Crystal structure and properties ofYSiO2N

Abstract

The crystal structure and electronic properties of the quaternary compound ${\mathrm{YSiO}}_{2}\mathrm{N}$ are investigated theoretically. It is shown that one of the possible low-symmetry structures obtained by an ab initio geometry optimization has a space group of $\mathrm{P}{6}_{1}22.$ This is different from the space group of $\mathrm{P}{6}_{1}$ that was suggested more than 25 years ago based on intensity calculations of the measured powder diffraction pattern. Using the newly determined structure, the electronic structure and bonding of this complex ceramic crystal is studied by the first-principles OLCAO method. It is shown that this crystal is an insulator with a calculated direct band gap of 4.35 eV within the local density approximation and strong Si-O and Si-N bonding. There is also considerable bonding between the Y ions and anions. The specific ${\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{O}}_{2}{\mathrm{N}}_{2}$ local bonding unit introduces several distinctive features to the occupied density of states. An interband optical absorption calculation shows major absorption peaks at 7.9 and 25.7 eV. The estimated optical dielectric constant is 3.75 while the plasma excitation frequency for ${\mathrm{YSiO}}_{2}\mathrm{N}$ is 20.0 eV.