Strontium Dialuminate SrAl4O7: Synthesis and Stability

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Up to now, strontium dialuminate, SrAl4O7 (SA2), could be synthesized only by solidification from the high‐temperature liquid state. We describe its synthesis from a spray‐dried amorphous precursor, and specify its stability domains. Its kinetics of formation is very low. It can be crystallized in the 900–1000°C temperature range either directly with a low heating rate or via two metastable solid solutions—hexagonal strontium monoaluminate (SrAl2O4 (SA)) and γ‐alumina—by annealing at 950–1000°C. As the temperature is raised beyond 1100°C, SA2 becomes metastable, its formation is no longer possible, and the crystallization of Sr4Al14O25 (S4A7) is favored. The latter compound, whose composition is close to that of SA2, is stable up to 1500°C. At higher temperature it decomposes into SA and SA2, which in its turn decomposes into SA and SA6 (SrAl12O19). There is again another stability domain for SA2, restricted to a narrow temperature scale close to its melting point (∼1800°C). The behaviors at crystallization from amorphous precursors at low temperature and from liquid at very high temperature are symmetrical: low heating or cooling rates produce pure SA2 while too rapid kinetics result in mixtures of phases.