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

Abstract

K-III, the high-pressure phase of potassium stable above $20\mathrm{GPa}$, 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 $22.0\left(2\right)\mathrm{GPa}$, 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 $1\mathrm{GPa}$, no lock-in to a commensurate structure is observed.