Deuterium content and site occupancy in iron sulphide at high pressure and temperature determined using in situ neutron diffraction measurements

Abstract

We have performed in situ time‐of‐flight neutron diffraction experiments to examine the uptake of deuterium in iron monosulphide at pressures up to 11.4 GPa and temperatures to 1300 K. A D₂ fluid was formed in the experiments through the decomposition of ND₃BD₃, resulting in an oxygen fugacity of approximately 1.2 log units below the iron‐wüstite buffer. Deuterium positions and site occupancies were determined in FeS V, using Rietveld refinements of the powder neutron diffraction patterns. Our structural model indicates that two normally unoccupied sites in the P6₃/mmc FeS V structure, at Wyckoff positions 6h and 4f, are partially occupied by D atoms, with the latter being more dominant. The deuterium content D_x in FeSD_X increases with both pressure and temperature over the experimental conditions explored, from 0.126(14) at 2.3 GPa and 787 K to 1.20(16) at 9.7 GPa and 1300 K. The unit‐cell volume expansion per deuterium atom is 1.53 ± 0.16 ų at 6.9 GPa and 960 K, which is smaller than that determined for metallic iron phases at similar conditions. The variation in unit‐cell volume indicates that most deuterium is lost from FeS V upon temperature quenching at high‐pressures. By fitting the obtained FeS V deuterium site occupancies to a thermodynamic model, estimates for the hydrogen contents of iron monosulphide at conditions and oxygen fugacities consistent with the base of the cratonic lithosphere can be made. This results in values in the range of 1700–2700 ppm, which contribute to approximately 2–3 ppm hydrogen in the bulk mantle.