Pressure-Induced Three- to Two-Dimensional Structural Transition in Light Lanthanide Trichlorides

Rare-earth chlorides exhibit three polymorphs at ambient pressure, among which the UCl₃-type three-dimensional (3D) framework with 9-fold Ln coordination is the dominant structural motif for the light lanthanides. Here, we report the high-pressure synthesis and structural characterization of LnCl₃ (Ln = La, Ce, Pr, Nd, Gd, and Y) obtained at 5 GPa and 1000 °C. All high-pressure polymorphs adopt the two-dimensional (2D) NdBr₃-type structure (Cmcm) built from LnCl₈ polyhedra. For YCl₃, the ambient-pressure AlCl₃-type phase (CN = 6) transforms into the NdBr₃-type structure (CN = 8) under compression. In contrast, La-Gd trichlorides undergo an unusual reduction from CN = 9 to 8. This counterintuitive behavior is rationalized by pressure-induced Ln-Cl bond shortening, which maintains reasonable bond-valence sums, together with enhanced packing density arising from pronounced out-of-plane contraction, as supported by density functional theory (DFT) calculations. These results demonstrate that high pressure can stabilize recoverable 2D polymorphs, expanding the compositional space of NdBr₃-type layered structures and offering opportunities for the exploration of functional van der Waals-type materials.