[SIT26-P09] In situ X-ray and neutron diffraction studies of hcp iron hydride
Keywords:iron hydride, hexagonal closed packing phase, X-ray, neutron
The phase diagram changes abruptly when pure metallic iron is hydrogenated. In the case of pure metallic iron, fcc (face-centered cubic) or hcp (hexagonal closed packing) phase can be present under deep Earth conditions. On the other hand, fcc or dhcp (double hexagonal close packing) phase appear in the case of iron hydride. The hcp phase of iron hydride is a metastable phase. Conventional phase diagram of iron hydride has been determined only in experiments under hydrogen saturated conditions. However, our recent studies have revealed that hcp iron hydride is stable under low-concentration hydrogen condition [e.g., Machida et al., 2019]. Therefore, experiments under low-concentration hydrogen condition are important to discuss iron hydride under the deep Earth.
We conducted in-situ neutron diffraction experiments under high pressure and high temperature at J-PARC MLF BL11 to determine hydrogen-induced volume expansion coefficient of hcp-FeDx. Deuterated Ammonia borane (ND3BD3) was used as hydrogen source This material decomposed to D2 and BN at high pressure and high temperature conditions. The experimental conditions were 10-12 GPa, 300-900 K. Hydrogen content was controlled to D/Fe = 0.45. In addition, we conducted in-situ X-ray diffraction experiment under high pressure and high temperature at SPring-8 BL04B1 and BL14B1 to determine P-T-x phase diagram, especially maximum hydrogen content of hcp-FeHx at a high pressure and high temperature condition. The experimental conditions were 3-16 GPa, 300-1100 K. Hydrogen content was controlled to D/Fe =0.5-0.8.
In the in-situ X-ray experiments, we observed coexistence of hcp and fcc-FeHx. The hydrogen content is x = 0.4 and 0.7 for hcp and fcc-FeHx at 15 GPa and 700 K, respectively. There is an immiscibility gap between hcp and fcc-FeHx. In this presentation, we will discuss hcp iron hydride based on the results of neutron diffraction experiments.