Paramagnetic defects induced by mechanical stress in calcium sulfide phosphor

The effect of uniaxial pressing and grinding on pure and europium-doped CaS powders prepared by the alkaline polysulfide flux method is studied by electron spin resonance (ESR). F+ centers are generated in the bulk of CaS particles and their ESR spectra can be easily separated from those of other paramagnetic species by using a 90° out-of-phase detection. This is possible because of the very long transverse relaxation time T2 of F+ centers. It is thought that the primary defects induced by mechanical stress are bulk F centers, which are next partially converted into F+ centers by reduction of metal ion impurities (Eu3+,Cr3+,Cu2+) and native hole centers. Heat treatments in air of mechanically stressed powders show that F+ centers firstly diffuse to the particle surface forming low-symmetry F+s centers for temperatures ranging from 300 to 700 K. Atmospheric oxygen then transforms F+s centers into ESR silent O− ions except if they are stabilized by Na+ impurities originating from the flux. For temperatures higher than 700 K, these centers are then converted into paramagnetic SO−2 centers. The reason for the strong decrease of Eu2+ emission intensity (λ=645 nm) following mechanical stress is discussed. The study of the fine structure of Mn2+ natural impurity shows that local order is strongly disturbed in CaS particles by mechanical stress.