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Oxidation dynamics and optical properties of oxygen-containing yttrium hydride thin films
Abstract
The synthesis of the photochromic YHO films is based on the oxidation of deposited yttrium hydride in ambient conditions. The actual state of the films during the deposition process, which is influenced by the deposition pressure and the oxidation caused by the residual gases, is not completely known. We report on the YHxOy thin films deposited by reactive pulsed-DC magnetron sputtering. Since the visible light transmittance is closely related to the phase and chemical composition of the films, in situ transmittance measurements during and after deposition are performed to investigate the oxidation in more detail. Spectroscopic ellipsometry is used to determine the optical constants of YHxOy throughout the film thickness. In order to obtain metallic YH2-x films with a low oxygen content, a low sputtering pressure (<1 Pa) is required, otherwise the films are already partially transparent during the deposition. The oxidation is faster when higher deposition pressures are used. This is due to the more porous growth of the microstructure at higher pressures that is observed at the surface and cross-section images of the films. The films exhibit a refractive index gradient perpendicular to the substrate surface, which is related to the porosity and variation of the chemical composition.
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Thin films of rare-earth metal oxyhydrides, such as yttrium oxyhydrides (YH3−2xOx), show a photochromic effect where the transparency of the films decreases reversibly upon exposure to UV light. However, the exact mechanism behind this effect is unknown. In this paper, we describe the behavior of YH3−2xOx thin films, with different O2−:H− ratios, under dark and illuminated conditions using in situ muon spin relaxation (μ+SR), and compare that to an oxygen-free reference compound, yttrium dihydride (YH2−δ). The muon acts as a local magnetic probe in our compounds, giving information related to electronic, structural, and photochromic properties. Although YH2−δ is the parent compound to YH3−2xOx, the muon behavior in these two materials is different—the muon electrostatically interacts primarily with H− (dihydride) or O2− (oxyhydride)—leading to the use of different theoretical models. For YH2−δ, we observed the formation of an entangled H-μ complex and the onset of Mu+ diffusion and H− rearrangement above 150 K (EA,Γ=67±13meV). For the oxyhydrides, we adopted a transition state model, where Mu0 formation and gradual Mu+ recovery take place, accompanied by the formation of a Mu+−O2− complex and a polaron at the Y cation. The activation energy (EA,dia) associated with Mu+ recovery is dependent on lattice relaxation and is lower for thin films of higher H content (EA,dia=29–45meV). In situ illumination further reduces this energy barrier for all measured oxyhydrides, suggesting that the photochromic effect involves a reversible structural rearrangement during photodarkening.
We study the dependence of the photochromic effect on environment and triggering light. We demonstrate that the first darkening/bleaching cycle of freshly grown films is accompanied by a release of weakly bound hydrogen, most likely present at the grain boundaries. For consecutive photochromic cycles, we do not find further exchange of material with the environment. Moreover, we report bleaching kinetics dependent on the gas environment after darkening with light of energies below the optical bandgap of the film. For darkening with photon energies above the bandgap of the film, we report a linear relation between the degree of darkening and bleaching relaxation time irrespective of gas environment.
Rare-earth oxyhydride REO
x
H3-2x thin films prepared by air-oxidation of reactively sputtered REH2 dihydrides show a color-neutral, reversible photochromic effect at ambient conditions. The present work shows that the O/H anion ratio, as well as the choice of the cation, allow to largely tune the extent of the optical change and its speed. The bleaching time, in particular, can be reduced by an order of magnitude by increasing the O/H ratio, indirectly defined by the deposition pressure of the parent REH2. The influence of the cation (RE = Sc, Y, Gd) under comparable deposition conditions is discussed. Our data suggest that REs of a larger ionic radius form oxyhydrides with a larger optical contrast and faster bleaching speed, hinting to a dependency of the photochromic mechanism on the anion site-hopping.
