Role of coordination site in governing the structural, electronic and optical properties of Ca-doped strontium barium niobate

The structural, electronic and optical properties of Ca doped SBN60 are studied via first principle calculations based on density functional theory. Influence of Sr site substitution by Ca at square and pentagonal sites has been studied. The bandgap of Ca incorporated SBN is found to depend largely on the coordination of site of doping, where the bandgap increases for the square site substitution and decreases for the pentagonal site substitution. The formation energy calculations reveal that the Ca is more favorable to occupy the pentagonal site which is supported by the UV-Visible spectra study of the thin films of doped and undoped SBN60 grown using PLD technique in which a decrease in the bandgap can be seen in Ca doped SBN. A detailed analysis is provided for the changes in structural parameters upon doping at square and pentagonal sites. Dependence of the optical properties like dielectric constant, refractive index, reflectance and absorption of doped and undoped SBN60 on different polarization directions '(100), (010) and (001)' are reported. The anisotropy in the optical properties can be observed which makes Ca doped SBN a promising material for optoelectronic device applications.