28 August 2022 to 2 September 2022
Science and Technology Campus, University of Parma
Europe/Rome timezone
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New insights into the photochromism of yttrium oxyhydride thin films from in-situ muon spin rotation (MuSR) and positron annihilation spectroscopy (PAS) studies

30 Aug 2022, 12:30
20m
Science and Technology Campus, University of Parma

Science and Technology Campus, University of Parma

University of Parma, Italy
Oral Energy materials Oral contributions

Speaker

Dr Stephan Eijt (Delft University of Technology)

Description

Thin films of rare-earth metal oxyhydrides, such as yttrium oxyhydrides (YH$_{3-2x}$O$_x$), show a pronounced photochromic effect where the transparency of the films decreases reversibly over a large range of sub-bandgap wavelengths upon exposure to UV light. This makes these materials suitable candidates for applications in smart windows. However, the exact mechanism behind the photochromic effect is unknown. We investigated the behavior of YH$_{3-2x}$O$_x$ thin films, with different O:H ratios, under dark and illuminated conditions using in-situ muon spin relaxation, employing low energy muons at the LEM spectrometer. Transverse Field (TF) measurements, complemented by ZF and LF experiments, revealed that the muonium (Mu$^0$) formation, inferred from the missing fraction in the TF depolarization curves, increases with increased O:H ratio corresponding to a larger semiconductor band gap. The temperature dependence of the muonium fraction was well described by a transition-state model, where Mu$^0$ formation and gradual Mu$^{+}$ recovery takes place, accompanied by the formation of a Mu$^+$-O$^{2-}$ complex and a polaron at the Y cation. The activation energy (E$_{A,dia}$) associated with Mu$^+$ recovery is dependent on lattice relaxation and is lower for thin films of higher H content (E$_{A,dia}$ $=29$–$45$ meV). In-situ illumination further reduces this energy barrier for all measured oxyhydrides, suggesting that the photochromic effect involves a reversible structural rearrangement during photodarkening. In the light of our muon spin rotation studies, we discuss several proposals for the identity of the light-absorbing species generated by the electron-hole pairs created upon UV illumination, such as the formation of metallic domains by H$^-$ diffusion, hydroxide formation, color centers, and dihydrogen formation. We complement our discussion with recent findings from in-situ positron annihilation studies on similar films, that suggest that hydrogen vacancies are formed, as well as metallic domains that may play an important role in the mechanism of the photochromic effect.

Primary authors

Ms Diana Chaykina (Delft University of Technology) Mr Tom de Krom (Delft University of Technology) Ms Sophie Vlaar (Delft University of Technology) Dr Giorgio Colombi (Delft University of Technology) Ms Ziying Wu (Delft University of Technology) Mr Herman Schreuders (Delft University of Technology) Dr Andreas Suter (PSI) Dr Thomas Prokscha (PSI) Prof. Bernard Dam (Delft University of Technology) Dr Stephan Eijt (Delft University of Technology)

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