Yttrium oxyhydride (YHO) is a rare-earth-metal oxyhydride that has attracted considerable attention due to its outstanding photochromic properties. The transparency of YHO thin films across the infrared and visible spectral regions is reduced considerably under UV illumination (photodarkening) and recovers when the illumination is removed (bleaching). Although oxygen diffusion has been shown to be necessary for these processes, the exact mechanism for the photochromic behavior is not yet understood. In this work, infrared spectroscopy is utilized to investigate the effect of temperature on the photochromic properties of YHO thin films. The measurements show that YHO can photodarken at temperatures as low as 5 K, where anion diffusion is expected to be severely limited. The bleaching of the films is small, but not zero, for temperatures between 5 and 50 K. A stepwise recovery of the transmittance is observed as the temperature of the films is increased above 100 K up to 250 K. The temperature-dependent data show that anion diffusion is not required to explain the photochromic behavior of YHO, and that an additional mechanism (or mechanisms), e.g., electronic charge transfer, contributes to the photochromic behavior of YHO, as well as other rare-earth-metal oxyhydrides.
The phase formation of a photochromic Gd0.31(H0.55O0.45)0.69 thin film, grown by reactive magnetron sputtering, is critically evaluated. Oxygen is preferably incorporated into the underdense columnar grain boundaries, when the as‐deposited gadolinium hydride film is exposed to ambient conditions. Two phases, Gd2O3 and GdH2, are formed with significant compressive residual stress of 5.9 ± 1.5 GPa. These findings, extracted from transmission electron microscopy, X‐ray diffraction and atom probe tomography, provide a straightforward explanation for the photochromic effect. The mechanism can be understood as photon‐induced hydrogen transfer between the two phases, identical in nature to the photochromic effect in bulk yttrium hydride at high pressure.
When exposed to air, metallic yttrium dihydride YH2 films turn into insulating and transparent yttrium oxyhydride (YHO). The incorporation of oxygen causes the lattice expansion of YH2 and the emergence of photochromic properties, i.e., YHO darkens reversibly when illuminated with light of adequate energy and intensity. However, the adequate bleaching of the photodarkened samples once the illumination has stopped is much faster in air than in inert atmosphere. According to this experimental evidence, the photochromic mechanism has to be related to an oxygen diffusion and exchange process. Since this process is accompanied by a lattice expansion/contraction, it can be said that YHO “breathes” when subjected to illumination/darkness cycling. Another interesting side effect of the breathing is the unexpected enhancement of the hydrophobicity of the YHO samples under illumination. A theoretical model able to explain the breathing in YHO is presented, together with the discussion of other alternative explanations.
Oxygen-containing yttrium hydride thin films exhibit photochromic behavior: Transparent thin films reversibly switch from a transparent state to a photodarkened state after being illuminated with UV or blue light. From optical spectrophotometry and ellipsometry measurements of the transparent state and photodarkened state, it is concluded that the photochromic effect can be explained by the gradual growth, under illumination, of metallic domains within the initial wide-band-gap semiconducting lattice. This conclusion is supported by Raman measurements.
Thin films of oxygen-containing yttrium hydride show photochromic effect at room temperature. In this work, we have studied structural and optical properties of the films deposited at different deposition pressures, discovering the possibility of engineering the optical band gap by variation of the oxygen content. In sum, the transparency of the films and the wavelength range of photons triggering the photochromic effect can be controlled by variation of the deposition pressure. (C) 2014 AIP Publishing LLC.
The applicability of standard methods for compositional analysis is limited
for H-containing films. Neutron reflectometry is a powerful, non-destructive
method that is especially suitable for these systems due to the large negative
scattering length of H. In this work we demonstrate how neutron reflectometry
can be used to investigate thin films of yttrium hydride. Neutron reflectometry
gives a strong contrast between the film and the surface oxide layer, enabling
us to estimate the oxide thickness and oxygen penetration depths. A surface
oxide layer of 5-10 nm thickness was found for unprotected yttrium hydride
films.
Metal hydrides have earlier been suggested for utilization in solar cells.
With this as a motivation we have prepared thin films of yttrium hydride by
reactive magnetron sputter deposition. The resulting films are metallic for low
partial pressure of hydrogen during the deposition, and black or
yellow-transparent for higher partial pressure of hydrogen. Both metallic and
semiconducting transparent YHx films have been prepared directly in-situ
without the need of capping layers and post-deposition hydrogenation. Optically
the films are similar to what is found for YHx films prepared by other
techniques, but the crystal structure of the transparent films differ from the
well-known YH3 phase, as they have an fcc lattice instead of hcp.
