Equation of state of the high-pressure polymorph of FeSi to 67 GPa

The compressibility of high-pressure phase FeSi was investigated by in situ high-pressure X-ray powder diffraction. Pressure up to 67 GPa was generated using the diamond anvil cell technique. FeSi transformed to a B2-type structure during laser heating, and remained stable up to the maximum pressure. This first-order phase transformation showed a volume reduction of 4 % at 25 GPa. The fit of a Birch-Murnaghan equation-of-state to the pressure-volume data resulted in V-0 = 21.32 (3) angstrom(3) and K-0 = 225 (2) GPa when K-0' was fixed at 4. Our results are in good agreement with those of previous numerical studies using a GGA method employing ab initio calculations. This indicates that a small amount of B2-type FeSi phase may contribute to the decrease in seismic velocity at the base of the lower mantle.