Optical Phonons in Mixed Sodium Potassium Tantalates

The infrared and Raman spectra of members of the mixed-crystal system ${\mathrm{Na}}_x{\mathrm{K}}_{1-x}\mathrm{Ta}{\mathrm{O}}_3$ have been studied at temperatures from 4 to 600°K for $x=0.0,0.12,0.40,0.65, \mathrm{and} 0.85$. The infrared data indicate that the low-frequency phonons are "soft" modes and are responsible for the ferroelectric phase change. The temperature dependence of the modes has been correlated with the dielectric behavior and is in agreement with current theories of displacive ferroelectrics. In the paraelectric phase, the Raman spectrum is entirely second order and qualitatively independent of cation concentration. First-order Raman bands are observed in the ferroelectric phase for $x<\mathrm{}0.70\mathrm{}$ and a $C_{4v}$ structure is strongly indicated. The frequencies of the optical modes at the center and at the boundary of the Brillouin zone have been deduced as a function of temperature and composition.