Laser-ablation studies of highly-oriented thin films of the electon-doped infinite-layer copper-oxide compounds Sr1 - xCuO2 are reported. We observe significant variations in film properties with substrate or buffer layer material. X-ray diffraction, atomic force microscopy (AFM), Rutherford back-scattering (RBS), and electrical resistivity were used to characterize the films. Films were deposited on strontium titanate (001) or on buffer layers of T′-phase copper oxides (Ln2CuO4 with Ln = Pr, Nd, Sm), Sr3FeNb2O9, and La1.8Y0.2CuO4 on SrTiO3 (001). The in-plane lattice constants of such buffer layers (a = 0.390-0.400 nm) should provide the bond tension required for electron doping. Extremely flat, epitaxial buffer layers with X-ray rocking curves as narrow as 0.08° were obtained from stoichiometric targets of Ln2CuO4; the other buffer layers yielded poor epitaxy. A linear dependence of infinite-layer c-axis plane spacing on substrate or buffer-layer in-plane a-axis lattice constant is observed.@en

Mg-Al thin films with a compositional gradient are co-sputtered from off-centered magnetron sources and capped with a thin Pd layer. We study their hydride formation by monitoring their optical transmission during hydrogenation under defined pressure and temperature conditions. We find that Mg(AlH 4)2 is already formed from the elements at p(H2)=1 bar and T=100 ° C. A thin layer of Ti acts as a catalyst. Doping of Mg-Al with Ti has a negative influence on hydrogen absorption.@en

Hydrogenography is a new optical thin film combinatorial method that follows hydrogenation and determines its associated thermodynamic properties. Due to clamping to the substrate, stresses generated in thin films are larger than in bulk. This must be taken into account for a comparison between these two types of systems. In this article, we follow the microstructure, surface morphology and in-plane stress changes of thin polycrystalline PdHx films upon several hydrogen ab/desorption cycles and correlate them to the evolution in shape and hysteresis of pressure-optical transmission isotherms (PTIs) recorded by hydrogenography. The in-plane stress in the first instance is relaxed inhomogeneously by buckling, and a more complete, homogeneous relaxation is only reached after the creation of a buckle-and-crack network that is the two-dimensional analogue of bulk decrepitated grains. This sequence of changes is clearly visible in the PTIs, demonstrating another useful facet of hydrogenography for characterizing metal-hydrogen systems.@en

The optical properties of Mg-TM (TM: Ni, Co, Fe; Mg:TM ≈ 2) thin films during hydrogenation are investigated using reflection and transmission measurements. Mg-Ni and Mg-Co show an unusual behavior upon exposure to hydrogen. The nucleation of the hydride starts at the film substrate interface and not - as intuitively expected - at the Pd covered top side of the film. Consequently, a double layer structure is formed. Reflection measurements are used to identify the loading behavior as the double layering yields to an remarkable optical black state at intermediate hydrogen concentrations. Photometric measurements on samples loaded at H2-pressures of 10 5 Pa show that in Mg-NiHx and Mg-CoHx the complex hydrides Mg2NiH4 and Mg2CoH5 are formed, whereas in Mg-Fex the formation of MgH2 is prevalent.@en

Hydrogenography is an advanced combinatorial and standard method used for the search of new hydrogen-storage materials to synthesize bulk samples and to use volumetric or gravimetric techniques to follow their hydrogenation reaction. Hydrogenography, with a straightforward optical setup, makes it possible to monitor hydrogen absorption and desorption simultaneously on thousands of samples under exactly the same experimental conditions. Hydrogenography is much more than a monitoring technique, as it also provides a high-throughput method to measure quantitatively the key thermodynamic properties of hydride formation. The continuous change of optical transmission with hydrogen concentration was used to measure the pressure-concentration isotherms and determine the enthalpy of hydride formation. Hydrogenography is valuable for the search for catalytic caplayers promoting hydrogen uptake, electrode materials for batteries, and smart coatings for adaptive solar collectors.@en

