An optical and dielectric spectroscopy study of Er3+ -doped KTaO3

We report the results of a multifaceted study of optical and dielectric properties of erbium-doped (500 ppm) potassium tantalate incipient ferroelectric single crystals. Studies of optical absorption and photoluminescence spectra allowed us to determine the system of energy levels that control the main internal optical transitions in the 4f electronic shell of Er3+ in KTaO3. It was found that at least two types of Er3+ centres, major and minor ones, are present. The major centres are non-cubic, formed by Er3+ substituting for K+ sites; these are responsible for the n-type conductivity of KTaO3:Er crystals. Wideband optical absorption with a maximum at 1.13 eV (λ = 1097 nm) was observed in the near-IR range and was attributed to polaron formation. Low-temperature far-infrared reflectivity studies revealed an increase in the frequency of the lowest transverse optical mode and a decrease in the dielectric permittivity in comparison with undoped KTaO3 crystals. This stiffening means the suppression of the ferroelectric instability in the system.