Composite incommensurate K-III and a commensurate form: Study of a high-pressure phase of potassium

K-III, the high-pressure phase of potassium stable above 200.3em0exGPa, is reported to have a host-guest composite structure comprising the same 16-atom host structure as Rb-IV and a C-face-centered, rather than a body-centered, tetragonal, guest structure. At 22.0(2)0.3em0exGPa, the incommensurate ratio of the host and guest c-axis lattice parameters passes through the commensurate value of $8∕5$ but, within the experimental resolution of approximately 10.3em0exGPa, no lock-in to a commensurate structure is observed.