Discovery of an Ideal HgO ₁₂ Icosahedron and Magnetodielectric Coupling in HgCu ₃ Ti ₄ O ₁₂

We report the first successful high‐pressure synthesis and comprehensive characterization of A‐site‐ordered quadruple perovskite HgCu 3 Ti 4 O 12 . The compound crystallizes in the cubic Im ‐3 structure with Hg 2+ Cu 2+ 3 Ti 4+ 4 O 2− 12 valence configuration. A remarkable structural feature is the observation of a perfectly regular and undistorted HgO 12 icosahedron, representing the highest and most symmetric oxygen coordination environment reported for Hg 2+ in any oxide material. Magnetic and specific heat measurements reveal the antiferromagnetic ordering at T N = 31 K, driven by indirect super‐exchange interaction via the Cu 2+ ‐O 2− ‐Ti 4+ ‐O 2− ‐Cu 2+ pathway. The dielectric properties of HgCu 3 Ti 4 O 12 display a significantly lower, more intrinsic permittivity (∼10 2 ) with two distinct Debye‐type relaxation processes: a high‐temperature process related to oxygen vacancy–defect dipole reorientation, and a low‐temperature process attributed to low‐energy local lattice vibrations of the TiO 6 or CuO 4 units. Crucially, a dielectric anomaly around T N suggests the presence of magnetodielectric coupling, likely mediated by the aforementioned indirect super‐exchange interaction pathway. This work elucidates the structure–property relationships in HgCu 3 Ti 4 O 12 , highlights the critical role of cation chemistry in tailoring functional properties, and demonstrates the potential of A‐site‐ordered quadruple perovskites as a versatile platform for exploring coupled electronic, magnetic, and dielectric phenomena.