The structural, optical and dc electrical properties of Mg xAl1-x (0.2 ≤ x ≤ 0.9) gradient thin films covered with Pd/Mg are investigated before and after exposure to hydrogen. We use hydrogenography, a novel high-throughput optical technique, to map simultaneously all the hydride forming compositions and the kinetics thereof in the gradient thin film. Metallic Mg in the MgxAl1-x layer undergoes a metal-to-semiconductor transition and MgH2 is formed for all Mg fractions x investigated. The presence of an amorphous Mg-Al phase in the thin film phase diagram enhances strongly the kinetics of hydrogenation. In the Al-rich part of the film, a complex H-induced segregation of MgH2 and Al occurs. This uncommon large-scale segregation is evidenced by metal and hydrogen profiling using Rutherford backscattering spectrometry and resonant nuclear analysis based on the reaction 1H(15N, αγ)12C. Besides MgH2, an additional semiconducting phase is found by electrical conductivity measurements around an atomic [Al]/[Mg] ratio of 2 (x = 0.33). This suggests that the film is partially transformed into Mg(AlH4)2 at around this composition.@en

The peculiar optical patterns observed in Mg2NiHx films during hydrogenation with their applicability as selective tunable absorbers were discussed. These films showed large change in reflection that yielded very low reflectance at different hydrogen contents. It was shown that these films are suitable as smart coatings due to their energy tunability, high reflectance contrast, and switching capabilities. It was observed that these films can be used as a switchable solar absorber in solar collectors as the film in this state emits only 16% of the 100°C blackbody radiation. It was found that.@en

In many experimental investigations of thermodynamic equilibrium or kinetic properties of series of similar reactions it is found that the enthalpies and entropies derived from Van ′t Hoff or Arrhenius plots exhibit a strong linear correlation. The origin of this Enthalpy-Entropy compensation, which is strongly related to the coalescence tendency of Van ′t Hoff or Arrhenius plots, is not necessarily due to a physical/chemical/biological process. It can also be a merely statistical artefact. A new method, called Combined K-CQF makes it possible both to quantify the degree of coalescence of experimental Van ‘t Hoff lines and to verify whether or not the Enthalpy-Entropy Compensation is of a statistical origin at a desired confidence level. The method is universal and can handle data sets with any degree of coalescence of Van ‘t Hoff (or Arrhenius) plots. The new method requires only a standard least square fit of the enthalpyΔH versus entropy ΔS plot to determine the two essential dimensionless parameters K and CQF. The parameter K indicates the position (in inverse temperature) of the coalescence region of Van ‘t Hoff plots and CQF is a quantitative measure of the smallest spread of the Van ‘t Hoff plots. The position of the (K, CQF) couple with respect to universal confidence contours determined from a large number of simulations of random Van ‘t Hoff plots indicates straightforwardly whether or not the ΔH-ΔS compensation is a statistical artefact.@en

Enthalpy-entropy compensation (EEC) is very often encountered in chemistry, biology and physics. Its origin is widely discussed since it would allow, for example, a very accurate tuning of the thermodynamic properties as a function of the reactants. However, EEC is often discarded as a statistical artefact, especially when only a limited temperature range is considered. We show that the likeliness of a statistical origin of an EEC can be established with a compensation quality factor (CQF) that depends only on the measured enthalpies and entropies and the experimental temperature range. This is directly derived from a comparison of the CQF with threshold values obtained from a large number of simulations with randomly generated Van ‘t Hoff plots. The value of CQF is furthermore a direct measure of the existence of a genuine isoequilibrium or isokinetic relationship.@en

We report on the implementation of Pd-capped chemo-chromic metal hydrides as a sensing layer in fiber optic hydrogen detectors. Due to the change in optical properties of Mg-based alloys on hydrogen absorption, a drop in reflectance by a factor of 10 is demonstrated at hydrogen levels down to 15% of the lower explosion limit. The switching takes place in only a few seconds. Comparing Mg-Ni and Mg-Ti based alloys, we find that the latter has superior optical and switching properties. Using a 50 nm thick Mg70Ti30 film capped with a 30 nm Pd catalytic layer as the sensing layer, we report on the sensitivity of the fiber optic detector for hydrogen both in argon and oxygen, the temperature behavior and the reversibility.@en

