Raman spectroscopy of KTN thin films

Abstract

The renewed interest in the KTa_1-x-Nb_xO (KTN) mixed perovskite materials, especially in single crystalline thin film forms, is connected with their remarkable dielectric properties in the dilute compositions. Off-center Nb ions in the highly polarizable KTaO₃ lattice provide a drastic increase in the dielectric peak up to 20 times in comparison with pure KTaO₃ and KNbO_3. KTN thin films with several Nb concentrations were prepared in the range 0 ≤ × ≤ 1 by pulsed laser deposition. The films were deposited onto MgO substrate. The effect of a substrate and symmetry-breaking defects was studied by micro-Raman spectroscopy. An anomalous residual intensity of the forbidden first-order scattering modes in the cubic paraelectric phase of the KTN films was connected with formation of polar microregions even far above the bulk T_c. On the whole, the KTN film behavior shows the existence of specific defects enhancing the unit perovskite cell in the film so that the activity of off-center Nb ions increases in producing larger electric dipoles and extending the precursor phase above T_c.