Abstract
The aim of this paper is a comparative study of structural stability and mechanical and optical properties of fluorapatite (FA) (Ca-5(PO4)(3)F) and lithium disilicate (LD) (Li2Si2O5), using the first principles pseudopotential method based on density functional theory (DFT) within the generalized gradient approximation (GGA). The stability of fluorapatite and lithium disilicate compounds has been evaluated on the basis of their formation enthalpies. The results show that fluorapatite is more energetically stable than lithium disilicate. The independent elastic constants and related mechanical properties, including bulk modulus (B), shear modulus (G), Young's modulus (E) and Poisson's ratio (m) as well as the Vickers hardness (Hv), have been calculated for fluorapatite compound and compared with other theoretical and experimental results. The obtained values of the shear modulus, Young's modulus and Vickers hardness are smaller in comparison with those of lithium disilicate compound, implying that lithium disilicate is more rigid than fluorapatite. The brittle and ductile properties were also discussed using B/G ratio and Poisson's ratio. Optical properties such as refractive index n(omega), extinction coefficient k(omega), absorption coefficient alpha(omega) and optical reflectivity R(omega) have been determined from the calculations of the complex dielectric function epsilon(omega), and interpreted on the basis of the electronic structures of both compounds. The calculated values of static dielectric constant epsilon(1)(0) and static refractive index n(0) show that the Li2Si2O5 compound has larger values compared to those of the Ca-5(PO4)(3)F compound. The results of the extinction coefficient show that Li2Si2O5 compound exhibits a much stronger ultraviolet absorption. According to the absorption and reflectivity spectra, we inferred that both compounds are theoretically the best visible and infrared transparent materials.