Silicon Nitride - Crystal Structure and Properties

Crystal Structure and Properties

There exist three crystallographic structures of silicon nitride (Si₃N₄), designated as α, β and γ phases. The α and β phases are the most common forms of Si₃N₄, and can be produced under normal pressure condition. The γ phase can only be synthesized under high pressures and temperatures and has a hardness of 35 GPa.

The α- and β-Si₃N₄ have trigonal (Pearson symbol hP28, space group P31c, No. 159) and hexagonal (hP14, P6₃, No. 173) structures, respectively, which are built up by corner-sharing SiN₄ tetrahedra. They can be regarded as consisting of layers of silicon and nitrogen atoms in the sequence ABAB... or ABCDABCD... in β-Si₃N₄ and α-Si₃N₄, respectively. The AB layer is the same in the α and β phases, and the CD layer in the α phase is related to AB by a c-glide plane. The Si₃N₄ tetrahedra in β-Si₃N₄ are interconnected in such a way that tunnels are formed, running parallel with the c axis of the unit cell. Due to the c-glide plane that relates AB to CD, the α structure contains cavities instead of tunnels. The cubic γ-Si₃N₄ is often designated as c modification in the literature, in analogy with the cubic modification of boron nitride (c-BN). It has a spinel-type structure in which two silicon atoms each coordinate six nitrogen atoms octahedrally, and one silicon atom coordinates four nitrogen atoms tetrahedrally.

The longer stacking sequence results in the α-phase having higher hardness than the β-phase. However, the α-phase is chemically unstable compared with the β-phase. At high temperatures when a liquid phase is present, the α-phase always transforms into the β-phase. Therefore, β-Si₃N₄ is the major form used in Si₃N₄ ceramics.