Thermal equation of state of iron and Fe0.91Si0.09

We have carried out an in situ synchrotron X-ray diffraction study on iron and an iron-silicon alloy Fe0.91Si0.09 at simultaneously high pressure and temperature. Unit-cell volumes, measured up to 8.9 GPa and 773 K on the bcc phases of iron and Fe0.91Si0.09, are analyzed using the Birch-Murnaghan eq uation of state and thermal pressure approach of Anderson. Equation of stat e parameters on iron are found to be in agreement with results of previous studies. For both iron and Fe0.91Si0.09, thermal pressures show strong depe ndence on volume; the (partial derivative K-T/partial derivative T)(V) valu es are considerably larger than those previously reported for other solids. The present results, in combination with our previous results on epsilon-F eSi, suggest a small dependency of the room-temperature bulk modulus upon t he silicon content, less than 0.3 GPa for 1 wt.% silicon. We also find that substitution of silicon in iron would not appreciably change the thermoela stic properties of iron-rich Fe-Si alloys. If this behavior persists over l arge pressure and temperature ranges, the relative density contrast between iron and iron-rich Fe-Si alloys at conditions of the outer core of the Ear th could be close to that measured at ambient conditions, i.e., 0.6% for 1 wt.% Si.