The anisotropy of dielectric properties in the orthorhombic and hexagonal structures of Anhydrite -: an ab initio and hybrid DFT study

The effects of an electric field on the electronic properties of a typical ionic-covalent compound, Anhydrite (CaSO4), are investigated within the HF level of theory and two different formulations of hybrid DFT functionals (B3LYP [A. Becke, J. Chem. Phys. 98, 5648 (1993)] and F-0.6-BLYP [F. Cord et al., Structure and Bonding 113, 171-232 (2004)]). An external electric field is applied along each of the three periodic lattice vectors of the orthorhombic and hexagonal structures in order to detect the anisotropy of the response. The perturbation introduced by the field is analysed in terms of Mulliken charges and electron density maps. The largest response is due to a polarisation of the covalent SO bonds of the sulfate ions. The high frequency dielectric tensor, epsilon(infinity), is computed and compared with the experimental value; its anisotropy can be rationalised by the orientation of the sulfate ions relative to the three crystallographic directions. We find that the calculated value of epsilon(infinity) decreases on increasing the percentage of HF exchange in the Hamiltonian; the best match with experiment is given by B3LYP, but a higher percentage of HF exchange is required to reproduce the anisotropy in epsilon(infinity) a feature that we attribute to the better representation of the equilibrium geometry and bond distances in the latter case. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.