Formation and structural anomaly of the metastable phases in an immiscible Ag-Mo system studied by ion beam mixing and molecular dynamics simulation.

For the equilibrium immiscible Ag-Mo system characterized by a large positive heat of formation, the nanosized Ag-Mo multilayered samples are designed and prepared to include sufficient interfacial free energy to elevate their initial energetic states to be higher than that of either the amorphous phase or solid solution and then subject to 200 keV xenon ion irradiation. The results show that a uniform amorphous alloy can be obtained within a composition range, at least, from 25 to 88 atom % of Mo. Interestingly, in the intermediate stage of ion irradiation, a bcc phase, an amorphous phase, and an order (bcc)-disorder coexisting state appear simultaneously in the Ag12Mo88 multilayered sample and extend over the entire bright field image with unanimously homogeneous composition. In thermodynamic modeling, a Gibbs free energy diagram of the Ag-Mo system is constructed, based on Miedema's model, and suggests that within a narrow composition regime of 85-90 atom % of Mo, the energy difference between the bcc and the amorphous phases is extremely small, which is probably the very reason for the order-disorder coexisting state to appear. In atomistic modeling, an ab initio derived Ag-Mo potential is applied to perform molecular dynamics simulations. The simulations not only determine an intrinsic glass-forming ability/range (GFA/GFR) of the Ag-Mo system to be from 10 to 88 atom % of Mo but also reveal the possibility of the formation/appearance of a crystalline and amorphous mixture in a narrow composition regime of 88-92 atom % of Mo. Apparently, the theoretical results are in excellent agreement and/or compatible with the experimental observations in ion beam mixing.

Observations of distinct atomic packings in Cu-Nb metallic glasses synthesized by ion beam mixing.

The structural transformation and disordered atomic packing of metallic glasses in a selected immiscible system at equilibrium, i.e. the Cu-Nb system characterized by a positive heat of formation, are studied using ion beam mixing far from equilibrium. The experimental results indicate that the Cu-Nb metallic glasses could be formed in a composition range from 30 to 85 at.% of Nb and that the Cu-Nb metallic glasses are formed through two different structural phase transition routes, i.e. from the Nb-based body centred cubic and face centred cubic solid solutions, in which the two distinct predominant atomic packings have icosahedral and icositetrahedral orderings, respectively, revealed by the respective diffraction patterns. These observations not only help in formulating a general atomic structural spectrum for the binary metallic glasses, but also suggest an important concept of structural heredity: that the crystalline structure of the constituent metals plays a decisive role in determining the atomic structure of the resultant metallic glasses.

Tetrodotoxin and derivatives in several species of the gastropod Naticidae.

The organs of 195 specimens of 4 species of the gastropod Naticidae, collected from fish markets in Taiwan, were assayed for toxicity. The calf moon shell Natica vitellus contained weak toxicity (10-99 MU/g) in the muscle and digestive gland. The bladder moon shell Polinices didyma contained moderate (100-999 MU/g) and weak toxicity in the muscle and digestive gland, respectively. The digestive gland in 1 out of 20 specimens of the pear-shaped moon shell P. tumidus was toxic (4 MU/g). All tissues of the butterfly moon shell N. alapapilionis were non-toxic. The toxins were partially purified from the toxic specimens of the calf moon shell and the bladder moon shell, and identified to be tetrodotoxin and anhydrotetrodotoxin.