Ab initio thermochemistry study of polymorphism in the Si₂N₂(NH) analog of Si₂N₂O

We present a systematic density functional theory (DFT) study of the structural, bonding and electronic properties of the Si₂N₂(NH) polymorphs. This recently synthesized compound is a direct analog of Si₂N₂O in which the oxygen is replaced by an NH group. We show that a local-density approximation (LDA) description in the static lattice approximation gives a good account of the lowest energy orthorhombic phase of both the parent oxide and imide analog. In this work we extend our systematic comparisons between Si₂N₂O and Si₂N₂(NH) to denser tetragonal and monoclinic polymorphs. For Si₂N₂(NH) we find that the tetragonal and monoclinic polymorphs have energies 0.8 and 1.8 eV higher than (respectively) that of the orthorhombic ground state, similar to their Si₂N₂O counterparts (0.6 and 1.4 eV, respectively). We elucidate differences in chemical binding in Si₂N₂O and Si₂N₂(NH) polymorphs using Bader charge analysis and conclude that the imide systems possess slightly less ionic character than their oxide counterpart. Using the modified Becke-Johnson (MBJ) meta-LDA approach we find that orthorhombic Si₂N₂O and Si₂N₂NH have direct gaps of 5.65 and 5.75 eV, respectively, in excellent agreement with experiment. Relative to the orthorhombic Si₂N₂O (5.65 eV, direct), the band gaps in the denser polymorphs increase and become indirect (tetragonal: 5.72 eV, monoclinic: 6.06 eV). We find more complex trends in Si₂N₂NH in which the band gaps are also indirect in character, but decrease to 3.91 eV in the tetragonal phase, and then increase to 4.18 eV (Γ → D) in the monoclinic case.