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) approximate to 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 +/- 1.3 GPa. The pressure coefficient of Delta T-N/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) approximate to 15 GPa within the AFM ordered phase with monoclinic symmetry. The monoclinic and triclinic lattice distortions increase with pressure up to gamma = 90.64( 1)degrees, much larger than the maximum value of 90.1 degrees, 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.

Automated summary

This study investigates the magnetic order and structural modulation in van der Waals layered FeOCl under different pressures. It reveals the relationship between the optimization of chlorine atom packing and the modulation of the orthorhombic symmetry. The research also highlights the importance of strong lattice distortion in understanding the properties of single-layer FeOCl materials.