Potassium under pressure: Electronic origin of complex structures

Recent high-pressure X-ray diffraction studies of alkali metals revealed unusual complex structures that follow the body-centred and face-centred cubic structures on compression. The structural sequence of potassium under compression to I Mbar 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 stability. Formation of Brillouin zone planes close to the Fermi surface is related to opening an energy gap at these planes and reduction of crystal energy. Under pressure, this mechanism becomes more important leading to appearance of complex low-symmetry structures. The stability of the post-fcc phases in K is attributed to the changes in the valence electron configuration under strong compression. (C) 2014 Published by Elsevier Masson SAS.