Optical and electrical changes of hydrogenated Dy films

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Abstract

Electrical and optical measurements on hydrogenated Pd coated (20 nm) Dy films (250–300 nm) are presented. The films are prepared in a high vacuum chamber and hydrogenated, at room temperature, by exposing them to a hydrogen atmosphere. Maximum hydrogen concentration of the order of 3 is achieved as measured by QCM. Changes in electrical resistance and optical transmission (at λ=632.8 nm) in the films indicate the dielectric nature of the DyH3. The Hall coefficient measured also as a function of concentration, gives an insight into the evolution of the density of charge carriers as hydrogen is absorbed.

Introduction

The discovery in 1996 that it is possible to achieve high hydrogen absorption in Y and La films, with dramatic changes in their optical properties [1] has set off a flourish of studies in the rare earth hydrogen systems. In particular, trying to characterize films of these materials which, in contrast with the bulk samples, do not pulverize as the di- and trihydride are formed [2], [3], [4], [5], [6]. However, some questions in understanding the nature of the trihydride phase are still open [7], [8], [9].

Dysprosium, as other rare earths, absorbs large quantities of hydrogen and the system is well-characterized [10], [11], although, to our knowledge, thin film data are not available. Hydrogen sits initially in the tetrahedral sites until the stoichiometric dihydride is formed. Further hydrogen atoms go into the octahedral sites. However, there seems to be a broad border in concentration when the trihydride, γ phase, begins to form [12]. This is particularly true in thin film samples because the clamping to the substrate, the polycrystalline nature of the films and the tensions involved during the absorption, affect the formation of the different phases.

In this work the change in transmittance, T, electrical resistance, R, and Hall coefficient, CH, of Pd-coated Dy films exposed to a hydrogen atmosphere are studied.

Section snippets

Experimental details

The films are prepared in a high vacuum chamber at 4×10−6 Torr base pressure by electron beam deposition on glass substrates at room temperature. The Dy (4N) films are coated with a thin Pd (3N) layer (20 nm) to protect them from oxidation while allowing hydrogen to pass. With this protection it is possible to perform ex-situ measurements. Dy films with thicknesses between 250 and 300 nm were studied. Films with appropriate configurations for each type of measurement are prepared simultaneously.

Results and conclusions

As shown in Fig. 1 the Pd coated Dy films absorb large amounts of hydrogen, achieving concentrations close to 3 hydrogen atoms per metal atom, [H]/[Dy]=3, with relatively low hydrogen pressures (less than 5 Torr). These concentrations indicate that the hydride β and γ phases are formed.

The most interesting feature observed in this system is the large change in the optical transmission, as shown in Fig. 2. This change is similar to the one observed in the Y–H system [1] although it occurs at a

Acknowledgements

Financial support for this research provided by the University of Costa Rica and the Costa Rica National Research Council (CONICIT) is gratefully acknowledged.

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