In situ synchrotron radiation X-ray diffraction measurements of Fe–Mo alloy hydrides formed under high pressure and high temperature

•Hydrogenation reaction processes of Fe–Mo at 6 GPa and 750 °C were investigated.•Two kinds of novel hydrides were synthesized.•The hydrogenation reaction process was classified into three types. [Display omitted] Conventional hydrides obtained thus far contain at least one type of metal with high hydrogen affinity, which can form metal hydrides near ambient pressure. In contrast, we synthesized metal hydrides comprising only metals with low hydrogenation affinity which are metals that do not form metal hydrides MHx (M stands for metal element, x > 0.5) below 1 GPa. This is because such hydrides are rare and would demonstrate novel properties and functionalities. To provide a guideline for synthesizing such hydrides, we clarified the hydrogenation reaction processes of FexMo1−x alloys at 6 GPa and 750 °C over a wide composition range of 0.41 ≤ x ≤ 0.88, where both Fe and Mo demonstrate low hydrogen affinities. Depending on the alloy composition, the hydrogenation reaction process can be classified into three types. Two kinds of novel hydrides were synthesized herein. First, FexMo1−xH (0.50 ≤ x ≤ 0.74) with metal elements at the vertices of hexagonal close-packed (hcp) lattices, and second, Fe0.88Mo0.12H with a complex crystal structure, which is likely to be a long-period close-packed structure.