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.

我们对 Si ₂ N ₂ (NH) 多晶型物的结构、键合和电子特性进行了系统的密度泛函理论 (DFT) 研究。这种最近合成的化合物是 Si ₂ N ₂ O的直接类似物，其中氧被 NH 基团取代。我们表明，静态晶格近似中的局部密度近似 (LDA) 描述很好地说明了母体氧化物和酰亚胺类似物的最低能量正交相。在这项工作中，我们将 Si ₂ N ₂ O 和 Si ₂ N ₂ (NH)之间的系统比较扩展到更密集的四方和单斜多晶型物。对于 Si ₂ N ₂(NH) 我们发现四方晶和单斜晶型的能量比（分别）高出正交基态的能量 0.8 和 1.8 eV，类似于它们的 Si ₂ N ₂ O 对应物（分别为 0.6 和 1.4 eV）。我们使用贝德电荷分析阐明了 Si ₂ N ₂ O 和 Si ₂ N ₂ (NH) 多晶型物的化学结合差异，并得出结论，酰亚胺系统的离子特性略低于其氧化物对应物。使用改进的 Becke-Johnson (MBJ) 元 LDA 方法，我们发现正交 Si ₂ N ₂ O 和 Si ₂ N ₂NH 的直接间隙分别为 5.65 和 5.75 eV，与实验非常吻合。相对于斜方晶 Si ₂ N ₂ O（5.65 eV，直接），致密多晶型物中的带隙增加并变为间接（四方晶系：5.72 eV，单斜晶系：6.06 eV）。我们在 Si ₂ N ₂ NH 中发现了更复杂的趋势，其中带隙在性质上也是间接的，但在四方相中降低到 3.91 eV，然后在单斜晶情况下增加到 4.18 eV (Γ → D)。