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

Abstract

An ilmenite-like monoclinic phase of HgMnO 3 with space group P2 1 /c was prepared using high-pressure and hightemperature methods at 18 GPa and 1473 K. The MnO 6 octahedra form a two-dimensional (2D) network in the bc plane, leading to a longrange antiferromagnetic ordering with a low Neél temperature of T N ∼ 32 K. As the synthesis pressure increases to 20 GPa, a new perovskitelike rhombohedral phase with space group R3̅ c was found to occur. The rhombohedral phase exhibits a three-dimensional (3D) network for the MnO 6 octahedra, giving rise to an antiferromagnetic ordering at T N ∼ 60 K. X-ray absorption spectroscopy confirms the invariable Mn 4+ charge state in these two polymorphic phases, in agreement with the Curie−Weiss and bond valence sum analysis. HgMnO 3 provides an interesting example to study the magnetic properties from 2D to 3D by varying synthesis pressure.

High-Pressure Synthesized Perovskite CdMnO3 with C-Type Antiferromagnetic Spin Configuration

“…), 5 and R-HgMnO 3 (4.50 μ B /f.u.). 15 The AFM transition at T N ∼ 86 K is consistent with the negative Weiss constant (θ CW = −171(3) K) in the Curie− Weiss behavior of Mn 4+ spins. This transition temperature is lower than that (125 K) of the isostructural CaMnO 3 .…”

“…The Mn ion is located at the center of the slightly distorted octahedron with long Mn–O(1) bonds [1.931(1) Å] along the b axis and short Mn–O(2) bonds [1.894(5) and 1.930(4) Å] in the ac plane. The average Mn–O bond distance, 1.917 Å, is typical for Mn 4+ in oxides, − and the bond valence sum (BVS) actually gives +3.85. Significant tilting of the octahedra (the bond angles of 148.7(3)° for Mn–O(1)–Mn and 152.5(3)° for Mn–O(2)–Mn) is similar to that in CaMnO 3 but the tilting distortion of the present CdMnO 3 is much enhanced (corresponding bond angles for CaMnO 3 ; 152.68° and 159.47°).…”

“…On the other hand, relatively small divalent ions like Hg 2+ , Zn 2+ , and Mg 2+ can be included in A 2+ Mn 4+ O 3 with the ilmenite structure rather than the perovskite structure. , Hg 2+ , Zn 2+ , and Mg 2+ prefer octahedral oxygen coordination, and 12-fold oxygen coordination for those ions is rare. This explains why HgMnO 3 , ZnMnO 3 , and MgMnO 3 crystalize in the ilmenite structure with the 6-fold oxygen coordinated A-site ions.…”

High-Pressure Synthesis and Magnetism of the 4H-BaMnO3 Single Crystal and Its 6H-Type Polymorph

“…Perovskite structure oxides with a chemical formula of ABO 3 have diverse structural derivatives and A-B charge combinations and thus exhibit a series of interesting physical properties in magnetism, electrical transport, magnetoelectric coupling, and chemical catalysis. − The detailed crystal structure of perovskite is closely related to the relative size of cations at both A and B sites. Goldschmidt’s tolerance factor, t = ( r A + r O )/(√2)­( r B + r O ), is often used to describe such a relationship. , Here r A , r B , and r O denote the ionic radii of A, B, and O ions, respectively.…”

“…y 0.3817(8) Mn−O2 (Å) ×3 1.879(3) Uiso(Ba1) (100 × Å 2 ) 0.184(2) ∠Mn−O1−Mn (deg) 180.00 Uiso(Ba2) (100 × Å 2 ) 0.060(2) ∠Mn−O2−Mn (deg) 84.5(2) Uiso(Mn) (100 × Å 2 )…”