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 ErFe 3 (BO 4 ) 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 P 3 1 21 to either P 321 or R 32 across the 3.8 GPa transition. The BO 3 3– units are robust, whereas the Fe helix is soft and flexible. Analysis of the crystal field excitations reveals that the local symmetry of Er 3+ 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.