Pressurizing the van der Waals magnet FeOCl at low temperatures: Phase transitions and structural evolution

Magnetic order is frustrated on the orthorhombic lattice of van der Waals layered FeOCl. Antiferromagnetic (AFM) order is attained at ambient pressure upon cooling through ${T}_{N}=81$ K, due to an accompanying monoclinic lattice distortion lifting the magnetic frustration. Within the paramagnetic state at 293 K, an incommensurate structural modulation appears above a critical pressure of ${p}_{c}\ensuremath{\approx}15$ GPa, while orthorhombic symmetry is retained. This modulation is related to an optimization of the packing of chlorine atoms within the van der Waals gap. Here, we report four new phases in the pressure-temperature $(p,T)$ phase diagram of FeOCl below room temperature. High-pressure--low-temperature single-crystal x-ray diffraction (SXRD) up to 37.8 GPa reveals that, at 100 K, the AFM transition occurs at $p=7.3\ifmmode\pm\else\textpm\fi{}1.3$ GPa. The pressure coefficient of $\mathrm{\ensuremath{\Delta}}{T}_{N}/\mathrm{\ensuremath{\Delta}}p=2.13$ K/GPa explains that FeOCl remains paramagnetic up to the highest measured pressure of 33.3 GPa at 293 K. At 6 and 100 K, the structural modulation appears around ${p}_{c}\ensuremath{\approx}15$ GPa within the AFM ordered phase with monoclinic symmetry. The monoclinic and triclinic lattice distortions increase with pressure up to $\ensuremath{\gamma}=90.64{(1)}^{\ensuremath{\circ}}$, much larger than the maximum value of $90.{1}^{\ensuremath{\circ}}$, that can be reached upon cooling at ambient pressure. The structural evolution provides the geometrical basis for the increase with the pressure of direct $3d\ensuremath{-}3d$ exchange and superexchange interactions. It is proposed, that a strong monoclinic lattice distortion may be of importance for understanding the properties of single-layer FeOCl materials.