  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.
