Photochromic oxygen$-$containing yttrium$-$hydride thin films are synthesized by argon$-$magnetron sputtering on microscope slides. Some of them are encapsulated with a thin, transparent and non$-$photochromic diffusion-barrier layer of either Al2O3 or Si3N4. Ion beam-based methods prove that these protective diffusion barriers are stable and free from pinholes, with thicknesses of only a few tens of nanometers. Optical spectrophotometry reveals that the photochromic response and relaxation time for both $-$ protected and unprotected $-$ samples are almost identical. Ageing effects in the unprotected films lead to degradation of the photochromic performance (self$-$delamination) while the photochromic response for the encapsulated films is stable. Our results show that the environment does not play a decisive role for the photochromic process and encapsulation of oxygen containing rare-earth hydride films with transparent and non-organic thin diffusion barrier layers provides long-time stability of the films, mandatory for applications as photochromic coatings on e.g., smart windows.
When exposed to air, metallic yttrium dihydride YH2films turn into insulating and transparent yttrium oxy-hydride (YHO). The incorporation of oxygen causes the lattice expansion of YH2 and the emergence of photochromic properties. However, the oxidation of YH2 is not completely irreversible: under illumination some oxygen atoms move towards the YHO surface, leaving behind an oxygen-deficient bulk (responsible for the photochromic darkening and observed lattice contraction). Under illumination, and according to experimental evidence, some oxygen atoms can effectively leave the film, being repla…
Oxygen-containing yttrium hydride thin films exhibit a photochromic behavior: transparent thin films switch from a transparent state to a photodarkened state after being illuminated with UV or blue light. This feature has attracted much attention in recent years due to its potential applications in smart fenestration or in any device in which a response to intense light radiation is needed. However, the process responsible for the reversible change of the optical properties upon illumination is still not well understood. The objective of the present work is to shed some light on the photochrom…
In this work we report on photochromism in transparent thin film samples of oxygen-containing yttrium hydride. Exposure to visible and ultraviolet (UV) light at moderate intensity triggers a decrease in the optical transmission of visible and infrared (IR) light. The photo-darkening is colour-neutral. We show that the optical transmission of samples of 500 nm thickness can be reduced by up to 50% after one hour of illumination with light of moderate intensity. The reaction is reversible and samples that are left in the dark return to the initial transparent state. The relaxation time in the da…
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 YH$_\mathrm{x}$, followed by a controlled reaction with air that causes incorporation of oxygen into the material. An in-situ study of the YH$_\mathrm{x}$ 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 YH$_\mathrm{x}$ went from dark and opaque to transparent and yello…
Substrate-controlled epitaxial-like growth of photochromic yttrium oxyhydride thin films
Institute of Solid State Physics, University of Latvia（拉脱维亚大学固体物理研究所）；Institute of Physics, University of Tartu（塔尔图大学物理研究所）；Department of Physics, University of Helsinki（赫尔辛基大学物理系）；KTH Royal Institute of Technology（瑞典皇家理工学院）
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 unprotec…
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. Ex-situ spectroscopic …
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 te…
Inspired by the high critical temperature in yttrium superhydride and the low stabilized pressure in superconducting cerium superhydride, we carry out four independent runs to synthesize yttrium-cerium alloy hydrides. The phases examined by the Raman scattering and x-ray diffraction measurements. The superconductivity is detected with the zero-resistance state at the critical temperature in the range of 97-140 K at pressures ranging from 114 GPa to 120$\pm$4 GPa. The maximum critical temperature of the synthesized hydrides is larger than those reported for cerium hydrides, while the correspond…
We report ab-initio calculations of the superconducting properties of two high-Tc sodalite-like clathrate yttrium hydrides, YH6 and YH10, within the fully anisotropic ME theory, including Coulomb corrections. For both compounds we find almost isotropic superconducting gaps, resulting from a uniform distribution of the electron-phonon coupling over phonon modes and electronic states of mixed Y and H character. The Coulomb screening is rather weak, resulting in a Morel-Anderson pseudopotential mu*= 0:11, at odds with claims of unusually large Tc in lanthanum hydrides. The corresponding critical …
The discovery of high-temperature conventional superconductivity in H3S with a critical temperature of Tc=203 K was followed by the recent record of Tc ~250 K in the face-centered cubic (fcc) lanthanum hydride LaH10 compound. It was realized in a new class of hydrogen-dominated compounds having a clathrate-like crystal structure in which hydrogen atoms form a 3D framework and surround a host atom of rare earth elements. Yttrium hydrides are predicted to have even higher Tc exceeding room temperature. In this paper, we synthesized and refined the crystal structure of new hydrides: YH4, YH6, and…
