Dielectric Properties and Soft Modes in the Ferroelectric Mixed CrystalsK1−xNaxTaO3

Mixed crystals of ${\mathrm{K}}_{1-x}{\mathrm{Na}}_x\mathrm{Ta}{\mathrm{O}}_3$ undergo ferroelectric transitions for $0.05<x<\mathrm{}0.\mathrm{}72$. A maximum in the paraelectric-ferroelectric transition temperature is observed for $x\simeq 0.55$. For $0.05<x<\mathrm{}0.\mathrm{}55$, the transition is to a tetragonal ferroelectric phase. For $0.55<x<\mathrm{}0.\mathrm{}72$, the transition appears to be from cubic perovskite to an orthorhombic phase. Underdamped "soft" modes have been observed by means of Raman scattering in the paraelectric and ferroelectric phases. Raman line shapes are considerably different from those previously found in ferroelectrics, and a simple empirical expression for line shape is presented.