Influence of the central mode and soft phonon on the microwave dielectric loss near the strain-induced ferroelectric phase transitions in S rn+1 T in O3n+1

Recently, Lee et al. [Nature (London) 502, 532 (2013)NATUAS0028-083610.1038/nature12582] used ∼1% tensile strain to induce a ferroelectric instability in thin films of Srn+1TinO3n+1(n=1-6) phases. They showed that the Curie temperature TC gradually increased with n, reaching 180K for Sr7Ti6O19(n=6). The permittivity of this (n=6) sample could also be tuned significantly by the application of an electric field with exceptionally low dielectric loss at 300 K, rivaling all known tunable microwave dielectrics. Here, we present microwave (MW), terahertz, and infrared spectra of strained Srn+1TinO3n+1 thin films deposited on (110) DyScO3. Near the ferroelectric phase transitions, we observe the splitting and shifting of phonon and central mode frequencies, demonstrating the change of crystal symmetry below TC. Moreover, our spectra reveal that the central mode contribution dominates MW loss. In the Sr7Ti6O19 thin film, the central mode vanishes at 300 K, explaining its low MW loss. Finally, we discuss the origin and general conditions for the appearance of central modes near ferroelectric phase transitions.