Transformation of Hexagonal Lu to Cubic LuH_2+x Single-Crystalline Films

With the recent report of near ambient superconductivity at room temperature in the N-doped lutetium hydride (Lu–H–N) system, the understanding of cubic Lu–H compounds has attracted worldwide attention. Generally, compared to polycrystals with non-negligible impurities, the single-crystalline form of materials with high purity can provide an opportunity to show their hidden properties. However, the experimental synthesis of single-crystalline cubic Lu–H compounds has not been reported so far. Here, we develop an easy way to synthesize highly pure LuH_2+x single-crystalline films by the post-annealing of Lu single-crystalline films (purity of 99.99%) in H₂ atmosphere. The crystal and electronic structures of films were characterized by x-ray diffraction, Raman spectroscopy, and electrical transport. Interestingly, Lu films are silver-white and metallic, whereas their transformed LuH_2+x films become purple-red and insulating, indicating the possible formation of an unreported electronic state of Lu–H compounds. Our work provides a novel route to synthesize and explore more single-crystalline Lu–H compounds.