THz and IR dielectric response of BaTiO3 core-shell composites: evidence for interdiffusion

Time-domain THz transmission and FTIR reflectivity spectra of dense core-shell composites of the BaZr0.2Ti0.8O3, BaZr0.4Ti0.6O3, Ba0.66Sr0.34TiO3 and Ba0.45Sr0.55TiO3 compositions were studied in a broad temperature range of 10-900 K. The spectra were evaluated to obtain the complex dielectric functions and at room temperature they were compared with the prediction of the generalized brick-wall model based on the effective medium approximation. The model was analysed concerning the dielectric losses and electric-field tunability below the polar phonon frequency range. Whereas the predicted losses are much reduced compared with those of the cores, the observed enhanced losses in the THz range give evidence of an interdiffusion of the BaTiO3 cores into the shells.