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detail and reports data on spectral coloration efficiency (the change in optical
density per amount of charge exchange) for a few compositions (Green et al.
2012
). The favorable properties of a small Ni addition to W oxide stand out again.
Another study showed that the bleached-state transmittance of Ni oxide could be
improved by several additives to Ni oxide (Avendaño et al.
2004
).
2.2 Nanostructures are Crucial for Electrochromic
Thin Films
Nanostructural features are essential for electrochromic devices, as discussed next.
The majority of the electrochromic oxides comprise octahedral units in various
arrangements (Granqvist
1995
), and the spaces between these units are large
enough to permit some transport of small ions. Clusters of the octahedra can form
disordered and more or less loosely packed aggregates with significant porosity.
Hence, nanostructure enters at two or more length scales. The discussion to follow
is focused on W oxide, which has been studied in considerable detail. Its generality
might be questioned for the anodic electrochromic oxides, though, and film
porosity and processes at grain boundaries may then be essential, but the situation
is not yet clear.
Figure
5
shows nanostructural features of W oxide and delineates WO
6
octa-
hedra comprising six oxygen atoms and a centrally located tungsten atom. Stoi-
chiometry corresponds to a state wherein each octahedron shares corners with
neighboring octahedra so that WO
3
is formed. Substoichimetry is easily formed in
so-called Magnéli phases (Magnéli
1950
) with the presence of some edge-sharing
octahedra. The three-dimensional structure formed by the octahedra leads to a
three-dimensional network of ''tunnels,'' which are wide enough to act as conduits
for ion transport.
The crystalline structure above is somewhat simplified, though, since it refers to
a cubic structure, and a tetragonal structure is more appropriate for WO
3
at normal
temperature and pressure. This is, in fact, favorable for ion transport since the
Fig. 5 Schematic image of
corner-sharing and
edge-sharing octahedra in
slightly substoichiometric
crystalline W oxide. From
Granqvist (
1995
)
