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

Abstract

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 YH x O y and GdH x O y thin films deposited by magnetron sputtering using positron annihilation techniques and x-ray diffraction, in comparison with Y, YH ∼1.9 , Y 2 O 3 , Gd, GdH ∼1.8 , and Gd 2 O 3 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 YH x O y and Y 2 O 3 films. Doppler broadening positron annihilation spectroscopy (DB-PAS) of the Y-and 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 O 2-) 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 e g 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 e g electrons may occur, leading to an Anderson-Mott insulator-metal transition like the case of yttrium trihydride in these domains.

Aliovalent Calcium Doping of Yttrium Oxyhydride Thin Films and Implications for Photochromism

“…7,35,36 When a significant amount of Ca metallic domains would have been formed, a larger increase in the positron Doppler broadening S parameter is expected than what is observed here. 7,37 Two samples with either 0 or 20% Ca have nearly the same S parameter (Figure S5 and Table SII), suggesting that no metallic phases are present in either case and that their cation vacancy structure is very similar. Next, it is also possible that CaH 2 forms a secondary phase within the Y-oxyhydride matrix.…”

“…Had there been a metallic phase, this value would also be lowered. As well, positron annihilation spectroscopy (Figure S5) can be used to rule out the presence of small metallic secondary phases because the positron may annihilate preferentially in metallic centers. ,, When a significant amount of Ca metallic domains would have been formed, a larger increase in the positron Doppler broadening S parameter is expected than what is observed here. , Two samples with either 0 or 20% Ca have nearly the same S parameter (Figure S5 and Table SII), suggesting that no metallic phases are present in either case and that their cation vacancy structure is very similar.…”

“… 5 , 7 One explanation for photochromism has emerged involving a structural rearrangement, 8 , 9 perhaps by local diffusion, 10 to segregate a metallic phase 1 , 11 , 12 of high H – content. 7 , 11 , 12 On the other hand, some suggest the formation of in-gap states by H 2 13 or OH – 14 generation as well as H – exchange between phases. 15 …”

“…Many properties have been evaluated for their influence on photochromism such as the anion and cation compositions , and the defects or inhomogeneities present in the film. , One explanation for photochromism has emerged involving a structural rearrangement, , perhaps by local diffusion, to segregate a metallic phase ,, of high H – content. ,, On the other hand, some suggest the formation of in-gap states by H 2 or OH – generation as well as H – exchange between phases …”

Correlating Photoconductivity and Optical Properties in Oxygen‐Containing Yttrium Hydride Thin Films

“…Both effects can be visible for the energy of light slightly lower than their bandgap which indicates the involvement of other processes such as defect formations, during illumination. [8] Furthermore, we have seen that both effects recover close to the initial value almost at similar time constants, but with different scaling, indicating that they might follow different kinetics or might scale differently with the induced change in concentrations, charge carriers, etc. Moreover, the recovery time scales for both effects being in the range of minutes shows that they are not purely electronic processes, but rather multistep mechanisms as proposed by Wu et al [8] as previously mentioned.…”