Aus I-Brom-emodin-trimethylather, dessen Synthese sich ebenso wie die des Emodins erheblich verbessern IieR, wurde das 4.5.7.4'.5'.7'-Hexahydroxy-2.?'-dimethyl-mcso-naphthodianthron aufgebaut. Es erwies sich als identisch mit Hypericin aus Hypcricum hirsutum. Damit ist die Konstitutionsformel des Hypericins auch durch Totalsynthese gesichert.
We investigated the changes in composition and structure induced by heat treatment of cleaved fivefold icosahedral Al-Pd-Mn quasicrystal surfaces by scanning electron microscopy and calibrated Auger electron spectroscopy. With increasing temperature we observed five different composition ranges of the surface coupled with distinct changes in the surface morphology. The changes in composition can be explained by successive evaporation and diffusion processes occurring with increasing temperatures. First, Mn evaporates from the uppermost surface layer, then Al evaporation starts and induces a further Mn evaporation leading to an enrichment in Pd. The enrichment is reduced by Mn diffusion from the bulk, Al diffusion from the bulk, and probably simultaneously occurring Pd desorption. Finally, a recrystallization of the surface is induced by the full diffusive mobility of all elements and preferential evaporation of Mn and to a lesser degree of Al. We also discuss the signature of the bonding configuration of Al in the Auger spectra of all investigated compositions.
We identified the presence and chemical nature of bulk vacancies in Al-Pd-Mn quasicrystals by measuring the structure and composition of two-and fivefold cleavage surfaces of different preannealed quasicrystals subjected to postcleavage heat treatments using scanning electron microscopy and Auger electron spectroscopy. A strong dependence of the surface structure from the preannealing is observed and explained by varying concentrations of bulk vacancies. The analysis of the data shows that as-grown Al-Pd-Mn quasicrystals contain Al, Pd, and Mn vacancies in supersaturated but near stoichiometric concentrations, while long-term preannealed samples contain a much lower vacancy concentration, with a tendency to have excess Al vacancies. We found that Al and Mn vacancies are more mobile at lower temperatures than Pd vacancies, and that the diffusion of Mn vacancies is directly coupled to the mobility of Al vacancies. The results demonstrate that the evolution of the surface structure is primarily affected by bulk vacancies migrating toward the surface during heat treatments and provide a new methodology to characterize the vacancies in intermetallic alloys. We identified the presence and chemical nature of bulk vacancies in Al-Pd-Mn quasicrystals by measuring the structure and composition of two-and fivefold cleavage surfaces of different preannealed quasicrystals subjected to postcleavage heat treatments using scanning electron microscopy and Auger electron spectroscopy. A strong dependence of the surface structure from the preannealing is observed and explained by varying concentrations of bulk vacancies. The analysis of the data shows that as-grown Al-Pd-Mn quasicrystals contain Al, Pd, and Mn vacancies in supersaturated but near stoichiometric concentrations, while long-term preannealed samples contain a much lower vacancy concentration, with a tendency to have excess Al vacancies. We found that Al and Mn vacancies are more mobile at lower temperatures than Pd vacancies, and that the diffusion of Mn vacancies is directly coupled to the mobility of Al vacancies. The results demonstrate that the evolution of the surface structure is primarily affected by bulk vacancies migrating toward the surface during heat treatments and provide a new methodology to characterize the vacancies in intermetallic alloys.
Keywords
Ames Laboratory
Disciplines
Physical Chemistry
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The spatial distribution of negatively charged Zn dopant atoms in GaAs has been investigated by cross-sectional scanning tunneling microscopy. At high densities, the dopant atoms exhibit clear clustering behavior, suggesting the existence of an effective attractive interaction in addition to the screened Coulomb repulsion between two dopants. By analyzing the data through Monte Carlo simulations, we have extracted the intrinsic screening length at different dopant densities and attributed the origin of the effective attraction to strong many-body effects in the dopant-dopant repulsion.
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