Towards fully-relativistic simulations of the adsorption of super-heavy elements on -SiO2 surfaces

While in the last two decades super-heavy elements with Z ≤ 112 have been studied, the focus of the present work is on the chemical properties of element 114. Our theoretical calculations have been motivated by two conflicting gas-chromatography experiments, which aimed on studying the interaction strength of element 114 with a gold surface. The experiment by Eichler et al. [1] reported adsorption in the chromatography column at only very low temperatures of approximately −90◦C, from which they concluded a weak interaction between element 114 and the gold surface. In contrast, experiments performed at GSI [2] observed adsorption at room temperature, indicating a much stronger bond between element 114 and gold. To resolve this conflict further experiments will be performed at GSI within the next two years, where besides gold SiO2 will be used as detector material. Besides the previously mentioned experiments, extensive theoretical studies on the adsorption of element 114 on gold surfaces were performed using fully-relativistic DFT methods [3], while the adsorption on inert surfaces such as SiO2 were estimated using semi-empirical methods in conjunction with computed properties of atoms, dimers, or small molecules [3, 4]. Unfortunately a fully-relativistic treatment of the entire adsorption process is beyond the capacities of nowadays computing ressources. However, this work can be divided into two steps: (i) extensive studies on SiO2 bulk and surface properties (e.g. stable bulk-phases or possible surface structures and terminations) using a nonrelativistic approach; (ii) fully-relativistic calculations on the adsorption process of element 114 on SiO2, where the most stable and interesting surface structures obtained in the first step serve as basis. So far we have focused on the first task, understanding the surface structure of SiO2 under realistic experimental conditions. For these calculations, the CASTEP code [6] with Vanderbilt-type ultrasoft pseudopotentials [7] and the PBE exchange–correlation functional has been used. The obtained DFT-energies were then used in conjunction with the ab initio atomistic thermodynamics approach [5] to evaluate the surface phase diagrams, providing information of the surface stability as function of surrounding temperature and pressure. Starting with bulk systems, our calculations show that at experimental conditions (pO2 = 10−13 atm, 100 K<T<320 K) the most stable bulk structure is the so called α-quartz. Using this crystal structure as basis, various surface orientations and morpholo-