Valence evaluation of cerium ions in Ce-doped alpha-sialon by electron energy loss spectra

For a long time cerium ions, both Ce3+ and Ce4+, have not been considered t o be able to enter the interstitial sites in an alpha -sialon structure unt il recent successes in the preparation of cerium-doped alpha -sialon materi als using CeO2 as the starting powder. It is then of great importance and n ecessity to know the chemical valence of these incorporated cerium ions. Me anwhile, the specific structure of the interstices in alpha -sialon provide s quite a different chemical environment upon which the electronic structur es of cerium ions could be different from those ever reported in, for insta nce, metallic compounds and oxides. Electron energy loss spectra (EELS) in the M-edge region of rare-earth elements carries information on the initial state 4f occupancy. We have acquired and examined the M-4,M-5-edge spectra of cerium at different locations in the microstructure. It has been found that all the Ce(IV) ions in the intergranular glass and the majority inside the alpha -sialon crystals have been reduced to Ce(III) while there exist inside the crystal cerium ions showing 4f(0) initial-state peaks on the EEL S spectrum. The 4f(0) peak locates at about 0.6 eV higher in energy than th at for CeO2. An increased and possibly by far the highest weight of f(0) st ate is derived for Ce(IV) ions in alpha -sialon. The spectral evidences con firm the general suggestion of the unstability of Ce(IV) ions with silicon nitride. (C) 2001 American Institute of Physics.