On the geometric structure of the (0 0 0 1) hematite surface

A theoretical study of the surface structural relaxation of the (0 0 0 1) hematite is presented which explores the influence of the size of a slab model, the number of relaxed surface layers and various computational approaches, all of them based on density functional theory (DFT). Four different slab models including 9, 12, 15, and 18 layers were analyzed. In each model, sequences from 1 to 7 layers were relaxed while keeping the bottom layers fixed to mimic the constraints imposed by the bulk. In the geometry optimization procedure, four different DFT approximation levels were employed, namely, the non-selfconsistent Harris functional, the local spin-density approximation, LSDA, the PW91 or BP meta GGA functionals and the hybrid B3LYP method. It was found that the relaxation predicted for up to four layers is independent of the slab thickness. Except for the Harris functional, all methods and models predict consistent values for the relaxation of the innermost layers. It is concluded that the Harris functional can be used to explore the adequacy of a given model but not to provide an accurate enough structure of the relaxed hematite (0 0 0 1) surface.