The effect of uniaxial pressing and grinding, upon pure or Eu-doped powders that had been prepared by using the alkaline polysulfide flux method, was studied by using electron spin resonance techniques. It was found that F+ centers were generated in the bulk of the particles, and that their electron spin resonance spectra could be easily separated from those of other paramagnetic species by using 90 out-of-phase detection. This was possible because of the very long transverse relaxation times of F+ centers. It was suggested that the primary defects which were introduced by the mechanical stresses were bulk F centers, which were then partially converted into F+ centers by the reduction of metal ion impurities (Eu3+, Cr3+, Cu2+) and native hole centers. Heat treatment in air, of mechanically stressed powders, showed that F+ centers first diffused to the particle surface and formed low-symmetry Fs+ centers, at temperatures which ranged from 300 to 700K. Atmospheric O then transformed the Fs+ centers into electron spin ‘resonance-silent’ O- ions, except when they were stabilized by Na+ impurities which originated from the flux. At temperatures above 700K, these centers were then converted into paramagnetic SO2- centers.

D.Caurant, D.Gourier, N.Demoncy, I.Ronot, M.Pham-Thi: Journal of Applied Physics, 1995, 78[2], 876-92