High-Pressure Synthesis, Crystal Structure, and Semimetallic Properties of HgPbO.

The crystal structure of HgPbO 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 Pb is octahedrally coordinated by six oxygen atoms as usual, while Hg 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 HgPbO. Although HgPbO has a carrier concentration (=7.3-8.5 × 10 cm) 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 emu mol at 300 K) that is in the same order of that of Bi powder and the remarkably small Sommerfeld coefficient [=1.6(1) × 10 J mol K] implied that it is semimetallic in nature. HgPbO 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 RbOsO. The mechanism of the anharmonic mode of the HgPbO has yet to be identified, however.