Determination of the crystallization temperature of mullite by luminescence spectra of europium and chromium ions

The luminescence spectra of Cr$^{3+}$ and Eu$^{3+}$ ions embedded in mullite ceramic samples are studied. The samples were annealed at temperatures from 600 to 1200$^\circ$С. From the temperature of 800$^\circ$С, the spectra demonstrated the presence of a newly-formed crystal phase. The appearing of the crystalline phase is revealed by significant changes in some parameters of the luminescence spectrum. At temperatures from 920 to 1200$^\circ$С, the crystalline form manifested itself as a stable mullite phase.