Ordered Superstructure of InSb-IV

The crystal structure of InSb phase IV has been determined at 5 GPa using angle-dispersive powder-diffraction techniques. The proposed solution is a site-ordered orthorhombic superstructure, spacegroup $\mathrm{Cmcm}$, composed of six NaCl-like planes stacked along the $c$ axis and alternately displaced approximately $\pm 0.1b$. Two of the planes are flat, and the others are slightly puckered due to a small relative displacement of the In and Sb atoms along the $c$ axis. The results add a remarkably complex example to growing evidence that (generally much simpler) $\mathrm{Cmcm}$-type structures are common to most of the more ionic II-VI and III-V semiconductors under pressure.