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Significance Statement
The elements of the Group IIB are generally considered post-transition metals. With their d-orbitals completely filled, Zn, Cd and Hg only form naturally occurring compounds where their oxidation state goes up to +2, from their s-orbitals.
However, the relativistic effects over the orbital energy for mercury are very strong: Hg-Hg bonds are weaker than Cd-Cd, or its neighbor Au-Au, and in gas phase mercury is typically found in its monoatomic form. The weakness of this bond also explains how mercury is liquid at ambient conditions. Consequently a case can be made that the d-orbitals could be high enough in energy to play a role in the chemistry of mercury.
The chemistry of mercury has been researched extensively, both from theoretical and experimental approaches. Molecules containing Hg(IV) and Hg(III) have been predicted by theory, although these molecules are thermodynamically unstable. Recently, HgF4 has been observed experimentally in using matrix-isolation infrared (IR) spectroscopy. All the research shows that Hg can indeed achieve higher oxidation states, especially when paired with strong oxidizing elements like F, but these compounds fail to be stabilized under ambient conditions.
Coming from our previous experience with Cs-F compounds, we realized that high pressure could well be the method that would stabilize those exotic oxidation states for Hg, as it had done for Cs. By means of DFT calculations using VASP, coupled with the novel structure search algorithm of CALYPSO, we examined the HgFn (n=2…6) compounds and found that pressure indeed stabilizes both Hg(IV) and Hg(III). Beyond the ambient pressure form HgF2, at pressures comparable to the Earth’s mantle, HgF4 is the most thermodynamically stable HgFn compound. Furthermore, at pressures comparable to the Earth’s core, HgF3 becomes the most stable compound.
Journal Reference
Angewandte Chemie International Edition (Impact Factor: 11.26). 07/2015; 54(32).
Dr. Jorge Botana1,Dr. Xiaoli Wang1,2,Dr. Chunju Hou1,3,Prof. Dr. Dadong Yan4,Prof. Dr. Haiqing Lin1,Prof. Dr. Yanming Ma1,5,Prof. Dr. Mao-sheng Miao1,6,7
[expand title=”Show Affiliations”]- Beijing Computational Science Research Center, Beijing 100094 (P.R. China)
- Institute of Condensed Matter Physics, Linyi University, Linyi 276005 (P.R. China)
- School of Science, JiangXi University of Science and Technology, Ganzhou 341000 (P.R. China)
- Department of Physics, Beijing Normal University, Beijing 100875 (P.R. China)
- State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (P.R. China)
- Department of Chemistry and Biochemistry, California State University, Northridge, CA 91330 (USA)
- Materials Department and Materials Research Laboratory, University of California, Santa Barbara, CA 93106-5050 (USA)
Abstract
The inclusion of mercury among the transition metals is readily debated. Recently, molecular HgF4 was synthesized in a low-temperature noble gas but the potential of Hg to form compounds beyond a +2 oxidation state in a stable solid remains unresolved. We propose high-pressure techniques to prepare unusual oxidation states of Hg-based compounds. Using an advanced structure search algorithm and first-principles electronic structure calculations, we find that under high pressure Hg in HgF compounds transfers charge from the d orbitals to the F, thus behaving as a transition metal. Oxidizing Hg to +4 and +3 yielded the thermodynamically stable compounds HgF4 and HgF3. The former consists of HgF4 planar molecules, a typical geometry for d8 metal centers. HgF3 is metallic and ferromagnetic owing to the d9 configuration of mercury, with a large gap between its partially occupied and unoccupied bands under high pressure.
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