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The unusual structure of the mercury chalcogenides : relativistic effects in the solid state : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Massey University, Albany, New Zealand
Mercury oxide in its solid state crystallizes in a rather unusual structure and at ambient pressure two equally intriguing yet different polymorphs are distinguished. Both the low-temperature so-called montroydite form and the high-temperature cinnabar modification consist of planar O-Hg-O zigzag or spiral chains. These sophisticated structures significantly differ from those found for the lighter group 12 chalcogenides zinc oxide and cadmium oxide, which under ambient conditions are known to crystallize in rather simple hexagonal wurtzite and cubic rocksalt or zinc blende structures. Descending the chalcogenide group in the periodic table, the cinnabar structure disappears as an equilibrium modification for mercury selenide and telluride but is still present as a high-pressure modification. However, the deviations in the crystal arrangement between the mercury chalcogenides as opposed to the corresponding zinc and cadmium congeners are still obvious in terms of a different coordination. Considering this curious behaviour of the mercury chalcogenides the question arises: What causes the occurrence of the unusual structures in the mercury chalcogenides? To this end relativistic as well as nonrelativistic density functional calculations have been carried out to investigate this question with respect to the influence of relativistic effects. Even though relativistic effects in atoms and molecules are well-known and understood, little attention has been given to their influence on the electronic structure and properties of the solid state yet. The study at hand for the first time demonstrates that the structural differences between the mercury chalcogenides and their lighter zinc and cadmium congeners are a result of relativistic effects. The montroydite and cinnabar modifications of HgO and HgS disappear if relativity is neglected due to a substantial decrease of the cohesive energies by up to 2.2 eV. This deviation becomes smaller for HgSe and HgTe, yet a slight change in the coordination can be attributed to the influence of relativity. Furthermore, the electronic structure and density of states of the mercury chalcogenides are discussed with respect to relativistic effects including the consideration of spin-orbit effects. It was found that relativistic effects have a major impact on the electronic structure. In mercury selenide and telluride the neglect of relativity goes as far as changing the experimentally observed semimetallic behaviour to the restoration of semiconducting properties.