In this work simple cost-effective physical vapor deposition method for obtaining high-quality Bi2Se3 and Sb2Te3 ultrathin films with thicknesses down to 5 nm on mica, fused quartz and monolayer graphene substrates is reported. Physical vapor deposition of continuous Sb2Te3 ultrathin films with thicknesses 10 nm and below is demonstrated for the first time. Studies of thermoelectrical properties of synthesized Bi2Se3 ultrathin films deposited on mica indicated opening of hybridization gap in Bi2Se3 ultrathin films with thicknesses below 6 nm. Both Bi2Se3 and Sb2Te3 ultrathin films showed Seebeck coefficient and thermoelectrical power factors comparable with the parameters obtained for the high-quality thin films grown by molecular beam epitaxy method. Performance of the best Bi2Se3 and Sb2Te3 ultrathin films is tested in the 2-leg prototype of a thermoelectric generator.
Transparent yttrium oxyhydride (YHO) thin films exhibit photochromic effects under ambient conditions. The optical transmission in such materials can be strongly modulated upon light absorption. This switchable optical property is interesting for technological applications, such as energy-saving smart windows and optical sensors. For the practical use of photochromic YHO materials, it is crucial to understand how the kinetics of coloration and the resulting photochromic contrast are influenced under different illumination conditions at both room and elevated temperatures. In the present study, we have prepared yellow, transparent, photochromic YHO thin films on glass substrates by using reactive magnetron sputtering. We investigated the photochromic performance of both as-deposited and annealed YHO thin films using illumination sources with varying photon energies and intensities. The photochromic contrast and the coloration were found to decrease considerably with increasing annealing temperature and with decreasing intensity and photon energy of the illumination sources. Moreover, time-resolved optical transmission measurements revealed a nearly logarithmic time dependence of the coloration of YHO thin films, indicating rapid coloration in the beginning of light exposure, followed by slow coloration with increasing illumination time.
Oxygen-containing rare-earth metal hydride, YHxOy, is a newly found photochromic material showing fast photoresponse. While its preparation method, optical properties and structural features have been studied extensively, the photochromic mechanism in YHxOy remains unknown. Here, using excited-state molecular dynamics simulation based on the recently developed real-time time-dependent density functional theory (RT-TDDFT) method, we study the photochemical reactions in YHxOy. We find that under photoexcitation, dihydrogen defects are formed within 100 fs. The dihydrogen defect behaves as a shallow donor and renders the material strongly n-type doped, which could be responsible for the photochromic effect observed in YHxOy. We also find that oxygen concentration affects the metastability of the dihydrogen species, meaning that the energy barrier for the dihydrogen to dissociate is related to the oxygen concentration. The highest barrier of 0.28 eV is found in our model with O/Y=1:8. If the oxygen concentration is too low, the dihydrogen will quickly dissociate when the excitation is turned off. If the oxygen concentration is too high, the dihydrogen dissociates even when the excitation is still on.
Photochromic yttrium oxyhydride (YHO) films of different thicknesses but similar chemical composition were grown by reactive magnetron sputtering. Photochromic response of the films defined as the relative change in transmittance upon illumination increases almost linearly with thickness for films below 600 nm and saturates (≈ 50%) for thicker ones. These results suggest that the photochromic effect on YHO films has a bulk nature and might be limited by material transport on the microscale.
The synthesis of stable yttrium oxyhydride-type compounds raised a principal question regarding the key factors that may be responsible for formation routes and structural features of these attractive materials. For solving this challenge the interplay between chemical composition and crystalline architectures has been theoretically explored via modeling structural transformations caused by the gradual oxidation of the host metal-hydride system. The close combination of group-theory methods, mixed-anion chemistry arguments, and relevant DFT calculations provided us with the opportunity to suggest and characterize the candidate models for most probable stoichiometric versions of yttrium oxyhydrides. The predicted chemical compositions along with the crystallization results have been summarized in the phase diagram. It is shown that the cation-ligand interaction governs the structural stability which is achieved by matching favorable crystallographic positions of the oxygen and hydrogen atoms at the metal center.
