Isostructural phase transition and tunable electrical properties of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>La</mml:mi> <mml:msub> <mml:mi mathvariant="normal">I</mml:mi> <mml:mn>3</mml:mn> </mml:msub> </mml:mrow>

<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi>La</a:mi> <a:msub> <a:mi mathvariant="normal">I</a:mi> <a:mn>3</a:mn> </a:msub> </a:mrow> </a:math> 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 <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mi>La</c:mi> <c:msub> <c:mi mathvariant="normal">I</c:mi> <c:mn>3</c:mn> </c:msub> </c:mrow> </c:math> . In this work, the crystal structure and properties of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mi>La</e:mi> <e:msub> <e:mi mathvariant="normal">I</e:mi> <e:mn>3</e:mn> </e:msub> </e:mrow> </e:math> are extensively investigated under high pressure up to 50 GPa. The ambient layered - <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:mi>La</g:mi> <g:msub> <g:mi mathvariant="normal">I</g:mi> <g:mn>3</g:mn> </g:msub> </g:mrow> </g:math> is found to undergo an unexpected isostructural phase transition at ∼11 GPa, characterized by an anomalous change in the ratio of the <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mi>a</i:mi> </i:math> to <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:mi>b</j:mi> </j:math> 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 <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"> <k:mrow> <k:mi>La</k:mi> <k:msub> <k:mi mathvariant="normal">I</k:mi> <k:mn>3</k:mn> </k:msub> </k:mrow> </k:math> 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 <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mi>La</m:mi> <m:msub> <m:mi mathvariant="normal">I</m:mi> <m:mn>3</m:mn> </m:msub> </m:mrow> </m:math> under high pressure.