Equation of state of K+Fe at high pressure: K in the Earth's core?

X-ray diffraction to 60 GPa of potassium heated with iron reveals a 2-3% inflation of the hexagonal close-packed unit-cell volume of Fe, suggesting that K dissolves into the ɛ-Fe phase at pressures above ~25 GPa. The equation of state of this K-Fe phase is well described by the Birch-Murnaghan formalism [Birch, 1978], with a zero-pressure isothermal bulk modulus slightly higher than that of ɛ-Fe: K_0T = 240 (+/-10) GPa, assuming a pressure derivative K_0T' = 4. Our results are compatible with both theoretical [e.g., Bukowinski, 1979] and experimental work [Parker et al., 1996] indicating that K transforms from alkali- to transition-metal character under pressure. The implication is that potassium can indeed be incorporated into the core early during Earth history, and thus provide an additional source of heat for the geodynamo and mantle dynamics.