Yttrium metal is quite a
good electrical conductor. A thin layer (500 nm) of it deposited on a glass substrate
reflects light, it forms a mirror. 
However, yttrium oxidizes easily. This
can be prevented by coating the yttrium with a thin layer of palladium. This layer can be
so thin (20 nm) that it is optically transparent. In 1996 the following interesting
observation was made in the Free University in Amsterdam. An yttrium film prepared in
the above manner was exposed to hydrogen gas. Yttrium metal spontaneously absorbs two
atoms of hydrogren for each metal ion to form a dihydride which is an even better metal
than yttrium itself and is an even better mirror. In the figure on the left hand side the
knight is reflected very well; the chess-board pattern behind the mirror cannot be seen
through the mirror. 
When the yttrium film absorbs sufficient hydrogen,
about 2.85 hydrogen atoms per yttrium ion, the thin film becomes transparent; in the
figure on the right we can see the chess board pattern through the yttrium film and the
reflection of the knight has become much weaker. Apparently the yttrium trihydride phase
is transparent.
What makes this example of a "Metal-Insulator Transition" special is (i) that it is reversible; on pumping off the hydrogen the transparent state becomes opaque and (ii) it occurs at room temperature and is visible to the naked eye. This offers the prospect of a number of practical applications.
Metal-insulator transitions play a particularly important role in condensed matter physics. They offer us the opportunity to study a transformation between two fundamentally different phases of matter occurs as some external parameter is changed. The occurance of a MI transition in the yttrium hydride system posed a particular problem as state-of-the-art density functional theory calculations predicted that the trihydride phase should be metallic.
Our research centres on understanding why yttrium trihydride is an insulator by performing quantitative investigations of various mechanisms proposed to explain the experimental observations such as:
- Strong electron-phonon coupling
- Strong electron correlations