Predicted Pressure-Induced $s$-Band Ferromagnetism in Alkali Metals

First-principles density-functional-theory calculations show that compression of alkali metals stabilizes open structures with localized interstitial electrons which may exhibit a Stoner-type instability towards ferromagnetism. We find ferromagnetic phases of the lithium-IV-type, simple cubic, and simple hexagonal structures in the heavier alkali metals, which may be described as $s$-band ferromagnets. We predict that the most stable phases of potassium at low temperatures and pressures around 20 GPa are ferromagnets.

Figure 1

Enthalpy-pressure curves for the FM and PM phases of K, Rb, and Cs. Differences in enthalpy from the fcc phase are plotted. FM phases are shown as solid lines and PM phases as dashed lines.

Figure 2

Electronic density of states, $g(E)$, of K-sc at 20 GPa. Data for the PM phase are shown in brown or dark gray and for the FM phase in light gray. The Fermi energy is shown as a vertical dotted line.

Figure 3

Spin density of FM K-sc (top) and FM K-sh (bottom) at 20 GPa. The isosurfaces are at 0.05 and 0.045 electrons per atom per ${\AA }^3$, respectively. The small spheres (purple) represent atoms.