Thin films of yttrium oxyhydride (YHO) exhibit reversible light-induced resistivity and transmission changes at room temperature and ambient pressure. Establishing how the physical properties of YHO films are influenced by their chemical composition is an important challenge that can open the way toward practical applications. In this work, we have prepared YHO thin films with lateral gradient of oxygen and hydrogen concentrations by reactive magnetron sputtering deposition. This enables us to efficiently investigate the effect of changes in the composition of the films on their structural, electrical and optical properties. The as-deposited YHO film appeared to be black opaque and non-photochromic in the oxygen-poor part of the film, and it changed to yellow transparent and photochromic in the oxygen-rich part of the film. We report a gradual increase in the lattice constant with increasing oxygen content of the film, as revealed by grazing incidence X-ray diffraction measurements. Electrical resistivity measurements unveiled that persistent photoconductivity was strongly enhanced as the oxygen content decreased in the photochromic yellow transparent film. Analysis of the kinetics of the photochromic reaction indicated that the bleaching speed increased with increasing oxygen content in the photodarkened film. Unusual large persistent photochromism was discovered in the same yellow YHO film with lowest oxygen content which lasted for almost 10 days. Moreover, it was shown that the optical constants can be tuned by varying the oxygen content in the photochromic film as well.
Thin films of rare-earth (RE)-oxygen-hydrogen compounds prepared by reactive magnetron sputtering show a unique color-neutral photochromic effect at ambient conditions. While their optical properties have been studied extensively, the understanding of the relationship between photochromism, chemical composition, and structure is limited. Here we establish a ternary RE-O-H composition-phase diagram based on chemical composition analysis by a combination of Rutherford backscattering and elastic recoil detection. The photochromic films are identified as oxyhydrides with a wide composition range described by the formula REO xH3-2 x where 0.5 ≤ x ≤ 1.5. We propose an anion-disordered structure model based on the face-centered cubic unit cell where the O2- and H- anions occupy tetrahedral and octahedral interstices. The optical band gap varies continuously with the anion ratio, demonstrating the potential of band gap tuning for reversible optical switching applications.
It has been recently demonstrated that yttrium oxyhydride (YHO) films can exhibit reversible photochromic properties when exposed to illumination at ambient conditions. This switchable optical property enables their utilization in many technological applications, such as smart windows, sensors, goggles, and medical devices. However, how the composition of the films affects their optical properties is not fully clear and therefore demands an investigation. In this paper, the composition of YHO films manufactured by reactive magnetron sputtering under different conditions is deduced in a ternary diagram from time-of-flight elastic recoil detection analysis. The results suggest that stable compounds are formed with a specific chemical formula—YH2−δOδ. In addition, optical and electrical properties of the films are investigated, and a correlation with their compositions is established. The corresponding photochromic response is found in a specific oxygen concentration range (0.45<δ<1.5) with maximum and minimum of magnitude on the lower and higher border, respectively.
Yttrium is known to form two hydrides: YH2, a metal, and YH3, which is dielectric. However, the stability of YH3 is not fully understood, especially in the context of thin films, where the yttrium layer must be coated to protect it from oxidation. In this work, we show that the stability of a YH3 thin film depends on the capping layer material. Our investigation reveals that YH3 appears to be stabilized by hydrogen that is adsorbed to the capping layer surface. This is evidenced by the YH3-YH2 transition temperature, which was found to be correlated with the desorption temperature of hydrogen from the surface. We posit that surface-adsorbed hydrogen prevents hydrogen from diffusing out of the thin film, which limits YH3 dissociation to the solubility of hydrogen in the YH2/YH3 thin film.
