Pressure Triggers Structural Phase Transitions in Chiral ErFe ₃ (BO ₃ ) ₄

We combined diamond anvil cell techniques with Raman scattering and near-infrared spectroscopies plus lattice dynamics calculations and a symmetry analysis to reveal the properties of ErFe3(BO4)3 under compression. Overall, the system is surprisingly soft, with a series of structural phase transitions (but no metallicity) up to 20 GPa. Bringing frequency vs pressure trends together with a group–subgroup analysis and examination of the energy landscape, we uncover a structural phase transition from P3121 to either P321 or R32 across the 3.8 GPa transition. The BO33– units are robust, whereas the Fe helix is soft and flexible. Analysis of the crystal field excitations reveals that the local symmetry of Er3+ remains trigonal prismatic until after 8.6 GPa with significantly lower symmetries realized at higher pressures. Evaluation of spin–phonon coupling across the 40 K magnetic ordering transition and spectral modifications through the 450 K structural phase transition place this material on a firm foundation, paving the way for a combination of external stimuli including magnetic field, pressure, and strain.