A case study of complex metallic alloy phases: structure and disorder phenomena of Mg-Pd compounds

International audienceThe Mg-Pd phase diagram has been re-determined in the range 60 to 100 at-% Mg. The existence of the intermediate phases Mg₆Pd (β), Mg₅₇Pd₁₃ (γ), Mg_56.4Pd_13.5 (δ), Mg₃₀₆Pd₇₇ (ε), Mg_78.5Pd_21.5 (ζ), Mg₃Pd (η), Mg₅Pd₂ (θ) and Mg₂Pd (ι) has been reconfirmed. The β-phase melts congruently whereas the CMAs γ, δ, ε and ζ form in a cascade of peritectoid and peritectic reactions in a narrow window of approx. 30 °C and 3 at-%. It is assumed that the stability of the Mackay icosahedron plays an important role in the phase formation of these CMAs. However, the β-phase reveals an intricate pattern of disorder at the atomic positions of the Mackay icosahedron (MI). Therefore, the concept of the stability of the Mackay cluster should be used only as a rule of thumb

Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films

We report positron depth-profiling studies on the hydrogen sorption behavior and phase evolution of Mg-based thin films. We show that the main changes in the depth profiles resulting from the hydrogenation to the respective metal hydrides are related to a clear broadening in the observed electron momentum densities in both Mg and Mg2Ni films. This shows that positron annihilation methods are capable of monitoring these metal-to-insulator transitions, which form the basis for important applications of these types of films in switchable mirror devices and hydrogen sensors in a depth-sensitive manner. Besides, some of the positrons trap at the boundaries of columnar grains in the otherwise nearly vacancy-free Mg films. The combination of positron annihilation and x-ray diffraction further shows that hydrogen loading at elevated temperatures, in the range of 480–600 K, leads to a clear Pd–Mg alloy formation of the Pd catalyst cap layer. At the highest temperatures, the hydrogenation induces a partial delamination of the ? 5?nm thin capping layer, as sensitively monitored by positron depth profiling of the fraction of ortho-positronium formed at interface with the cap layer. The delamination effectively blocks the hydrogen cycling. In Mg–Si bilayers, we investigated the reactivity upon hydrogen loading and heat treatments near 480 K, which shows that Mg2Si formation is fast relative to MgH2. The combination of positron depth profiling and transmission electron microscopy shows that hydrogenation promotes a complete conversion to Mg2Si for this destabilized metal hydride system, while a partially unreacted, Mg-rich amorphous prelayer remains on top of Mg2Si after a single heat treatment in an inert gas environment. Thin film studies indicate that the difficulty of rehydrogenation of Mg2Si is not primarily the result from slow hydrogen dissociation at surfaces, but is likely hindered by the presence of a barrier for removal of Mg from the readily formed Mg2Si.Radiation, Radionuclides and ReactorsApplied Science