SiAlONs and the Representation of Phase Relationships

SiAlONs are solid solutions based on the α- and β-silicon nitride structures. β′-SiAlON is formed when oxygen (O2−) replaces nitrogen (N3-) and silicon (Si4+) is replaced by aluminum (Al3+) to maintain overall charge neutrality. In α′-SiAlON, partial replacement of Si4+ by Al3+ occurs if, at the same time, charge compensation is effected by the accommodation of other ions, M = Li+, Ca2+, Y3+ or rare earth lanthanide ions (RE3+), in the two interstitial sites in the unit cell. The phase diagram representation of the Si-Al-O-N and related systems is based on equivalent concentrations so the Si3N4–AlN–Al2O3–SiO2 system is a square and M-Si-Al-O-N (M = 3rd cation) systems are represented using Jänecke׳s triangular prism. This article provides an introduction to SiAlONs and reviews the development of these phase diagrams.