Large relaxation effects have been observed in many high Tc superconductors and the time dependence of the magnetization is usually found to follow a logarithmic time decay. This is in agreement with the results of simple Monte Carlo simulations which lead to M(t, T, B) = M(O, T, B) [1 - (kT/U)In(1 + t/τ)], where M(O, T, B) is the magnetization at time t = 0, temperature T and magnetic field B, and τ is a relaxation time related to the attempt frequency for hopping of flux lines from one pinning region to an adjacent one. The activation energy, U, depends on both T and B. The purpose of this paper is to discuss some of the difficulties encountered in measurements of magnetization relaxation effects in high Tc superconductors in general and thin films in particular. First, how information about the energy barriers for flux motion may be extracted from experimental data on M(t, T, B) is discussed and experimental evidence is given for the necessity to introduce a distribution of activation energies to reproduce the observed temperature dependence of the relaxation rate, S = -dlnM/dlnt. Second, by means of Monte Carlo simulations, the influence of an initial partially critical state (where the critical current, Jc, flows only in a thick surface layer of the sample) on the time dependence of the magnetization relaxation is studied. The results of these simulations are compared with the predictions of an approximate analytical treatment of the relaxation problem. Third, it is shown that the initial conditions have a crucial influence on the magnetization relaxation. As an example, the effect is considered of a small overshoot during the application of a magnetic field before measuring M(t, T, B), which leads to remarkable micro-creep behaviour. Finally, some data are presented which imply that magnetization relaxation is still large at low temperatures (down to 1.5 K) in good quality epitaxial thin films and the possibility of flux line tunnelling at low temperatures is discussed.@en

We succeeded to image with magnetic force microscopy individual vortices in thin Nb films as well as in thin YBa2Cu3O7-δ (YBCO) films. Relying on the imaging of microfabricated gold loops, we are able to identify the exact location of the vortices within 10 nm with respect to topographic features. The center of most of the vortices is located in between the grains appearing at the film surface. For the Nb films with a larger coherence length, the pinning can be linked to the reduced film thickness in between the protruding grains. For the YBCO films with a very small coherence length, the pinning is likely to be dominated by line defects at the trenches in between the growth islands. On the other hand, Fourier analysis of the imaged vortex distribution shows that for thicker Nb films not all of the vortices are pinned in between the grains. Due to the vortex-vortex repulsion, the vortex lattice reveals short-range correlations which become more pronounced at higher fields.@en

Hydrogen absorption by a Pd capped thin Mg2Ni film results in the nucleation of the Mg2NiH4 phase at the film/substrate interface and thus induces a self-organized two-layer system. This leads to the optical black state in Mg2Ni thin films upon hydrogenation. This unusual hydrogenation behaviour is completely unexpected since the hydrogen enters through the top film surface. To explain the nucleation of Mg 2NiH4 close to the substrate/film interface we performed scanning tunneling microscopy (STM) on as-prepared Mg2Ni films with various thicknesses (20-150 nm). For films thinner than 50 nm, the film consists of small grains and clusters of small grains whereas on further growth the grain size increases and a columnar microstructure develops. We propose, therefore, that close to the substrate, the relatively porous structure of the film with small Mg2Ni grains locally reduces the nucleation barrier for Mg2NiH4 formation.@en

The temperature dependent optical properties of certain thin films are used in so-called thermochromic devices. As an alternative to a single layer of, e.g. VO2, which displays a temperature dependent optical transmission, we propose the use of an encapsulated metal-hydride bilayer (i.e. a switchable mirror and a hydrogen storage layer). Due to differences in the thermodynamic properties of the two metal-hydride layers, a temperature change results in the migration of hydrogen from one layer to the other, thereby changing the optical properties of the switchable mirror metal-hydride. Depending on the materials chosen the transition can be optimized for transmission, reflection or absorption. Hence, the use of metal-hydride bilayers allows the fabrication of a thermochromic absorber. The thermodynamical background and the first results of such an all-solid-state thermochromic device are presented.@en

Mgy Ni (2 ≤ y ≤ 10) thin films covered with a Pd cap layer are hydrogenated in 1 05 Pa H2 between room temperature and 80 ° C and their dielectric function over(ε, ̃) is determined from reflection and transmission measurements. The hydrogenated Mgy NiHx thin films show a continuous shift of the optical absorption towards higher photon energies with increasing y. Comparison of the obtained dielectric functions with predictions from an effective medium theory show that a considerable doping of the Mg2 NiH4 host takes place at least for y ≤ 3.5 while no signature of MgH2 is observed in that composition range in the optical spectra. This is in contrast to the predictions from the bulk phase diagram where a mixture of semiconducting Mg2 NiH4 (energy gap Eg = 1.6 eV) and MgH2 (Eg = 5.6 eV) is expected.@en

