Tuning Octahedral Tilting of Ruddlesden-Popper Chalcogenide via Pressure

Transition metal perovskite chalcogenides (TMPCs) are currently of substantial interest as a class of emerging semiconductors for optoelectronic and photonic applications. Ruddlesden-Popper (RP) phases of perovskite chalcogenides have been predicted to be promising candidates to achieve ferroelectric semiconductors with static polar order. Hybrid improper ferroelectricity can be realized in such n=2 RP compounds with a⁻a⁻c⁺ tilt system as Ba₃Zr₂S₇ via coupling of out-of-phase octahedral tilting around in-plane directions and in-phase rotation mode around the stacking direction. However, experimentally obtained Ba₃Zr₂S₇ is in a higher symmetry centrosymmetric phase with only one octahedral tilting mode. Pressure offers a unique perspective to drive such octahedral tilting and study the phase transitions. We report the high-pressure study of Ba₃Zr₂S₇ up to 50 GPa using diamond anvil cells. In-situ Raman spectroscopy and X-ray diffraction analysis are carried out to probe the structural changes. Structural transitions of Ba₃Zr₂S₇ under pressure have been observed. Efforts in stabilizing the low-symmetry phases with hybrid improper ferroelectricity will also be discussed.