Rectangular Single‐Crystal Mullite Microtubes

A novel single‐crystalline microtubular structure of Al‐rich mullite has been synthesized via a simple solid–vapor process. The microtubes have perfect rectangularly framed cross‐sections (see Figure and cover) as defined by the ± (110) and ± (1 $\bar 1$ 0) facets. The microtubes have a [001] growth axis, a dislocation‐free volume, a large, completely hollow interior channel 0.5–3 μm in width, and thin walls of 50–100 nm.