LATTICE DAMAGE AND AL-METAL PRECIPITATION IN 2.5 MEV ELECTRON-IRRADIATED ALH3

AIH(3) powder was bombarded with energetic electrons at 20 K and at ro om temperature and investigated by EPR, NMR, X-ray diffractometry, and microwave dielectric-constant measurements. The EPR spectra of the ir radiated powder and of a selected single crystal cuboid of similar to 10(-1) mm edge show a complex asymmetric line centered at g = 2.009, w ith a Curie-like temperature dependence, attributed to radiation-induc ed color centers and/or their agglomerates. At the same time, the grai ns, which have become shiny black after irradiation, exhibit an increa se of both the real and the imaginary part of E. Al-27-NMR spectra of the irradiated powder present a Knight-shifted line at 1600(50) ppm, c lose to the position of bulk metallic Al: and corresponding to a conce ntration of c(Al) similar to 10(-2). In addition, the main hydride lin e differs from that before irradiation, demonstrating an alteration of environmental symmetry. The irradiation induces also a change in shap e and width of the H-1-NMR line, another indication of symmetry change in the lattice. Finally, a refined X-ray single-crystal structure ana lysis of the irradiated cuboid indicates a change of structure from tr igonal R-3c to R-3, with a loss of mirror symmetry for the two Al site s caused by the introduction of Al-defects in the vicinity of one of t hem.