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=6.30.16em0exGPa and 100≤T≤8000.16em0ex^∘C based on the results of thermal desorption analysis of FeDₓ samples produced by quenching under high D₂ pressure to the temperature of liquid nitrogen. The experiment confirms the value of $x=0.64$ at T=7150.16em0ex^∘C proposed previously in a neutron diffraction work [Machida et al., Nature Commun. 5, 5063 (2014)] for {γ} iron deuteride under the assumption that deuterium atoms occupy both octa- and tetrahedral interstices in its fcc metal lattice. An estimate of ΔV/x=2.20.16em0ex³/atom D made in that work for the deuterium-induced volume expansion ΔV(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=6000.16em0ex^∘C and H₂ pressures from 4.3 to 7.4 GPa. The available ΔV(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 ΔV/x=2.20.16em0ex³/atom H. The transformation between the high-temperature fcc ({γ}) and low-temperature dhcp ({ε}{'}) deuterides of iron is shown to occur at 260 ^∘{}C, which is approximately 100 ^∘{}C lower than the temperature of the {γ} {↔} {ε}{'} transformation in the Fe-H system at the same pressure of 6.3 GPa.