We tune the thermodynamics of hydrogen absorption in Mg by means of elastic clamping. The loading isotherms measured by hydrogenography show that Mg films covered with Mg-alloy-forming elements, such as Pd and Ni, have hydrogen plateau pressures more than 2 orders of magnitude higher than bulk Mg at the same temperature. An elastic model allows us to interpret the Mg thickness dependence of the hydrogen plateau pressure. Our results suggest an alternative route for the development of new hydrogen storage materials with optimized thermodynamic properties.

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The thermodynamics of hydrogen absorption in Pd-capped Mg films are strongly dependent on the magnesium thickness. In the present work, we suppress such dependency by inserting a thin Ti layer between Mg and Pd. By means of optical measurements, we show that the surface energy contribution to the destabilization of MgH 2 is negligible. The inserted Ti layer prevents Mg-Pd alloy formation at the Mg/Pd interface, leading to quasifree Mg films and enhancing the kinetics of hydrogen desorption. Our observations are important for the development of thin film devices.

We report boron isotope effect on the E 2g phonon mode by micro-Raman spectroscopy on the ternary Mg 1−x Al x B 2 system, synthesized with pure isotopes 10 B and 11 B. The isotope coefficient on the E 2g mode frequency is nearly 0.5 in the wide range of Al, with a tendency to decrease at MgB 2 ͑x =0͒. The intraband electron-phonon ͑e-ph͒ coupling relative to the sigma band has been extracted from the E 2g line-shape parameters. By tuning the Fermi energy near the electronic topological transition ͑ETT͒, where the sigma Fermi surface changes from two-dimensional to three-dimensional topology ͑in range 0 Ͻ x Ͻ 0.28͒, the E 2g mode shows the Kohn anomaly accompanied with a splitting into a hard and a soft component. The results suggest that the intraband hardly plays any role to control the high T c of Mg 1−x Al x B 2 The common physical features of diborides with the multigap FeAs-based superconductors and cuprates are discussed.

PoisonsInhibitors Interferents a b s t r a c tThe resistance of several models of catalytic, workfunction-based metal-oxide-semiconductor and electrochemical hydrogen sensors to chemical contaminants such as SO 2, H 2 S, NO 2 and hexamethyldisiloxane (HMDS) has been investigated. These sensor platforms are among the most commonly used for the detection of hydrogen. The evaluation protocols were based on the methods recommended in the ISO 26142:2010 standard. Permanent alteration of the sensor response to the target analyte (H 2 ) following exposure to potential poisons at the concentrations specified in ISO 26142 was rarely observed.Although a shift in the baseline response was often observed during exposure to the potential poisons, only in a few cases did this shift persist after removal of the contaminants.Overall, the resistance of the sensors to poisoning was good. However, a change in sensitivity to hydrogen was observed in the electrochemical platform after exposure to NO 2 and for a catalytic sensor during exposure to SO 2 . The siloxane resistance test prescribed in ISO 26142, based on exposure to 10 ppm HMDS, may possibly not properly reflect sensor robustness to siloxanes. Further evaluation of the resistance of sensors to other Si-based contaminants and other exposure profiles (e.g., concentration, exposure times) is needed.

Magnesium thin films covered with a layer of Pd absorb hydrogen at much higher pressures than bulk Mg. Such an effect was originally explained as a consequence of elastic clamping on Mg by the capping Pd layer. An alternative interpretation later suggested that the pressure increase could originate from simple alloying between Mg and Pd. Here we resolve this controversy by measuring the hydrogenation and dehydrogenation isotherms of Mg-Pd thin film alloys over a wide range of compositions. Our results disentangle the effects of elastic clamping and alloying and highlight the role of plastic deformations.

A phase separation driven by the negative compressibility of the electron gas, near electronic topological transitions (ETT), could drive the system at the verge of a catastrophe. We show here that the metastable phases very close to the ETT transition are observed in a mesoscopic phase separation (MePhS) driven by the quenched lattice disorder. By using high resolution synchrotron radiation x-ray powder diffraction we have identified the MePhS for the intermetallic ternary Mg 1−x Al x B 2 in the proximity of two ETTs: the first at x 1 = 0.1 and the second at x 2 = 0.3. We have identified the competition between a first 'relaxed' (R) hole poor and a second 'tense' (T) hole rich phase, and by micro-Raman we observe the splitting of the in-plane phonon E 2g mode in the proximity of the first ETT at x = 0.1. The anisotropic quenched disorder due to a random distribution of Al 3+ and Mg 2+ ions both in the axial (c axis) direction and planar (ab plane) direction, probed by x-ray diffraction and Raman data, is proposed to be the physical variable that allows the formation of metastable phases near the critical points of electronic topological transitions, where Feshbach shape resonances in interband pairing amplifies the superconducting critical temperature.

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