Résumé:
We have used the pseudo-potential and plan wave method (PP-PW) based on density functional
theory (DFT) to examine the physical and mechanical properties of dental ceramics systems based
on the crystals Li2Si2O5, Ca5(PO4)3F and MgAl2O4, by calculating the structural properties, phase
stability, elastic properties, electronic and optical properties of these crystals (phases). The various
properties were calculated in the framework of the local density approximation (LDA) and the
generalized gradient approximation (GGA) using CASTEP code. The calculated structural
properties such as the equilibrium lattice parameters, cell volume and the bulk modulus are
generally in good agreement with the available experiment and theoretical results. The structural
stability of the different phases was evaluated by calculating their formation enthalpies. The
obtained results show that these phases are energetically more stable, thus suggesting the
thermodynamic stability of the studied materials. Our study of elastic properties indicates also that
these phases are mechanically stable in the normal conditions. Our values of the polycrystalline
elastic moduli are relatively close to the experimental values, thus indicating the reliability and the
predictive power of the applied method. We have found from these values and the hardness values
that the lithium disilicate phase (Li2Si2O5) exhibits a higher stiffness than that fluorapatite
(Ca5(PO4)3F), but it is less hard than the spinel phase (MgAl2O4). To further elucidate the insulating
nature of the different dental systems, we have calculated the band structure and density of states
(DOS) of Li2Si2O5, Ca5(PO4)3F and MgAl2O4 compounds. The calculations indicate that these phases have an insulator character. Optical parameters such as refractive index, extinction coefficient, absorption coefficient and optical reflectivity have been determined from the calculations of the complex dielectric function of these compounds. The results obtained allowed as to provide an insight into the optical behavior of the studied materials.
