Formation of microtubes in CdI$_2$ crystals doped with BiI$_3$

The formation of microtubes (MTs) with rectangular cross section in the volume of CdI$_2$ layered crystals doped with Bi$^{3+}$ trivalent impurity ions has been observed for the first time by scanning electron microscopy. The MTs grow predominantly in pores that appear in the volume of CdI$_2$–BiI$_3$ crystals as a result of a difference in the valence of host Cd$^{2+}$ ions and guest Bi$^{3+}$ ions substituted into regular crystal lattice sites. The formation of MT structures is related to the rolling of I–Cd–I trilayers containing impurity ions, which takes place as a result of the presence of uncompensated charges at the edges. Deviation of the MT cross section from circular is explained by nonuniformity of the elastic properties of I–Cd–I trilayers with Bi impurity, which is related to the appearance of vacancies in the cation subsystem upon replacement of Cd$^{2+}$ ions by Bi$^{3+}$ ions.