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Electronic Structure and Bonding of All Crystalline Phases in the Silica–Yttria–Silicon Nitride Phase Equilibrium Diagram
Author(s) -
Ching WaiYim
Publication year - 2004
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.2004.tb06352.x
Subject(s) - ternary operation , phase diagram , crystallography , materials science , ab initio , crystal structure , phase (matter) , melilite , chemistry , metallurgy , spinel , organic chemistry , computer science , programming language
This paper reviews the structures and properties of 10 binary, ternary, and quaternary crystals within the equilibrium phase diagram of the SiO 2 –Y 2 O 3 –Si 3 N 4 system. They are binary compounds SiO 2 , Y 2 O 3 , Si 3 N 4 ; ternary compounds Si 2 N 2 O, Y 2 Si 2 O 7 , and YSi 2 O 5 ; and quaternary crystals Y 2 Si 3 N 4 O 3 (M‐melilite), Y 4 Si 2 O 7 N 2 , (N‐YAM), YSiO 2 N (wallastonite), and Y 10 (SiO 4 ) 6 N 2 (N‐apatite, N‐APT). Although the binary compounds are well‐known and extensively studied, the ternary and the quaternary crystals are not. Most of the ternary and the quaternary crystals simply have been referenced as secondary phases in the processing of nitrogen ceramics. Their crystal structures are complex and not precisely determined. In the quaternary crystals, there exists O/N disorder in that the exact atomic positions of the anions cannot be uniquely determined. It is envisioned that a variety of cation–anion bonding configurations exist in these complex crystals. The electronic structure and bonding in these crystals are, therefore, of great interest and are indispensable for a fundamental understanding of structural ceramics. We have used ab initio methods to study the structure and bonding properties of these 10 crystals. For crystals with unknown or incomplete structural information, we use an accurate total energy relaxation scheme to obtain the most likely atomic positions. Based on the theoretically modeled structures, the electronic structure and bonding in these crystals are investigated and related to various local cation–anion bonding configurations. These results are presented in the form of atom‐resolved partial density of states, Mulliken effective charges, and bond order values. It is shown that Y–O and Y–N bonding are not negligible and should be a part of the discussion of the overall bonding schemes in these crystals. Spectroscopic properties in the form of complex, frequency‐dependent dielectric functions, X‐ray absorption near‐edge structure (XANES), and the electron energy‐loss near‐edge structure (ELNES) spectra in these crystals also are calculated and compared. These results are discussed in the context of specific bonding configurations between cations (silicon and yttrium) and anions (oxygen and nitrogen) and their implications on intergranular thin films in polycrystalline Si 3 N 4 containing rare‐earth elements.