Ion-implantation-induced crystalline krypton inclusions in aluminum, epitaxially aligned with the matrix, have been investigated with x-ray diffraction and Rutherford-backscattering channeling analysis. The data show a dual size distribution of the krypton bubbles. In samples annealed up to 620 K and subsequently cooled, the larger bubbles melt in a pressure-broadened transition at 114-118 K. The smaller bubbles remain solid upon heating but, as a result of a roughening transition on the aluminum facets of the cavities, the Kr crystallites gradually lose their epitaxial alignment. PACS numbers: 61.70.Qi, 64.70.DvThe discovery in 1984 that implanted gas atoms trapped in bubbles formed after ion implantation of heavy inert gases into metals are in solid form 1 " 10 finally solved the question discussed for more than thirty years regarding the pressure in these bubbles: In order to confine solid precipitates of the noble gases at room temperature, the pressure must exceed 1-2 GPa. As a rule, the structure of the crystallized gas bubbles is fee in an fee matrix, and despite very large misfits, the bubbles are aligned epitaxially with the crystal lattice of the matrix. 3,7 In situ heating experiments in the electron microscope have shown melting of the bubbles, which upon cooling solidify in epitaxial regrowth. 8,10,11 Heating to higher temperatures, where the size of the bubbles increases considerably, reduces the pressure in the bubbles, and solidification, then occurring at cryogenic temperatures, can only partially fill the cavities. 8,10,11 Nearly all the experiments on such solid inert-gas inclusions have up till now been carried out with the use of transmission electron microscopy and selected-area diffraction. In order to obtain quantitative information on the structure of the bubbles, we have carried out xray diffraction on aluminum single crystals implanted with krypton. This has been attempted previously under glancing-angle conditions, 12,13 where geometrical information alone was obtained. Aluminum single-crystal disks with a (111) normal were implanted at room temperature with 200-and 100-keV energy to fluences of 2xl0 20 m~2 and 1.5xlO 20 m~2, respectively, to give average implant concentrations of 8-10 at.% in a layer with a thickness of -150 nm. This is sufficient to produce a dense distribution of solid krypton bubbles aligned epitaxially with the aluminum matrix. 8 X-ray diffraction was carried out at high and low temperatures with Cu-A'a radiation from a rotating-anode source. To obtain further information on the fate of the implanted krypton, the crystals were also investigated by Rutherford-backscattering analysis of channeled 1.5-MeV He + ions.As-implanted crystals show distinct (111) krypton dif-fraction peaks which do not disappear after annealing to 620 K. At a temperature of 12 K, where all krypton particles are solid, the average krypton lattice parameter is 0.574 nm in both annealed and as-implanted samples.
