No abstract

No abstract

The electronic energy band structure, site- and angular-momentum-decomposed densities of states (DOS) and charge-density contours of perovskite SrTiO3 in 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 TiO3 is mainly ionic and that the TiO3 entities 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 SrTiO3 in 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 SrTiO3 is discussed.

We present a comprehensive theoretical investigation of paraelectric (cubic) and ferroelectric (tetragonal) BaTiO3. The atomic and electronic structure, piezoelectric tensor, Debye temperature, zone center phonon frequencies, and optical absorption are calculated for both phases from first principles. The structural and vibrational properties predicted from density functional theory are in good agreement with experiment and earlier theoretical work. The electronic structure and optical response are found to be very sensitive to quasiparticle and electron-hole attraction effects, which are accounted for by using the GW approach and by solving the Bethe-Salpeter equation, respectively. Electronic self-energy effects are found to open the band gap substantially, to 3.7 and 3.9 eV for the cubic and tetragonal phases, respectively. In contrast to earlier calculations, good agreement with the measured optical data is achieved. The ab initio thermodynamics predicts that the ferroelectric ordering will disappear at 419 K. It is shown that the phase transition is driven by the vibrational entropy of a variety of modes

No abstract