Pressure-induced thermal expansion anomalies in dhcp iron hydride associated with magnetoelastic coupling

Iron hydride with a double hexagonal close-packed structure (dhcp-FeH${}_x$) undergoes a ferromagnetic-paramagnetic transition without changing its crystal structure. Despite its relevance to metal-hydrogen interactions and magnetically driven elasticity, the extensive investigation of this phase is almost limited to room temperature. Here, we performed XRD measurements at high pressure and high temperature, identifying the singularity in the temperature-volume relationship as the Curie temperature ($T_{\text{C}}$). Pressurization lowered the $T_{\text{C}}$ of dhcp-FeH${}_x$, and pronounced volume anomalies, indicating that pressure enhanced magnetoelastic coupling. Density functional theory combined with dynamical mean-field theory (DFT+DMFT) reproduced the spontaneous magnetization and its negative pressure dependence of $T_{\text{C}}$, consistent with our experimental results. This establishes a methodology for determining magnetic transition temperatures and magnetoelastic coupling effects, and highlights dhcp-FeH${}_x$ as a unique model system for providing new insights into itinerant-electron magnetism.