Raman scattering investigations in tetragonal tungsten-bronze compounds. II.Ba2KxNa1−xNb5O15mixed crystals

Low-frequency optical modes of ${\mathrm{Ba}}_2{\mathrm{K}}_x{\mathrm{Na}}_{1-x}{\mathrm{Nb}}_5{\mathrm{O}}_{15}$ ceramics and crystals have been investigated for $0<~x<~1$ in both ferroelastic and paraelastic phases. The ferroelastic transition occurs only for $x<\mathrm{}0.5\mathrm{}$ and the Curie temperatures $T_0$ are determined as a function of $x$ from the splitting of the $E$ modes of the tetragonal phase into the $B_1$ and $B_2$ modes of the orthorhombic phase. These modes are then followed between $T_0$ and 1.5 K and comparisons are made with their already-known behavior in ${\mathrm{Ba}}_2$Na${\mathrm{Nb}}_5$ ${\mathrm{O}}_{15}$. The interest of Raman scattering as a tool of characterization of ${\mathrm{Ba}}_2{\mathrm{K}}_x{\mathrm{Na}}_{1-x}{\mathrm{Nb}}_5{\mathrm{O}}_{15}$ crystals, where the actual composition differs from that of the starting melt, is pointed out.