Influence of Cation (RE = Sc, Y, Gd) and O/H Anion Ratio on the Photochromic Properties of REO_xH_3–2x Thin Films

Abstract

Rare-earth oxyhydride REO x H 3–2 x thin films prepared by air-oxidation of reactively sputtered REH 2 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 REH 2 . 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.

Influence of Crystal Structure, Encapsulation, and Annealing on Photochromism in Nd Oxyhydride Thin Films

“…The dihydride phase of the films deposited at p dep < 0.6 Pa is further confirmed by the small transparency window observable in the transmission spectrum ( Figure S7 ), typical of RE dihydrides. 2 − 4 , 18 , 19 The transmission spectrum for the 0.5 Pa sample, however, shows a larger transparency window, extending toward longer wavelengths. This film could be very minimally oxidized, yet still maintaining the metallic properties of as-deposited NdH 1.9+δ .…”

“…Thin films of NdH 3–2 x O x deposited between p dep = 0.3–0.9 Pa result in various optical properties upon air oxidation, similar to our previous work using other rare-earth cations (RE = Sc, Y, Gd, Er, Dy). 2 − 4 Films deposited at low pressures ( p dep = 0.3–0.5 Pa) are opaque ( Figure 2 a), meaning low average transmittance ( Figure 2 b), and have no optical band gap, suggesting that they largely maintain the as-deposited NdH 1.9+δ composition. 13 − 17 …”

“…Eventually, this porosity is sufficient to allow for the oxidation of the as-deposited NdH 1.9+δ film in air and the appearance of semiconducting properties that are characteristic of oxyhydrides (NdH 3–2 x O x ). 2 , 3 …”

“…In this way, both the type of cation (RE) and the O 2– :H – ratio of these materials can be tuned, impacting their photochromic properties. 2 …”

Photochromic Response of Encapsulated Oxygen‐Containing Yttrium Hydride Thin Films

“…The exact stoichiometry, structures, and phases of possible photochromic systems in both thin films and bulk, are, however, still a matter of scientific debate. [4,12,13,16] In a recent work, [17] we have-for photochromic GdHO thin films-observed a columnar-type structure and a coexistence of fcc REH 2 -like and bixbyite-like RE 2 (H x O y ) 3 phases with compressive residual stress of the order of gigapascals in the films. As it is known that yttrium hydride is photochromic at high pressures, [18] we propose a similar mechanism of photodarkening in REHO, accompanied by a photon-induced hydrogen transfer from the bixbyite RE 2 O 3 phase into the fcc REH 2 phase.…”