Pressure-dependent distortions in quasi-two-dimensional magnetic CrOCl at low temperatures

The layered van der Waals compound CrOCl is frustrated for antiferromagnetic (AFM) order, because of its orthorhombic $Pmmn$ symmetry that supports a triangular lattice of magnetic ${\mathrm{Cr}}^{3+}$ ions. At ambient pressure, CrOCl develops incommensurate AFM order below ${T}_{c}=27.2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and it becomes AFM ordered with a fourfold magnetic supercell below the N\'eel temperature ${T}_{N,1\mathrm{bar}}=13.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This AFM order is facilitated by an $\mathbf{a}$-unique monoclinic lattice distortion. At room temperature, CrOCl remains orthorhombic up to at least 57 GPa. It develops a structural modulation that is the result of the optimization of the packing of Cl atoms within the van der Waals gap. Single-crystal x-ray diffraction (SXRD) at conditions of high pressure and low temperature (HPLT) uncover three novel HPLT phases of CrOCl. Instead of an $\mathbf{a}$-unique monoclinic lattice, major distortions are found for the lattice parameter angles $\ensuremath{\beta}$ and $\ensuremath{\gamma}$, resulting in a $\mathbf{c}$-unique HPLT phase at 35 K, a triclinic HPLT phase at both 14 and 35 K, and a structurally modulated, triclinic phase at yet higher pressures. The structurally modulated phases appear at nearly the same critical pressures of $15.0<{p}_{c}<16.0\phantom{\rule{4pt}{0ex}}\mathrm{GPa}$, and they are of a similar nature at 14, 35, and 293 K, despite the different lattice symmetries. The unmodulated triclinic phase is proposed to lead to more favorable distortions within the double layers CrO than is possible within the high-symmetry structures at equivalent $(p,T)$ conditions. Incommensurate magnetic order or commensurate AFM order may appear with the $\mathbf{c}$-unique HPLT phase at 35 K. This then implies a substantial pressure dependence of the magnetic transitions of $\mathrm{\ensuremath{\Delta}}{T}_{c}/\mathrm{\ensuremath{\Delta}}p=1.42$ K/GPa or $\mathrm{\ensuremath{\Delta}}{T}_{N}/\mathrm{\ensuremath{\Delta}}p=4.1$ K/GPa, respectively. The present finding of substantial lattice distortions involving the angle $\ensuremath{\gamma}$ may have ramifications for monolayer structures of CrOCl.