Rubidium-IV: A High Pressure Phase with Complex Crystal Structure

The crystal structure of the high-pressure phase rubidium-IV was investigated using synchrotron x-ray diffraction. Full profile refinements of angle-dispersive powder diffraction data resulted in a solution with the space group $I4\mathrm{}/\mathrm{mcm}$. The structure is made up of columns of face-sharing square antiprisms formed by one subset of Rb atoms (16 per unit cell). Quasi-one-dimensional channels in between these columns are occupied by a second subset of Rb atoms. These results demonstrate that a quite complex crystal structure is adopted by a heavy alkali metal during the progression of the pressure-driven electronic $s\to d$ transition. The structure of Rb-IV exhibits a close resemblance to the metal sublattice of the ${\mathrm{W}}_5{\mathrm{Si}}_3$-type structure.