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

Abstract

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.