The optical properties, the switching kinetics and the lifetime of hydrogen switchable mirrors based on Mg-Ni alloys are determined with particular regard to the composition of the optically active metal-hydride layer in combination with the thickness of the catalytic capping layer. For this, a high-throughput experiment is introduced. The switching kinetics and the reversibility of switchable mirrors are strongly thickness dependent, though the details hinge on the fine structure of the clustered capping layer. Therefore, the kinetics is correlated with the surface structures of Pd on Mg y Ni 1-y as investigated by scanning tunneling microscopy. The results are explained by the so-called strong metal-support interaction (SMSI) state, characterized by a complete encapsulation of the capping layer clusters by oxidized species originating from the support. The SMSI-effect is less important with increasing Pd-layer thickness, and is suppressed by a good wetting of the Pd-clusters on the optically active film. This explains the critical thickness for the catalyzed hydrogen uptake observed in many switchable mirror systems. Moreover, the degradation of the kinetics during cycling is found to depend on the Pd-layer thickness and on the gas environment. Only films, covered with at least 15 nm Pd, show small degradation caused by the SMSI-effect. The SMSI-effect is partly reversible: after changing the gas environment from hydrogen to oxygen, the oxide on the Pd-clusters can be partly removed.@en

The hydrogenation of metallic Mg2 Ni films was shown to proceed via a self-organized double layering of transparent Mg2 NiH4 and metallic Mg2 NiH0.3. For stoichiometries departing from Mg2 Ni we conclude from optical reflection, transmission and electrical measurements that the hydrogenation process involves two competing effects: (i) the nucleation of the initial Mg2 NiH4 layer near the substrate and the ensuing growth of this layer and (ii) the slow random nucleation of the same phase within the remaining part of the film. Which process dominates depends critically on the stoichiometry of the parent metal alloy.@en

MgHx thin films are grown by activated reactive evaporation in a Molecular Beam Epitaxy system fitted with an atomic hydrogen source. During deposition the electrical and optical properties are measured in-situ. The structural properties are determined ex-situ by Atomic Force Microscopy. These measurements confirm the growth of the MgH2 phase, however the presence of 10 vol.% of metallic Mg cannot be prevented. The metallic Mg grains cause an optical absorption edge at 2.0 eV, which has a completely different origin than the observed band gap of MgH2 at 5.6 eV. The observed optical spectra can be modelled using an effective medium theory. The Mg hydride films are electrically insulating despite the presence of metallic Mg particles. Upon re-hydrogenation of a de-hydrogenated in-situ grown MgHx thin film, the absorption edge at 2.0 eV disappears and the resistivity decreases to values normally observed for ex-situ hydrogenated films.@en

The reflection and transmission spectra of Pd capped Mgy Ti1 - y thin films (y = 0.7, 0.8 and 0.9) are measured in the 0.5-5.5 eV energy range, both in the as-prepared and hydrogenated states. Upon hydrogenation these films switch reversibly from a shiny metallic state into a "black" absorbing one. The composition and thicknesses can be tailored to achieve high solar absorptance and low thermal emittance in the hydrogenated state. The combination of these two characteristics is interesting for the application of this material as switchable absorber in solar collectors. The use of a Mgy Ti1 - y switchable absorber in solar collectors allows to lower the stagnation temperature from 180 to 80 °C. The collector efficiency is affected only minimally.@en

A detailed structural analysis of Mg-Ti-H thin films reveals the presence of a chemically partially segregated but structurally coherent metastable phase. By combining X-Ray Diffraction and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy on MgyTi1-yHx thin films we find non-zero Chemical Short-Range Order (CSRO) parameters for all the compositions measured. Despite the positive enthalpy of mixing of Mg and Ti the degree of ordering does not increase upon loading and unloading with hydrogen. The robustness of this system and the fast and reversible kinetics of hydrogen loading and unloading are caused by the formation of nanoscale compositional modulations in the intermetallic alloy. This microstructure is responsible for the exceptional properties of MgyTi1-yHx thin films. It also shows that reversible metastable metal-hydrides offer new possibilities for hydrogen storage, beyond the limits imposed by thermodynamic equilibrium.@en