Potassium under pressure: electronic origin of complex structures

Abstract: Recent high-pressure x-ray diffraction studies of alkali metals revealed unusual complex structures that follow the body-centered and face-centered cubic structures on compression. The structural sequence of potassium under compression to 1 Megabar is as follows: bcc - fcc - h-g (tI19*), hP4 - oP8 - tI4 - oC16. We consider configurations of Brillouin-Jones zones and the Fermi surface within a nearly-free-electron model in order to analyze the importance of these configurations for the crystal structure energy that contains two main contributions: electrostatic (Ewald) and electronic (band structure) energies. The latter can be lowered due to a formation of Brillouin zone planes close to the Fermi surface opening an energy gap at these planes. Under pressure, the band structure energy term becomes more important leading to a formation of complex low-symmetry structures. The stability of the post-fcc phases in K is attributed to the changes in the valence electron configuration implying the overlap of valence band with the upper core electrons. This effect offers an understanding of structural complexity of alkali metals under strong compression.