Luminescence emission from metastable Sm2+ defects in Y PO4:Ce,Sm

Ce(3+) and Sm(3+) both form stable defect centres in Y PO(4), and their emission properties are well known. However, by irradiating co-doped Y PO(4):Ce,Sm with x-rays or UV light the charge states of the defects can be modified to become Ce(4+) and Sm(2+), which are metastable, and their behaviour acts as a model system for understanding carrier dynamics in charge storage phosphors. Here we report on the luminescence emission behaviour of the Sm(2+) defects that can be observed after x-irradiation. Under suitable excitation conditions, emission from both the stable Sm(3+) and metastable Sm(2+) can be monitored simultaneously. The Sm(2+) luminescence is found to be comprised of a series of narrow lines in the energy range 1.5-1.8 eV, identifiable as internal 4f-4f transitions, accompanied by a series of phonon replicas (phonon energy, 20.4 meV). The intensity of the metastable Sm(2+) emission increases in proportion to the x-irradiation time as their population is increased; under 1.92 eV laser stimulation, the PL is found to fully quench at temperatures above 150 K due to photo-thermal ionization of the defect.     

Toughness of polymeric networks: The limited importance of crosslink density

Four series of polymeric model networks were prepared with bimodal chain length distribution between crosslink points and two types of dangling chains as network defects. In the last series the crosslink density was changed without a large change in the chemical composition. The fracture toughness of those networks were compared with that of the defect–free networks. The fracture toughness of the various networks is surprisingly little influenced by the introduction of defects. Neither bimodality, nor dangling chains, nor a high sol fraction alters the toughness of the network. A good correlation between K_Ic and the weight fraction of polyether is observed. A much smaller dependence of K_Ic on the strand density can be deduced. The yield stress is high and approximately invariant for all systems studied. It is concluded that the toughness of a polymeric network does not seem to be influenced by its perfection and only to a small extent by its degree of crosslinking.