Transformation of Hexagonal Lu to Cubic LuH2+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 LuH2+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 LuH2+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.