Soft optical phonon in Ba-doped Sr2Nb2O7 ceramics

The Raman scattering technique was applied to examine the soft optical phonon of the Ba0.32Sr0.68Nb2O7 in the temperature range from 26 to 1100 °C. The line shape of Raman spectra can be well fitted by multidamped–harmonic oscillator model. Two soft optical phonons were observed at 43.1, 52.4 cm−1 at room temperature. Both modes show softening approaching their corresponding phase transition temperature. It is found that damping factor γ of the lowest soft optical mode increases, while frequency of this mode decreases rapidly with temperature. Near Tc, the soft mode becomes over damped (γ/ω>1). The soft mode frequency related to the order parameter of this ferroelectric phase shows a non-classical behavior, and the critical exponent connected to the increase of the order parameters obtained as β=0.377. From the relation of soft mode frequency with temperature, the ferroelectric phase transition temperature Tc was estimated to be 1058 °C.