Electronic structure, optical and dielectric properties of BaTiO3/CaTiO3/SrTiO3 ferroelectric superlattices from first-principles calculations
The electronic structure, lattice vibrations, and optical, dielectric and thermodynamic properties of BaTiO3/CaTiO3/SrTiO3 (BT/CT/ST) ferroelectric superlattices are calculated by using first-principles calculations. After relaxation, the lattice parameters are in good agreement with the experimental and other theoretical values within an error of 1%. The band structure shows an indirect band gap with a value of about 2.039 eV, and a direct band gap of 2.39 eV at the Γ point. The density of states and the electron charge density along the  axis are calculated and show the displacement of Ti ions along the  axis. The strong hybridization between O 2p and Ti 3d contributes to the ferroelectricity of BT/CT/ST ferroelectric superlattices. The Γ modes are stable, while the vibration modes at A, M, R, and X points are unstable governing the nature of phase transition. The static dielectric tensor including the ionic contribution is calculated and the permittivity parallel to the optical axis is found to be almost eight times more than the permittivity vertical to the axis, exhibiting strong anisotropy. The thermodynamic enthalpy, the free energy, the entropy, and the heat capacity are also investigated based on the phonon properties.