Observation of the oP8 crystal structure in potassium at high pressure

Ab initio electronic structure calculations on potassium at high pressure reveal that the orthorhombic oP8 structure, found recently in sodium, has a lower enthalpy in potassium than do other candidate crystal structures recently reported to be stable at high pressure [Y. Ma, A. R. Oganov, and Y. Xie, Phys. Rev. B 78, 014102 (2008)]. Subsequent powder-diffraction studies to above 100 GPa confirm oP8 to be the stable phase of potassium from 54(2) to 90(2) GPa, where it transforms to the tetragonal tI4 structure, which in turn transforms to the orthorhombic oC16 structure at 96(3) GPa.