Chemically Defined <i>Ln</i> ₂ H _{3– <i>x</i>} N Nitride-Hydrides via High-Pressure Synthesis and Topochemical Hydrogen Intercalation

The controversial claim of near-ambient superconductivity in anion-excess fluorite-type nitrogen-doped lutetium hydride (LuH₃_-δN_ε) has renewed interest in mixed-anion hydrides as platforms for emergent electronic properties, while highlighting the need for systems with well-defined structural and compositional control. Here, we report the high-pressure synthesis of lanthanum nitride-hydride La₂H_3-<i>x</i>N at 5 GPa. The compound crystallizes in a layered anion-ordered Ba₂H₃Cl-type (<i>P</i>-3<i>m</i>1) structure derived from anion-excess fluorite-type LaH₃ via a 3:1 substitution of H^- by N^3-, accompanied by a reconstruction of the close-packed La sublattice. This pressure-stabilized phase can be accessed only under hydrogen-poor conditions, resulting in intrinsic hydrogen vacancies (∼23%), corresponding to La₂H_2.3N. These hydride vacancies can be further filled by a subsequent topochemical H^- intercalation under H₂ gas pressure after releasing the mechanical pressure, thereby driving a metal-semiconductor transition. This combination of high-pressure synthesis and post-synthetic topochemical intercalation provides a powerful yet underexplored route to access pressure-stabilized mixed-anion frameworks with precisely tunable anion stoichiometry. In addition, Nd and Gd analogues were synthesized at 8 GPa, and density functional theory calculations indicate that chemical pressure induced by lanthanide substitution systematically tunes the band structures from semiconducting to semimetallic regimes.