Pressure-induced ferroelectric-like transition creates a polar metal in defect antiperovskites Hg(3)Te(2)X(2) (X = Cl, Br)

Ferroelectricity is typically suppressed under hydrostatic compression because the short-range repulsions, which favor the nonpolar phase, increase more rapidly than the long-range interactions, which prefer the ferroelectric phase. Here, based on single-crystal X-ray diffraction and density-functional theory, we provide evidence of a ferroelectric-like transition from phase I2(1)3 to R3 induced by pressure in two isostructural defect antiperovskites Hg(3)Te(2)Cl(2) (15.5 GPa) and Hg(3)Te(2)Br(2) (17.5 GPa). First-principles calculations show that this transition is attributed to pressure-induced softening of the infrared phonon mode Γ(4), similar to the archetypal ferroelectric material BaTiO(3) at ambient pressure. Additionally, we observe a gradual band-gap closing from ~2.5 eV to metallic-like state of Hg(3)Te(2)Br(2) with an unexpectedly stable R3 phase even after semiconductor-to-metal transition. This study demonstrates the possibility of emergence of polar metal under pressure in this class of materials and establishes the possibility of pressure-induced ferroelectric-like transition in perovskite-related systems.