Multiple strong impacts of anion doping Λ (Λ = oxygen vacancy, nitrogen, or hydrogen) on the optical properties in the near-infrared to vacuum ultraviolet spectral range are experimentally revealed in cube-on-cube-type epitaxial perovskite BaTiO_3−δΛ_δ films (δ ≤ 0.01) grown on (001)SrTiO₃ substrates. Compared to the reference undoped films and bulk crystal, the anion-doped films exhibit massive changes of the index of refraction in the transparency range Δn of ∼0.1 to 0.2, a profound bandgap widening ΔE_g of ∼0.3 eV to 0.4 eV, a dopant-specific near-edge absorption, and doping-induced interband transitions. Both the conduction-band uplift and the changes in the index of refraction are ascribed to the internal stress and electric field, which result from epitaxially controlled dopant locations in the [Ba–O](001) planes. The dopant-induced in-gap states are responsible for the near-edge absorption, whereas the dopant-induced conduction-band states are suggested to modify the interband transitions and the intrinsic elasto- and electro-optic coefficients.

In this paper, we report the physical properties of ${\mathrm{Fe}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ single crystals. Fe-intercalated ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ exhibits superconductivity, pronounced Shubnikov–de Haas oscillations, high carrier mobility, and features of two-dimensional (2D) transport in the sample volume for $x$ as low as 0.02. The superconductivity of the samples is limited to the range $x=0.02–0.04$. We correlate all the transport properties with structural properties, namely, with mosaic structure of single crystals and point defects. This correlation in the context of the rich literature data allows us to build an intricate physical picture addressing the unique properties of the material under study within the framework of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ quasi-2D defect structure model. The ultimate aim of this paper is to show that many interesting properties of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ may be associated with inhomogeneous distribution of defects in the volume of the sample, which largely replicates the mosaicity of single crystals.