Tungsten and oxygen
While still maintaining that every octahedron shares
every corner with another, other arrays than the chess-board-like arrangement
are also possible. Low-temperature dehydration of WO3·1/3H2O yields a
metastable oxide (hexagonal WO3, or h-WO3), with a structure as shown in following
figure.
Figure the hexagonal framework of octahedra in h-WO3.
Hexagonal WO3 transforms to monoclinic WO3 above 400℃.
Another metastable modification is the pyrochlore WO3, modification is the
pyrochlore WO3, or p-WO3. It can be prepared from ammonium tungstate by removal
of NH4 ions via acid washing. The flexibility of the WO6 corner-linked
octahedral framework allows a cubic pyrochlore structure, too (the pyrochlore structure
will be presented under tungsten bronzes). The material is white and transforms
to monoclinic WO3 above 400℃.
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