P–V–T relation of the Fe–H system under hydrogen pressure of several gigapascals

Abstract We have measured the volumes of dhcp- and fcc-FeH x at temperatures of 300–950 K and hydrogen pressures of 4–7 GPa using in situ X-ray diffraction. For fcc-FeH x , the expanded volumes per Fe atom with respect to fcc-Fe metal were converted to hydrogen compositions x using hydrogen-induced volume expansion v H . A miscibility gap in the x – T diagram of fcc-FeH x was confirmed to be in agreement with an earlier study, and the critical pressure was located in a rather narrow span of 4.0–4.5 GPa with a critical composition of x  = ∼0.4. We examined Arrhenius plot analysis of the x – T relations to derive the enthalpy of solution of hydrogen ΔH s . However, an irregular behavior of the x – T relations did not allow derivation of a reliable value of ΔH s .