Structure-property relations in metal-substituted polar chalcogenides under pressure

Layered chalcogenides like CuInP2S6 are attracting attention for ferroelectricity at room temperature, enhanced properties under compression, and striking behavior in the monolayer limit. Less has been done to systematically uncover how the ground state changes with chemical substitution or unravel the combined effects of physical and chemical pressure. In this work, we combined diamond anvil cell techniques with synchrotron-based infrared absorbance and Raman scattering spectroscopies to explore symmetry and space group progressions of the CuM₂P₂S₆ (M₂ = Cr, Sc, V, In) family of materials under pressure. These materials host two structural phase transitions under compression; the first is near 3 GPa, and the second is near 12 GPa. Focusing on the Sc analog, we find that both even- and odd-symmetry modes are involved in the sequence of structural phase transitions. A group-subgroup analysis along with complementary lattice dynamics calculations and an analysis of the energy landscape reveals a Cc → P 31c progression across the first critical pressure. Taken together, these materials host a common set of transitions that follow trends in metal ion size. These structure-property relations can also be extended to ternary systems to support the development of pressure-tunable electronic devices.