Reversible loading of epitaxial Y„00.1… films with hydrogen
A. Remhof, G. Song, K. Theis-Bro
¨
hl, and H. Zabel
Ruhr-Universita ¨t Bochum, Institut fu ¨r Experimentalphysik/Festko ¨rperphysik, D-44780 Bochum, Germany
Received 16 May 1997
Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic
changes of the host lattice. We report on the reversibility of hydrogen loading in thin, monocrystalline Y films
grown by molecular beam epitaxy on Nb/sapphire substrates. During hydrogen loading, the Y film undergoes
structural transitions from the cubic dihydrid phase to the hexagonal trihydrid phase, while the structural
coherence is maintained. The transition from YH
2
to YH
3
occurs at room temperature at a hydrogen pressure
of 10 mbar and is completely reversible. S0163-18299751230-6
Within the last decades hydrogen in metals and alloys has
been studied in much detail.
1–3
Renewed interest in metal-
hydrogen systems stems from interesting structural and func-
tional properties which have recently been discerned in thin
films and superlattices. Among those are giant lattice expan-
sions caused by hydrogen in Mo/V Ref. 4 superlattices,
hydrogen induced changes of the exchange coupling in
Fe/Nb superlattices,
5
and hydrogen governed switching of
the optical properties in yttrium and lanthan-hydride thin
films.
6
These results indicate clearly that hydrogen in metals
is more than a structural ingredient and that it can be used as
a functional agent in high technology materials. This devel-
opment was only possible due to recent advances in thin-film
deposition techniques and hydrogen loading capabilities.
At low hydrogen concentrations, the hydrogen in rare-
earth RE metals can be described as a lattice gas. The metal
lattice expands in proportion to the hydrogen concentration,
while maintaining its structural and metallic properties
phase. At higher concentrations, stable dihydrid ( ) and
trihydrid ( ) phases form. While most RE metals crystallize
in the hcp structure, all dihydides of trivalent RE metals
transform to CaF
2
-like fcc structures. Apart from the divalent
lanthanides and from the systems La-H, Ce-H, and Pr-H, all
RE metals undergo a second phase transition at higher H
concentrations to the hexagonal phase.
Up until recently most work on hydrogen in RE metals
has been done using bulk or powder samples. Huiberts et al.
investigated polycrystalline thin Y films under hydrogen
loading. In the trihydrid phase, the formerly shiny metallic
film becomes a transparent semiconductor with a direct band
gap of 1.8 eV.
6
The optical changes are accompanied by a
metal-insulator transition at hydrogen concentrations above
2.8. The polycrystalline film can reversibly be loaded with
hydrogen and the switching time between a metallic reflect-
ing mirror and a transparent film has dropped from several
seconds to a fraction of a second.
The electronic structure of YH
2 +x
and LaH
2 +x
with
switchable optical properties have recently been described by
Ng et al.
7
in a local model assuming localized states centered
at vacancies in the RE-H
3
structure. In another density-
functional approach by Kelly et al.
8
small symmetry lower-
ing displacements of the hydrogen atoms are required for
driving the metal-insulator transition and for opening a large
band gap. In both models the main challenge is to understand
why only minute structural changes induced either by vacan-
cies or by collective displacements cause dramatic optical
switches. For a better understanding of the electronic prop-
erties it is therefore of utmost importance to prepare and
study single-crystalline RE films with varying hydrogen con-
centration. Because of the structural phase transitions men-
tioned above, it is, however, not clear whether the once de-
posited epitaxial RE film would maintain its structural
integrety up to x =3.
Here we report on our successful growth of epitaxial Y
films by molecular beam epitaxial MBE methods and on
hydrogen loading of these monocrystalline films with com-
plete reversiblity up to high hydrogen concentrations. We
have grown monocrystalline Y00.1 films with a typical
thicknesses of 50 nm by MBE using Al
2
O
3
11.0 substrates.
The Y film is separated from the sapphire substrate by a
Nb110 buffer.
Hexagonal closed packed RE metals are known to grow
epitaxially on Nb110.
9
The epiaxial relation follows the
Nishiama-Wassermann orientation, where the 10.0 axis of
the RE is parallel to the ( 1
¯
10) axis of the Nb. In the special
case of Y on Nb there is a 3:4 match, i.e., a supercell com-
mensuration between the Y10.0 distances of 0.316 nm and
FIG. 1. Radial Bragg scan in the direction normal to the yttrium
film after epitaxial growth. With increasing angle the Y00.2 peak,
the sapphire 11.0 peak, the Nb110, and the Pd111 reflection
can be seen. The inset shows the sample architecture.
RAPID COMMUNICATIONS
PHYSICAL REVIEW B 1 AUGUST 1997-II VOLUME 56, NUMBER 6
56 0163-1829/97/566/28973/$10.00 R2897 © 1997 The American Physical Society