Ambient Stabilization of Metastable Face-Centered Cubic Lanthanide Trihydrides

Achieving ambient stability of hydrogen-rich materials synthesized under extreme conditions remains a major barrier to unlocking their functional potential. Here, we report a general high-pressure-high-temperature (HPHT) strategy using large volume press (LVP) to stabilize metastable face-centered cubic (fcc) phases of lanthanide trihydrides (LnH₃, Ln = Sm, Gd, Tb, Dy, Ho, Er, and Tm) at ambient conditions. These fcc phases, which normally revert to trigonal structures upon decompression, are retained through controlled hydrogenation of LnH₂ precursors in the presence of ammonia borane. Structural characterization confirms the successful recovery of seven fcc-LnH₃ compounds, and systematic trends in lattice parameters and H-induced volume expansion are established across the lanthanide series. First-principles calculations reveal that these fcc phases are thermodynamically metastable, lying up to 70 meV/atom above the ground state, but dynamically stabilized at room temperature via strong anharmonic lattice vibrations. All fcc-LnH₃ compounds are wide-bandgap semiconductors with paramagnetic behavior, in sharp contrast to the metallic superconductivity of clathrate superhydrides. Our findings deliver a broadly applicable synthesis route and expand the functionality of hydrogen-rich materials beyond superconductivity, setting a new benchmark for exploring their diverse electronic and magnetic states at ambient conditions.