High-Pressure Mg₃Cl₇ Synthesized in a Diamond Anvil Cell as a Polar Metal with Second-Harmonic Generation

Polar metals have attracted growing interest due to both their significance in fundamental science and their potential functionalities. Here, we report the discovery of a novel polar metal, magnesium chloride Mg₃Cl₇, in which the metallicity of the polar structure is uniquely driven by attractive halogen interactions. Mg₃Cl₇ was synthesized in laser-heated diamond anvil cells and observed at pressures of 28(2)-93(3) GPa. Synchrotron single-crystal X-ray diffraction revealed that the structure of the new compound has polar hexagonal space group P6₃mc, representing an example of a previously unknown anti-Th₇Fe₃ structure type. Measurements of the physical properties have shown that the material is a metallic conductor capable of emitting second-harmonic generation light. Ab initio calculations support experimental findings and reveal complex halogen-halogen interactions, anionic metallicity, anisotropic electronic structure, and the presence of Dirac and Weyl points at the Fermi level. Our findings broaden the family of polar metals, provide new insight into halogen bonding under extreme conditions, and offer a platform for further exploration of materials' unconventional electronic behavior.