High-Pressure oC 16-YBr 3 Polymorph Recoverable to Ambient Conditions: From 3D Framework to Layered Material.

Exfoliation of graphite and the discovery of the unique properties of graphene─graphite's single layer─have raised significant attention to layered compounds as potential precursors to 2D materials with applications in optoelectronics, spintronics, sensors, and solar cells. In this work, a new orthorhombic polymorph of yttrium bromide, oC 16-YBr 3 was synthesized from yttrium and CBr 4 in a laser-heated diamond anvil cell at 45 GPa and 3000 K. The structure of oC 16-YBr 3 was solved and refined using in situ synchrotron single-crystal X-ray diffraction. At high pressure, it can be described as a 3D framework of YBr 9 polyhedra, but upon decompression below 15 GPa, the structure motif changes to layered, with layers comprising edge-sharing YBr 8 polyhedra weakly bonded by van der Waals interactions. The layered oC 16-YBr 3 material can be recovered to ambient conditions, and according to Perdew-Burke-Ernzerhof-density functional theory calculations, it exhibits semiconductor properties with a band gap that is highly sensitive to pressure. This polymorph possesses a low exfoliation energy of 0.30 J/m 2 . Our results expand the list of layered trivalent rare-earth metal halides and provide insights into how high pressure alters their structural motifs and physical properties.