Isostructural phase transition and tunable electrical properties of La I 3 under compression

La I 3 has emerged as a promising scintillator due to its relatively small band gap. While some studies have explored various dopants to enhance its performance, there remains significant room for optimization, highlighting the importance of further understanding and tuning the intrinsic characteristics of La I 3 . In this work, the crystal structure and properties of La I 3 are extensively investigated under high pressure up to 50 GPa. The ambient layered - La I 3 is found to undergo an unexpected isostructural phase transition at ∼11 GPa, characterized by an anomalous change in the ratio of the a to b lattice parameters. This transition is further supported by Raman spectroscopy, electrical transport measurements, and first-principles calculations. Additionally, ultraviolet-visible absorption experiments indicate that the band gap of La I 3 gradually narrows and tends toward closure, eventually leading to a metallic state at 37 GPa. This work uncovers an interesting second-order phase transition among lanthanide halides, providing new insights into the fundamental behavior of La I 3 under high pressure.