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 $p_c$≈15 GPa, while orthorhombic symmetry is retained. This modulation is related to an optimization of the

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... ing 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±1.3 GPa. The pressure coefficient of $ΔT_N/Δp=2$.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 $p_c$≈15 GPa within the AFM ordered phase with monoclinic symmetry. The monoclinic and triclinic lattice distortions increase with pressure up to $γ$=90.64(1)∘, much larger than the maximum value of 90.1∘, 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−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.