Pressure-Driven Dimensional Transition and Arise of Unconventional Phases in SbCl₃

At ambient pressure, nitrogen group trihalides predominantly crystallize in molecular configurations. However, under elevated pressures, these compounds undergo pressure-induced structural transitions to two-dimensional (2D) van der Waals layered phases. Despite this, their structural evolution under higher pressure regimes remains poorly characterized, particularly regarding potential dimensional phase crossovers. Here, we present a combined experimental and theoretical study of antimony trichloride (SbCl₃) using high-pressure in situ Raman scattering, synchrotron X-ray diffraction, alternating current (AC) impedance, and first-principles calculations. We found that SbCl₃ undergoes a phase transition at 12.4 GPa from a molecular phase (<i>Pnma</i>) to an eight-coordinated layered phase (<i>P</i>2₁/<i>m</i>) at room temperature, which remains stable up to at least 28.9 GPa. Notably, after laser heating to 1900 K at 20.7 GPa, SbCl₃ transforms into a noncentrosymmetric three-dimensional (3D) structure (<i>I</i>4̅), featuring a nine-coordinated SbCl₉ structural unit. Furthermore, impedance measurements and first-principles calculations, including electronic band structure and electron localization function, were performed and analyzed. Our findings reveal a novel high-pressure phase of nitrogen group trihalides, isostructural to the high-coordinated geometry found in transition-metal phosphides.