Electronic screening as a design lever in La-doped BaTiO₃: coupling between polarization, dielectric response, and...

Donor substitution is an effective strategy for modifying the coupled ferroelectric, dielectric, electronic, and optical properties of perovskite oxides by altering lattice distortion and electronic screening. In this work, first-principles calculations were performed to examine the microscopic effect of La donor substitution on BaTiO₃. A representative La-doped model, corresponding to 12.5% substitution at the Ba site, was investigated using density functional theory, density-functional perturbation theory, and the modern Berry-phase formalism. The reported lattice parameters indicate anisotropic contraction and a slight increase in tetragonality upon La incorporation, while the reported internal displacements and relative phase energies indicate decreased Ti off-centering and reduced energetic stabilization of the tetragonal structure relative to the cubic reference. The Berry-phase polarization is reduced accordingly, mainly due to enhanced donor-induced electronic screening and a decrease in anomalous dynamical charge transfer. Phonon dispersion calculations confirm that the La-doped structure remains dynamically stable, while the hardening of the low-frequency polar mode indicates suppression of the ferroelectric soft-mode instability. The dielectric response further reflects this behavior: the high-frequency dielectric constant increases moderately due to enhanced electronic screening, whereas the static dielectric response decreases as the lattice-mediated contribution is weakened. Electronic-structure calculations show that La doping shifts the Fermi level toward the conduction band without generating mid-gap defect states, leading to a Burstein-Moss-type shift of the optical absorption edge. The optical spectra also indicate improved near-edge transparency and reduced birefringence. The latter accompanies lower spontaneous polarization, although the reported lattice constants do not support attributing it to decreased tetragonality. Overall, the findings suggest a consistent microscopic connection between La-induced electronic screening, soft-mode modifications, reduced anomalous Born effective charges, polarization suppression, and decreased optical anisotropy. Since only one La concentration is explicitly considered, the 12.5% model is treated as a representative moderate-doping case rather than a complete concentration-dependent description. The results provide mechanistic insight into how donor substitution can tune the coupled ferroelectric and optical response of BaTiO₃-based oxides, while further calculations at multiple dopant concentrations would be valuable for establishing full quantitative doping trends.