Study on the thermodynamics of the gadolinium-hydrogen binary system (H/Gd = 0.0-2.0) and implications to metallic gadolinium purification

The thermodynamics of the gadolinium-hydrogen (Gd-H) binary system (H/Gd = 0.0-2.0) between 650 and 900 °C is studied by pressure composition isotherm measurement. Significant H dissolution in Gd is observed, up to H/Gd = 0.34 at 650 °C and 0.55 at 900 °C. The metal-rich phase boundary of nonstoichiometric gadolinium dihydride was found to occur with nominal composition of GdH1.80 at 650 °C and GdH1.53 at 900 °C. The results are in fairly good agreement with previous experimental work but with improved accuracy. The binary phase diagram is obtained using the CALPHAD method. The thermodynamic study here assists the understanding on the efficient deoxygenation effect by the H in Gd for Gd purification.