Oxygen-containing yttrium hydride, YHx:Oy, is an inorganic phothochromic material which exhibits promising features for its application in smart windows, ophthalmic lenses, and many more. In the present work the optical properties of photochromic and non-photochromic YHx:Oy thin films prepared by magnetron sputtering are studied by variable angle ellipsometry and spectrophotometry. These include the study of the complex refractive index, the transmittance, and luminous transmittance, as well as the reflectance of a selection of samples of diverse structural properties obtained under different deposition conditions. In the case of the photochromic YHx:Oy thin films, the refractive index in the near infrared and visible regions is found similar to the one corresponding to yttrium oxide, Y2O3. However, the YHx:Oy films exhibit a smaller band gap and hence higher absorption in the near ultraviolet than Y2O3. The photodarkening potential of the films are also quantified and characterized by the study and modeling of their optical properties before and after illumination. Our results show that photochromic YHx:Oy films, with a luminous transmittance above 80 % in the clear state, can reduce its transparency up to a 30% after 1 h illumination at 1 Sun.
Recently, thin films of yttrium oxy-hydride (YOxHy) were reported to show an unusual color-neutral photochromic effect promising for application in smart windows. Our present work demonstrates that also oxy-hydrides based on Gd, Dy, and Er have photochromic properties and crystal structures similar to YOxHy. Compared to YOxHy, the optical bandgaps of the lanthanide based oxy-hydrides are smaller while photochromic contrast and kinetics show large variation among different cations. Based on these findings, we propose that cation alloying is a viable pathway to tailor the photochromic properties of oxy-hydride materials. Furthermore, we predict that the oxy-hydrides of the other lanthanides are also potentially photochromic.
Photochromic oxygen-containing yttrium hydride (YHO) thin films with switchable optical properties have recently emerged as a promising material for the utilization in smart windows and sensor applications. In the present study, we have prepared YHO thin films with a lateral gradient of oxygen and hydrogen concentrations, which allows us to systematically investigate the effect of changes in chemical composition on their optical properties. We show that when the average lateral oxygen content exceeded a threshold level at a certain location in the as-deposited film, its appearance was abruptly changed from black opaque to yellow transparent, in which only yellow YHO exhibited photochromism. Moreover, we show that a small region (typically ∼5– 10 mm lateral size) in the black opaque part of the as-deposited film, located adjacent to the yellow transparent part of the film, was observed to transform permanently to yellow transparent upon exposure to oxygen in air in the dark for several weeks. The black to yellow color transformation was caused by an increase in the oxygen concentration, originating from the oxidation process. Optical characterization revealed pronounced photo-chromic response in the transformed region as compared to the rest of the yellow film. This finding demonstrates that the switchable optical properties can be tailored by changing the chemical composition of YHO films.
Photochromic oxygen-containing yttrium hydride has been synthesized using a two step process. The process consists of an initial sputter deposition of oxygen-free yttrium hydride , followed by a controlled reaction with air that causes incorporation of oxygen into the material. An in-situ study of the as it reacted with oxygen was made possible by applying an aluminium capping layer with a low but non-zero oxygen permeability prior to air exposure. During the reaction, the visual appearance of the went from dark and opaque to transparent and yellowish. This transition was accredited incorporation of oxygen into the material, as shown through analysis with energy dispersive x-ray spectroscopy (EDS) and x-ray photoemission spectroscopy (XPS). Grazing incidence x-ray diffraction (GIXRD) studies revealed an fcc structure both before and after oxygen exposure, with a lattice parameter that increased from 5.2 to 5.4 Å during the reaction. With the lattice parameter in the oxygen free being equal to that reported earlier for , our findings suggest that the non-reacted in this study is in-fact .
This paper investigates the influence of deposition conditions on the properties of yttrium oxide thin films. The paper focuses on the texture, optical and mechanical properties. With this objective, a series of yttrium oxide thin films with different thicknesses were deposited by direct current (DC) unbalanced reactive magnetron sputtering at high and low pumping speed. By changing the oxygen flow, depositions were performed in the three characteristic deposition modes for reactive magnetron sputtering, i.e., metallic, transition and poisoned mode. By using an oxygen flow directed to the substrate, full oxidation of the samples, as shown by X-ray photoelectron spectroscopy (XPS), in the three modes is obtained. Crystallographic characterization by X-ray diffraction (XRD) shows that films crystallize in the cubic phase with a strong (222) out-of-plane orientation at low oxygen flow. As the oxygen flow increases a mixture of cubic and monoclinic phase is obtained. In poisoned mode, the films consist of the cubic phase with preferred (420) orientation. Scanning electron microscopy (SEM) cross sections show, with increasing oxygen flow, a loss of the columnar structure. As the oxygen flow rates increase through the metallic, transition, and the poisoned mode, the grain size becomes gradually smaller. An overview diagram of all experimental results uncovers that the textural changes are closely linked to the oxygen partial pressure rather than the oxygen flow. The optical properties of films were investigated by spectroscopic ellipsometry (SE). The films with a columnar structure demonstrate superior hardness and modulus as well as the high plasticity.
