Hydrogenation of silicon-bearing hexagonal close-packed iron and its implications for density deficits in the inner core

To investigate silicon effects on the hydrogen-induced volume expansion of iron, neutron diffraction and X-ray diffraction experiments were conducted to examine hcp-Fe0.95Si0.05 under high pressures and high temperatures. Neutron diffraction experiments were performed on the deuterated hcp-Fe0.95Si0.05 at 13.5 GPa and 900 K, and at 12.1 GPa and 300 K. The hydrogen occupancy was determined from the obtained profiles using Rietveld analysis. By combining the P–V–T equation of state of hcp-Fe0.95Si0.05, the present results indicate that the hydrogen-induced volume expansion of hcp-Fe0.95Si0.05 is 10 % greater than that of pure hcp iron. Using the obtained values, we estimated the hydrogen content that would reproduce the density deficit in the inner core, which was 50 % less than that without the effect of silicon. Possible hydrogen content, x, in the inner core and the outer core was calculated to be 0.07 and 0.12–0.15, respectively when reproducing the density deficit of the inner core with hcp-Fe0.95Si0.05Hx.