High-Pressure Synthesis of Two Polymorphic HgMnO3 Phases and Distinct Magnetism from 2D to 3D

An ilmenite-like monoclinic phase of HgMnO3 with space group P21/c was prepared using high-pressure and high-temperature methods at 18 GPa and 1473 K. The MnO6 octahedra form a two-dimensional (2D) network in the bc plane, leading to a long-range antiferromagnetic ordering with a low Néel temperature of T N ∼ 32 K. As the synthesis pressure increases to 20 GPa, a new perovskite-like rhombohedral phase with space group R3̅c was found to occur. The rhombohedral phase exhibits a three-dimensional (3D) network for the MnO6 octahedra, giving rise to an antiferromagnetic ordering at T N ∼ 60 K. X-ray absorption spectroscopy confirms the invariable Mn4+ charge state in these two polymorphic phases, in agreement with the Curie–Weiss and bond valence sum analysis. HgMnO3 provides an interesting example to study the magnetic properties from 2D to 3D by varying synthesis pressure.