the N atoms. These results could be helpful for the understanding of disorder effects in ${\mathrm{MgSnN}}_2$, as well as the tuning of its properties through the control of cation ordering.