High-Pressure Synthesis, Crystal Structure, and Semimetallic Properties of HgPbO₃

The crystal structure of HgPbO₃ was studied using single-crystal X-ray diffraction and powder synchrotron X-ray diffraction. The structure was well characterized as a centrosymmetric model with a space group of R-3 m [hexagonal setting: a = 5.74413(6) Å and c = 7.25464(8) Å] rather than as a noncentrosymmetric model as was expected. It was found that Pb⁴⁺ is octahedrally coordinated by six oxygen atoms as usual, while Hg²⁺ is coordinated by three oxygen atoms in a planar manner, this being a very rare coordination of Hg in a solid-state material. The magnetic and electronic transport properties were investigated in terms of the magnetic susceptibility, magnetization, Hall coefficient, and specific heat capacity of polycrystalline HgPbO₃. Although HgPbO₃ has a carrier concentration (=7.3-8.5 × 10²⁰ cm^-3) that is equal to that of metallic oxides, the very weak temperature dependence of the electrical resistivity (residual-resistivity ratio ∼1.5), the significant diamagnetism (= -1.02 × 10^-4 emu mol^-1 at 300 K) that is in the same order of that of Bi powder and the remarkably small Sommerfeld coefficient [=1.6(1) × 10^-3 J mol^-1 K^-2] implied that it is semimetallic in nature. HgPbO₃ does not have a cage structure; nevertheless, at temperatures below approximately 50 K, it clearly exhibits phonon excitation of an anharmonic vibrational mode that is as significant as those of RbOs₂O₆. The mechanism of the anharmonic mode of the HgPbO₃ has yet to be identified, however.