Isotope Effects in Switchable Metal-Hydride Mirrors

Abstract

Measurements of optical reflectance, transmittance, and electrical resistivity on the switchable mirror systems YH x and YD x show that the absorption of hydrogen induces the same variations as that of deuterium. In both cases there is a weak transparency window for the metallic dihydride (dideuteride) phase and a yellowish transparency in the insulating trihydride (trideuteride) phase. The slightly higher electrical resistivity of the deuterides is related to the lower energy of their optical phonons. The absence of significant isotope effects in the optical properties of YH x ͑ YD x ͒ is at variance with Peierlslike theoretical models. It is, however, compatible with strong electron correlation models. PACS numbers: 71.30. + h, Spectacular changes in the optical and electrical properties were recently discovered [1,2] in metal-hydride films of yttrium and lanthanum near their metal-insulator transition: the dihydrides are excellent metals and shiny while the trihydrides are insulators and transparent in the visible part of the optical spectrum. The metal-insulator (MI) transition from a shiny to a transparent state is reversible and simply induced at room temperature by changing the surrounding hydrogen gas pressure or electrolytic cell potential [3-5]. All trivalent rare-earth hydrides and even some of their alloys exhibit switchable optical and electrical properties [4,6]. In the transparent state they have characteristic colors: for example, YH 3 is yellowish, LaH 3 red, while some alloys are colorless. These films can therefore be used as switchable mirrors.Soon after their discovery it was realized that the insulating state of the switchable mirrors could probably not be understood in terms of existing one-electron theories, since Wang and Chou [7,8] and Dekker et al. [9] had concluded a few years earlier from self-consistent band structure calculations, that YH 3 was a semimetal with, in fact, a very large band overlap (1.5 eV). By minimizing the total energy, these authors found that the HoD 3 -(P3c1)-type structure was energetically more favorable than the cubic BiF 3 -type structure originally assumed by Switendick [10]. By allowing for Peierls-like displacements of the hydrogen atoms near the metal planes, Wang and Chou [8] found that the total energy could be further lowered by an extra 30 meV per YH 3 . They finally obtained an electronic band structure with only one electron band and one hole band crossing the Fermi energy E F near the center of the Brillouin zone. These difficulties stimulated theorists to reconsider the YH x and LaH x systems. Somewhat similarly to the situation for high-T c superconductors, two clearly different lines of thought have been proposed so far: Peierls-like band structure models and strongly correlated electron models.(i) Strong electron correlation: Already in 1995 Sawatzky [12] suggested that electron correlation effects might explain the existence of the large optical gap in YH 3 . Along the same lines Ng et al. [13,14] discuss the MI transition in LaH x starting ...

Fermi-surface topology of rare-earth dihydrides

“…Only a structural deviation 4,5 led to a small gap via a symmetry lowering. The discrepancy in gap size between calculation and the experiment 6,7 is comparable to known strongly correlated systems and different models were proposed, which ascribed the Me-I transition to strong electron correlations on H sites. 8,9 Recently GW calculations were able to reproduce the experimental gap and it was claimed, that the insulating trihydride is similar to a conventional semiconductor.…”

Hysteresis and the single-phase metal-insulator transition in switchableYHxfilms

“…The resistivity values are corrected for the short circuit through the palladium cap layer using the parallel resistor model for the 15-nm-thick palladium layer and the 300-nm-thick yttrium layer. 26 When approaching xϭ3 this shunting effect becomes increasingly more important, owing to the strong rise of the resistivity of substoichiometric yttrium trihydride in this concentration range. 17 When desorbing hydrogen from the film the measured resistivity values are markedly lower, leading to hysteresis in the electrical characteristics.…”

Correlated Electromigration of H in the Switchable MirrorYH3−δ

“…In these materials [YH x , LaH x and the rare earth (RE) hydrides [10] ] a spectacular change in optical and electrical properties takes place with increasing H content. In YH x , both the hcp a-phase (YH x,0.21 ) and fcc b-phase (YH 1.8,x,2.1 ) are metallic, whereas the hcp g-phase (YH x.2.7 ) is transparent with an optical gap of 2.6 eV [11][12][13]. The precise nature of the insulating ground state has been a subject of recent debate.…”