Phase Formation and Hydrogen Ordering in Yttrium-Hydrogen System
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1216-W04-02
Phase Formation and Hydrogen Ordering in Yttrium-Hydrogen System
Ke Wang, Jason R. Hattrick-Simpers and Leonid A. Bendersky Materials Science and Engineering Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899, USA
ABSTRACT Phase transformations in epitaxial yttrium films grown on (0001)Ti//(0001)Al2O3 Ti-buffered sapphire substrates and hydrogenated for 10 min were characterized using transmission electron microscopy. After hydrogen charging, dense twin lamellae form during α(Y(H))-to-β(YH2) phase transition with twin boundaries predominately parallel to the interface between Y and a substrate. High densities of Shockley partial dislocations are present at the twin boundaries, their glides during phase transformation are responsible for the formation of twin lamellae. Electron diffraction from YH2 phase shows superlattice reflections, which suggests a new type of ordering on octahedral interstitial sites. INTRODUCTION The Y-H system has attracted considerable attention because of its remarkable physical properties and potential for optical applications [1]. The Y-H system has three phase, a metallic solid solution with the host h.c.p. lattice of Y (α-phase), dihydride YH2 with the host f.c.c. lattice (β-phase), and a h.c.p. trihydride YH3 (γ-phase). With increasing hydrogen loading, the Y atoms stacking switch from h.c.p. to f.c.c., then back to h.c.p. [2]. The Y-H system in the range of YH2+x has been investigated for its richness in electric and magnetic properties, which are sensitive to change in hydrogen concentration [3,4]. Hydrogen ordering in the dihydride lattice has been reported to be responsible to these phenomena. In the stoichiometric dihydride (x=0) H atoms occupy all tetrahedral interstitial sites (two tetrahedral sites per Y atom). When the hydrogen is loaded above the stoichiometry (x>0), excess of hydrogen atoms starts to fill octahedral interstitial sites of the f.c.c. lattice. Eventually, the superstoichiometric H atoms begin to interact and form either short-range or long-range ordered sublattices. Transmission electron microscopy (TEM) is well suited for detecting ordering Y-H structures because of the sensitivity of electron scattering to small displacements of Y, which are inevitable during ordering. TEM studies of Y-H systems are limited because the rare earths in general are very reactive and TEM sample preparation is difficult. The aim of the present work is to investigate microstructure and phase formation in hydrogenated epitaxial yttrium films.
EXPERIMENTAL DETAILS Y films were prepared by e-beam evaporation at room temperature under ultra-high-vacuum conditions (< 4.0×10-4 Pa). A Ti buffer layer with a thickness of 100 nm was first deposited directly on an α-Al2O3 (0001) substrate. After a 1.2 µm thick Y film was deposited on the Ti layer, a 5 nm thick Pd capping layer was used to protect the Y films against oxidation. The films were then hydrogenated at 523K under 0.490 MPa of ultra-high-purity H2 (>99.999%) for 10 min. In order to reduc
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