Oxygen containing yttrium hydride thin films show a unique reversible change in optical properties as a result of illumination. In this article, different solid state NMR methods are used to follow the changes during such a photochromic process. The novel stripline probe technology provides a high sensitivity for NMR experiments on thin film samples with a thickness of approximately 1 mu m. A key observation is that these photochromic films show spectral evidence of a highly mobile hydrogen species, which disappears upon illumination with white light. A relaxation back to the transparent phase is observed after a few days, accompanied by the reappearance of the mobile proton NMR signal. These results are compared with NMR experiments on pure yttrium hydride thin films. Magic-angle spinning H-1 and Y-89 experiments are performed to obtain detailed information in polycrystalline samples and flexible thin films. In particular, the Y-89 MAS experiments indicate changes in the local environment of the Y nucleus upon optical illumination. A detailed structural model for the photochromic change is not yet clear, but the presetn NMR observations may give some hints toward such a model.
For practical applications of the oxygen-containing yttrium hydride-based photochromic films, knowledge about deposition on substrates of different types and sizes is an important issue. In this article, we report on the dynamic reactive sputter deposition of the films on small and large-area glass and plastic substrates. By analysis of the optical properties we show that all the deposited films exhibited photochromic effect. Surface morphology and structural properties of the films have been studied. &
We describe an experimental method for in situ resistivity measurements during the electrochemical hydrogen loading of thin, switchable metal hydride films. Using an oxygen-free electrolyte we are able to measure the hydrogen concentration in the films quantitatively and to determine the pressure-composition isotherms and the hydrogen concentration dependence of the resistivity. Furthermore, the optical properties can be investigated by simultaneous transmission and reflection spectroscopy. The power of these in situ measurements is discussed on the basis of results obtained on YHx films; possibilities for additional experiments are briefly described
Extraordinary large hysteresis effects in optical, electrical, and structural properties are observed in switchable mirrors based on thin yttrium hydride (YHx) films, deposited on quartz glass or sapphire. The pressure-composition isotherms of the YHx system between x=2 and 3 for absorption and desorption, determined electrochemically, differ by approximately three orders of magnitude. The optical transmittance exhibits a distinct minimum when loading the films from the dihydride to the trihydride state; however, upon unloading this minimum is absent. The desorption data are in good agreement with literature data on bulk yttrium, but the absorption results show large deviations. Most important for the metal-insulator transition is that during hydrogen loading YHx remains in a single hcp phase for x>2.1. The hysteresis is discussed in terms of strains (and consequently stress) at the interface between fcc dihydride and hcp trihydride.
Thin La1-zYzHx films, in the composition range 0<z<1 and 0<x<3, are studied using x-ray diffraction, dc resistivity measurements, reflectance-transmittance measurements, and ellipsometry in the visible and near-infrared spectral range. For x=0 the structural phase diagram is similar to that of the bulk system. Upon hydrogen absorption and desorption, the La1-zYzHx films do not disproportionate. All dihydrides have a fcc structure with a continuous shift of the lattice parameter, whereas the trihydrides undergo a transition from a fcc lattice structure for 0<z<0.67 to a hexagonal lattice structure for 0.81<z<1. No significant thin-film effects occur in the structural, electrical, and optical properties, whereas disorder effects are observed in the x-ray coherence length, the electron relaxation time at both zero and optical frequencies, and in the optical properties of the trihydrides. In LaH2 a similar dihydride transmission window is observed as in YH2. The suppression of this window upon alloying is a disorder effect. As in the case of their parent materials, all La1-zYzHx alloys (both cubic and hexagonal) exhibit a metal-insulator transition for 2<x<3, which is a clear demonstration of the robustness of the metal-insulator transition in switchable mirrors. The optical band-gap shifts from 1.87±0.03 eV for LaH3 to 2.63±0.03 eV for YH3. The optical properties suggest that the fundamental band gap is 1–1.8 eV lower.
