Revealing pressure-driven structural transitions in the hybrid improper ferroelectric Sr3 Sn2 O7

We combine diamond anvil cell techniques and synchrotron-based infrared spectroscopy with a detailed symmetry analysis and lattice dynamics calculations to uncover a series of pressure-induced structural phase transitions in the hybrid improper ferroelectric Sr3Sn2O7. The microscopic character of each high-pressure phase is determined by comparing the measured spectrum with the predicted vibrational patterns of several related but distinct candidate space groups. Our analysis reveals a sequence of pressure-induced transitions from A21am↔Pnab↔Acaa↔I4/mmm at room temperature. Remarkably, this space group progression matches the sequence of temperature-dependent structural transitions observed in Sr3Sn2O7 between 77 and 1000 K. Other hybrid improper ferroelectrics display a similar set of temperature transitions, suggesting that pressure and probably strain will be very effective tuning parameters for this entire class of materials.