Photochromic Mechanism and Dual‐Phase Formation in Oxygen‐Containing Rare‐Earth Hydride Thin Films

Abstract

The photochromic effect was also observed in oxygen-containing gadolinium, dysprosium as well as erbium hydride and it has been concluded that reactively sputtered lanthanide-based hydrides behave similarly as the yttriumbased thin films, suggesting a common physical mechanism of the photochromic effect. [3] Based on charge neutrality considerations, the authors suggested a single-phase structure and introduced the term "metal oxyhydrides," [3] whereas previously such photochromic thin films were referred to as oxygen-containing metal hydrides. [1,2,4-7] The single-phase structure notion has been emphasized further by the proposal of a generalized simplified model of an aniondisordered fcc lattice. [8] This model was based on ion beam analysis and X-ray diffraction data, obtained from photo chromic oxygen-containing scandium, yttrium, and gadolinium hydride thin films. [8] Recently, diffraction data from photochromic oxygen-containing yttrium hydride were interpreted as indication for a multiphase nature. [9] Despite the fact that the phase formation has not been identified, [9] the interpretation of a multiphase nature is obviously in conflict with the notion of singlephase photochromic metal oxyhydride thin films. [3,8] Here, we critically evaluate the phase formation of photochromic oxygencontaining gadolinium hydride thin films. The evidence of a dual-phase structure in combination with a significant compressive residual stress state provides a straightforward explanation for the photochromic effect. Consequently, the notion of single-phase photochromic metal oxyhydrides, discussed in literature, is flawed.

Influence of Cation (RE = Sc, Y, Gd) and O/H Anion Ratio on the Photochromic Properties of REOxH3–2x Thin Films

“…The fact that we could rationalize (i) composition, (ii) optical transmittance, (iii) optical bandgap, (iv) lattice constant, and (v) trends in photochromic properties within the framework of a single-phase line-compound strongly indicates that the photochromism is an intrinsic property of the REO x H 3–2 x oxyhydride phase. We note that the alternative hypothesis of Hans et al, 31 who recently proposed the coexistence of an oxide and a dihydride phase, is not compatible with the well-established optical bandgaps and complete transmittance for E < E g of REO x H 3–2 x thin films. Conversely, our careful study of the oxidation behavior and its dependency on the deposition pressure can explain the specific composition reported by Hans et al as that of a partially oxidized REH 2 , a nongeneral case that we obtain only around p *.…”

Photochromic Response of Encapsulated Oxygen‐Containing Yttrium Hydride Thin Films

“…changes in the structure or electronic rearrangements. In our recent work [16] , we have observed a columnar-type structure and a coexistence of fcc REH2-like and bixbyite-like RE2(HxOy)3 phases with compressive residual stress on the order of GPa in the films. As it is known that yttrium hydride is photochromic at high pressures [17] we propose a similar mechanism of photodarkening in REHO, accompanied by a photon-induced hydrogen transfer from the bixbyite RE2O3 phase into the fcc REH2 phase.…”

Temperature‐Dependent Photochromic Performance of Yttrium Oxyhydride Thin Films

“…In YHO thin films, it was recently suggested that oxygen out‐diffusion from the lattice followed by formation of metallic YHO domains is necessary for the photodarkening process. [ 18,19 ] In contrast, it was proposed that photon‐induced hydrogen transfer between phases [ 22,23 ] could also explain the photochromic mechanism. In either case, the ability of the films to achieve the darkened state at 5 K suggests that anion transfer is not responsible for this process at low temperatures.…”