Electronic structure, chemical bonding, and optical properties of paraelectricBaTiO3

The electronic-energy band structure, site, and angular-momentum decomposed density of states (DOS) and charge-density contours of perovskite ${\mathrm{BaTiO}}_{3}$ in the paraelectric phase are calculated by the first-principles tight-binding linear muffin-tin orbitals method with the atomic-sphere approximation using density-functional theory in its local-density approximation. The calculated band structure shows a direct band gap of 1.2 eV at the \ensuremath{\Gamma} point in the Brillouin zone. The total DOS is compared to the experimental x-ray photoemission spectrum. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Ba and ${\mathrm{TiO}}_{3}$ is mainly ionic and that the ${\mathrm{TiO}}_{3}$ entities bond covalently. Using the projected DOS and band structure we have analyzed the interband contribution to the optical properties of ${\mathrm{BaTiO}}_{3}.$ The real and imaginary parts of the dielectric function and hence the optical constants (such as the reflectivity, refractive index, extinction coefficient, absorption coefficient, and the electron energy-loss spectrum) are calculated. The calculated spectra are compared with the experimental results for ${\mathrm{BaTiO}}_{3}$ at room temperature in the ferroelectric phase and are found to be in good agreement with the experimental data in the low-energy regions. The role of band-structure calculation as regards the optical properties of ${\mathrm{BaTiO}}_{3}$ is discussed.