Electronic Screening as a Design Lever in La-Doped BaTiO$_3$: Coupled Polarization, Dielectric Response, and Optical Anisotropy

Abstract

Donor substitution can tune the coupled ferroelectric, dielectric, electronic, and optical properties of perovskite oxides by modifying lattice distortions and electronic screening. We investigate these effects in La-doped BaTiO3 using density functional theory, density-functional perturbation theory, and the Berry-phase formalism for polarization. For a model with 12.5% La substitution at the Ba site, La incorporation induces anisotropic lattice contraction, reduces tetragonality and Ti off-centering, and weakens the energetic stabilization of the tetragonal ferroelectric phase. The accompanying reduction in polarization is associated with enhanced electronic screening and reduced anomalous Born effective charges. Phonon calculations indicate that the relaxed doped structure is dynamically stable, while hardening of a low-frequency polar mode is consistent with weakened ferroelectricity. The high-frequency dielectric response increases moderately, whereas the static dielectric response decreases as the lattice contribution is suppressed. Electronic-structure calculations show a Fermi-level shift toward the conduction band without mid-gap defect states. The optical spectra exhibit an absorption-edge shift consistent with a Burstein-Moss-type mechanism, enhanced near-edge transparency, and reduced birefringence. Together, these results suggest a microscopic connection between donor-induced screening, polar-mode hardening, reduced dynamical charges, polarization suppression, and decreased optical anisotropy. The conclusions apply to the investigated 12.5% composition; calculations at additional concentrations are needed to establish quantitative doping trends.