Jaarboek no. 89. 2010/2011 - Koninklijke Maatschappij voor ...

Natuurkundige voordrachten I Nieuwe reeks 89
Schakelbare spiegels: een samenspel van licht en waterstof
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(a) (b) (c)
Figure 1
(a)-(c) Photographs of an Yttrium-hydride switchable mirror in the as-deposited, dihydride and trihydride states. The
metal-insulator transition, which is occurs at low hydrogen pressures (at room temperature less than 1 bar), is not of
structural origin. It occurs within the hexagonal phase at a composition H/Y≈2.86.
have characteristic colors: for example, YH 3 is yel-
lowish, LaH 3 red, while some fully hydrogenated
alloys containing magnesium are colorless. One of
the most surprising results of these early measurements
was, however, that the films retained their
structural integrity even though they expanded
by typically 15% during hydrogenation of the pure
parent metal to the trihydride. This meant that for
the first time physical properties, such as electrical
resistivity, Hall effect, optical transmission, reflection
and absorption, were amenable to experimental
investigation. This led to the discovery of new
phenomena in the electrical, optical and mechanical
properties of these materials. Furthermore, the
possibility to fine-tune their properties by alloying
and the ease of continuously changing their
hydrogen content made them especially attractive
for fundamental condensed matter physics. Soon
after their discovery it became clear that switchable
metal hydride films would pose intriguing
questions. Their transparent appearance in the fully
loaded state posed a serious problem to theorists. In
particular, the yellow appearance of YH3 seemed to
be in contradiction with state-of-the-art band structure
calculations that predicted a metallic state for
this material. The physics of rare-earth switchable
mirrors is thus closely related to that of electron correlation
and metal-insulator transitions.
Three families of switchable mirrors
So far we have only described the properties of
Yttrium and Lanthanum hydrides. It turns out that all
Rare-Earth (RE) hydrides exhibit an optical switchability.
Two other families of hydrogen-based switchable
mirrors have been discovered. We have thus
the following families of switchable mirrors:
- rare-earth switchable mirrors
- color neutral magnesium-rare-earth (Mg-RE)
switchable mirrors
- magnesium-transition-metal (Mg-TM) switchable
mirrors
An example of the reversible optical switching of a
Mg-Gd layer is shown in figure 2 and that of a Mg-Ti
layer in figure 3.
The Mg-RE and the Mg-TM switchable mirrors both
exhibit the three fundamental optical states of
matter, i.e. shiny metallic, transparent and highly
absorbing, when their hydrogen content is modified.
The microscopic mechanisms responsible for
their ‘black state’ are, however, due to their structure
on a nanoscale. There is by now a large body of
articles on the remarkable properties of switchable
metal-hydride mirrors. 5, 6 Recently it was also shown
that alloys and multilayered samples can be used to
fine-tune all the essential properties of switchable
mirrors and induce new phenomena. 7