Structural and optical properties of paraelectric SrTiO₃

The electronic energy band structure, site- and angular-momentum-decomposed densities of states (DOS) and charge-density contours of perovskite SrTiO₃in the paraelectric cubic phase are calculated by the first-principles tight-binding linear muffin-tin orbitals method with atomic sphere approximation using density functional theory in its local density approximation. The calculated band structure shows a direct band gap of ~1.4 eV at the gamma point in the Brillouin zone. The total DOS is compared with experimental x-ray photoemission spectra. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Sr and TiO₃is mainly ionic and that the TiO₃entities bond covalently. Using the projected DOS and band structure we have analysed the interband contribution to the optical properties of SrTiO₃ . The real and imaginary parts of the dielectric function and hence the optical constants (such as the reflectivity, refractive index, extinction coefficient and absorption coefficient) and the electron energy-loss spectrum are calculated. The calculated spectra are compared with the experimental results for SrTiO₃in the cubic phase and are found to be in good agreement with the experimental results in low-energy regions. The role of band-structure calculation as regards the optical properties of SrTiO₃is discussed.