Rare-earth melilite solid solution and its phase relations with neighboring phases

Rare-earth (R) melilite M(R) and its solid solution M'(R) are investigated. Starting with compositions of R(2)Si(3)O(3)N(4) and R(2)Si(2)AlO(4)N(3), the formation conditions for M(R), M'(R), and their neighboring phases have been delineated, and their lattice parameters and stoichiometry have been determined. Systematic trends in the solubility of M'(R), the unit-cell dimensions, and the competition with neighboring phases with different rare-earth ions have been identified and rationalized in terms of size effects and acid-base reactions. The compatibility between melilite and neighboring phases, including alpha'- and beta'-SiAlON and polytypoids, is summarized in a series of tentative phase diagrams to elucidate the various phase relations for R-SI-(Al/O)-N systems.