Formation of vacancies and metallic-like domains in photochromic rare-earth oxyhydride thin films studied by in-situ illumination positron annihilation spectroscopy

Formation of vacancies and metallic-like domains in photochromic rare-earth oxyhydride thin films studied by in-situ illumination positron annihilation spectroscopy

Wu, Z. (TU Delft RST/Fundamental Aspects of Materials and Energy)
De Krom, Tom (Student TU Delft)
Colombi, G. (TU Delft ChemE/Materials for Energy Conversion and Storage)
Chaykina, D. (TU Delft ChemE/Materials for Energy Conversion and Storage)
Van Hattem, Gijs (Student TU Delft)
Schut, H. (TU Delft RST/Neutron and Positron Methods in Materials)
Dickmann, Marcel (University of the Federal Armed Forces Munich)
Brück, E.H. (TU Delft RST/Fundamental Aspects of Materials and Energy)
Dam, B. (TU Delft ChemE/Chemical Engineering)
Eijt, S.W.H. (TU Delft RST/Fundamental Aspects of Materials and Energy)

ChemE/Chemical Engineering

2022-06

Rare-earth (RE) oxyhydride thin films show a color-neutral, reversible photochromic effect at ambient conditions. The origin of the photochromism is the topic of current investigations. Here, we investigated the lattice defects, electronic structure, and crystal structure of photochromic YHxOy and GdHxOy thin films deposited by magnetron sputtering using positron annihilation techniques and X-ray diffraction, in comparison with Y, YH∼1.9, Y2O3, Gd, GdH∼1.8, and Gd2O3 films. Positron annihilation lifetime spectroscopy (PALS) reveals the presence of cation monovacancies in the as-deposited Y and YH∼1.9 films at concentrations of ∼10-5 per cation. In addition, vacancy clusters and nanopores are found in the as-prepared YHxOy and Y2O3 films. Doppler broadening positron annihilation spectroscopy (DB-PAS) of the Y- A nd Gd-based films reflects the transition from a metallic to an insulating nature of the RE metal, metal hydride, semiconducting oxyhydride and insulating oxide films. In-situ illumination DB-PAS shows the irreversible formation predominantly of di-vacancies, as PALS showed that cation mono-vacancies are already abundantly present in the as-prepared films. The formation of di-vacancies supports conjectures that H-(and/or O2-) ions become mobile upon illumination, as these will leave anion vacancies behind, some of which may subsequently cluster with cation vacancies present. In addition, in RE oxyhydride films, partially reversible shifts in the Doppler parameters are observed that correlate with the photochromic effect and point to the formation of metallic domains in the semiconducting films. Two processes are discussed that may explain the formation of these metallic domains and the changes in optical properties associated with the photochromic effect. The first process considers the reversible formation of metallic nanodomains with reduced O: H composition by transport of light-induced mobile hydrogen and local oxygen displacements. The second process considers metallic nanodomains resulting from the trapping of photoexcited electrons in an eg orbital at the yttrium ions surrounding positively charged hydrogen vacancies that are formed by light-induced removal of hydrogen atoms from octahedral sites. When a sufficiently large concentration, on the order of ∼10%, is reached in a certain domain of the film, band formation of the eg electrons may occur, leading to an Anderson-Mott insulator-metal transition like the case of yttrium trihydride in these domains.

http://resolver.tudelft.nl/uuid:ec848891-d2ad-482c-a670-59271ad4a112

https://doi.org/10.1103/PhysRevMaterials.6.065201

2475-9953

Physical Review Materials, 6 (6)

Institutional Repository

journal article

© 2022 Z. Wu, Tom De Krom, G. Colombi, D. Chaykina, Gijs Van Hattem, H. Schut, Marcel Dickmann, E.H. Brück, B. Dam, S.W.H. Eijt