Solubility of deuterium and hydrogen in fcc iron at high pressures and temperatures

An isobar x(T) of deuterium solubility in iron is constructed at P = 6.3GPa and 100 <= T <= 800 degrees C based on the results of thermal desorption analysis of FeDx samples produced by quenching under high D-2 pressure to the temperature of liquid nitrogen. The experiment confirms the value of x = 0.64 at T = 715 degrees C proposed previously in a neutron diffraction work [Machida et al., Nature Commun. 5, 5063 (2014)] for gamma iron deuteride under the assumption that deuterium atoms occupy both octa- and tetrahedral interstices in its fcc metal lattice. An estimate of Delta V/x = 2.2 angstrom(3) /atom D made in that work for the deuterium-induced volume expansion Delta V(x) of fcc iron is also confirmed. To prove that the absorption of protium leads to a similar volume expansion, we constructed an isotherm x(P) of hydrogen solubility in fcc iron at T = 600 degrees C and H-2 pressures from 4.3 to 7.4 GPa. The available Delta V (P, T) data of in situ x-ray diffraction studies of iron hydrides [T. Hiroi et al., J. Alloys Compd. 404-406, 252 (2005); H. Saitoh et al., J. Alloys Compd. 706, 520 (2017)] agree with this isotherm under the assumption that Delta V/x = 2.2 angstrom(3) /atom H. The transformation between the high-temperature fcc (gamma) and low-temperature dhcp (epsilon') deuterides of iron is shown to occur at 260 degrees C, which is approximately 100 degrees C lower than the temperature of the gamma <-> epsilon' transformation in the Fe-H system at the same pressure of 6.3 GPa.