High-pressure behavior of FeOCl

A pressure-induced phase transition of FeOCl is discovered to occur at ${P}_{c}=15\ifmmode\pm\else\textpm\fi{}1$ GPa. It is preceded by extremely anisotropic lattice compression, which is explained by a gradual collapse of the van der Waals gap between the chlorine atoms on the borders of the slabs of this layered compound. Single-crystal x-ray diffraction in a diamond anvil cell is used to show that the high-pressure phase can be described as a fourfold superstructure with monoclinic lattice distortion, described by a $4a\ifmmode\times\else\texttimes\fi{}1b\ifmmode\times\else\texttimes\fi{}2c$ supercell with space group $B{2}_{1}/m$ ($b$ unique). The high-pressure crystal structures have been used to uncover the mechanism of the phase transition, as the formation of alternating regions of increased packing density of chlorine atoms within a single layer and regions of interpenetrating layers. Raman and M\"ossbauer spectroscopies indicate that the phase transition is not related to electronic effects or magnetic order.