First-principles study of Cd impurities localized at and near the (001) α-Al2O3 surface

Abstract

Abstract A combination of two first-principles electronic structure calculation methods in the framework of density-functional theory was applied to investigate the (001) α-Al2O3 surface reconstruction and its structural, electronic, magnetic, and hyperfine properties when doped with Cd impurities at different depths from the surface. The SIESTA approach was used to obtain the equilibrium positions of all atoms and the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo) method was employed in order to obtain the electronic structure at these equilibrium positions and all the other physical properties. For the most stable (001) α-Al2O3 surface, we have demonstrated that the inclusion of the Cd atom at substitutional Al sites at and near the surface produces a ground state magnetic behavior. The largest principal component V33 of the electric-field-gradient (EFG) tensor at the Cd atom localized just above the α-Al2O3 terrace showed the same [001] orientation and a dominating p-character as Cd does when it is localized at bulk α-Al2O3, but exhibits an anomalous V33 magnitude four times larger than its value in bulk. Just below the surface, the non symmetric structural relaxation around the Cd impurity is responsible for the strong change in the asymmetry, magnitude, and orientation of the EFG tensor. The changes in the hyperfine properties have been correlated with the modifications observed on the electronic charge density at the different Cd sites and on the p-states of the Cd-projected partial density of states. Accordingly, the significant differences on the hyperfine parameters showed for different depths suggest that 111Cd probe-atoms used in Perturbed γ-γ Angular Correlation experiments could be used for evaluating geometrical and electronic distortions, particularly for positions quite close to the reconstructed surface, as well as contributing to studies of growth, adsorption, and diffusion of atoms in oxide surfaces and interfaces.