Transparent orange yttrium hydride turns to black when illuminated by visible laser light at pressures of several gigapascals at room temperature. The marked reduction in optical transmittance extends over the infrared region, suggesting that illumination creates persistent free carriers. The opaque black sample returns to the transparent orange hydride during room-temperature annealing for a few hours. Photochromism is pronounced for the coexistent state of the metallic fcc-YH2 and the insulating hexagonal-YH3 state but is depressed for the single phase of hexagonal-YH3. The results indicate that light illumination can modify the optical and possibly electronic properties during a certain period of times.
The crystal structure of YD1.96 and YH1.98 has
been examined at room temperature and 11 K by neutron-diffraction
techniques. Refinement of the data reveals the occupation of both
tetrahedral and octahedral interstices of the fcc metal lattice. These
results help to explain some features observed in the optical properties
of YHx.
Two cylindrically symmetric and complementary sputtering geometries, the post and hollow cathodes, were used to deposit thick (∼25-μ) coatings of various metals (Mo, Cr, Ti, Fe, Cu, and Al-alloy) onto glass and metallic substrates at deposition rates of 1000–2000 Å/min under various conditions of substrate temperature, argon pressure, and plasma bombardment. Coating surface topographies and fracture cross sections were examined by scanning electron microscopy. Polished cross sections were examined metallographically. Crystallographic orientations were determined by x-ray diffraction. Microstructures were generally consistent with the three-zone model proposed by Movchan and Demchishin [Fiz. Metal. Metalloved. 28, 653 (1969)]. Three differences were noted: (1) at low argon pressures a broad zone 1–zone 2 transition zone consisting of densely packed fibrous grains was identified; (2) zone 2 columnar grains tended to be faceted at elevated temperatures, although facets were often replaced by smooth flat surfaces at higher temperatures; (3) zone 3 equiaxed grains were generally not observed at the deposition conditions investigated. Hollow cathode deposition accentuated those features of coating growth that relate to intergrain shading.
A new method has been developed to synthesize compact yttriumtrihydride by making use of a thin film technique. For electrical measurements yttrium films of typically 500 nm thickness are covered under UHV conditions by a 5 nm thick palladium overlayer which consists of electrically disconnected islands. Loading of these films with hydrogen up to the trihydride phase can then be done ex-situ in a reasonably short time (around 20–40h) by applying gas pressures of about 60 × 105 Pa. For a thicker Pd layer (above 20 nm) this time can be considerably shorter (t ∼ 125 s). The film morphology stays intact during the loading process although the film thickness increases by approximately 11% and the crystal structure changes from h.c.p. to f.c.c. and back to h.c.p. These samples are, therefore, very well suited for an investigation of the remarkable electrical and optical properties of trihydrides, as recently reported by Huiberts et al. (Nature, 380, 1996, 231). In this article we give evidence for the island structure of the palladium overlayer and make a comparison of a number of physical properties of yttrium and its related hydrides as thin films with literature values for the same material in bulk form. These properties include lattice parameters for the different hydride phases, electrical resistivity for yttrium and its dihydride and Hall coefficient for yttrium. The characteristics of the yttriumhydride thin films are very similar to those of bulk material. Furthermore, we performed concentration measurements and resistivity measurements during hydrogen loading. It is shown that the resistivity rises three orders of magnitude when yttrium is loaded up to the trihydride phase at 60 × 105 Pa.
- B C Marstein
- S Z Hauback
- Karazhanov
Marstein, B.C. Hauback, S.Z. Karazhanov, A new thin film photochromic material: Oxygencontaining yttrium hydride, Solar energy materials and solar cells 95(12) (2011) 3596-3599.