Three-Tiered domain structure of Tungsten Trioxide: 

This is a backscatter electron image of an area within a single grain of a polished Tungsten Trioxide polycrystal. The third tier of domains, which might explain the anomalies in photoreactivity on the surface, is not visible in all grains, presumably due to channeling conditions. A high scan time of 5 minutes with a small spot size was necessary to bring out the third tier. The scale bar is 1 micron.