Rubidium-IV: A high pressure phase with complex crystal structure

The crystal structure of the high-pressure phase rubidium-IV was investigat ed using synchrotron x-ray diffraction. Full profile refinements of angle-d ispersive powder diffraction data resulted in a solution with the space gro up I4/mcm. The structure is made up of columns of face-sharing square antip risms formed by one subset of Rb atoms (16 per unit cell). Quasi-one-dimens ional 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-d riven electronic s --> d transition. The structure of Rb-IV exhibits a clos e resemblance to the metal sublattice of the W5Si3-type structure.