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 I213 to R3 induced by pressure in two isostructural defect antiperovskites Hg3Te2Cl2 (15.5 GPa) and Hg3Te2Br2 (17.5 GPa). First-principles calculations show that this transition is attributed to pressure-induced softening of the infrared phonon mode Γ4, similar to the archetypal ferroelectric material BaTiO3 at ambient pressure. Additionally, we observe a gradual band-gap closing from ~2.5 eV to metallic-like state of Hg3Te2Br2 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.

Cai, W., He, J., Li, H. et al. Pressure-induced ferroelectric-like transition creates a polar metal in defect antiperovskites Hg3Te2X2 (X = Cl, Br). Nat Commun 12, 1509 (2021). abstract

a Lattice parameters a, b, and c as a function of pressure obtained from single-crystal X-ray diffraction. b The third- and second-order Birch–Murnaghan equations of states (EOS) fit to the formula-unit volume (V/Z) data. These EOS have been used for calculating the ΔV collapse at the phase transition between phases I and II. The calculated bulk moduli are given in Supplementary Table 1. The inset in b indicates the c/a ratio of phase II in both compounds. Vertical black and blue dashed lines indicate phase transitions at 15.5 and 17.5 GPa for X = Cl and Br, respectively. The error bars are smaller than the symbols used in both a and b.