Raman-scattering investigations in tetragonal tungsten bronze compounds. I. ${\mathrm{Ba}}_2$Na${\mathrm{Nb}}_5$${\mathrm{O}}_{15}$ and related crystals

Raman-scattering experiments have been performed on ${\mathrm{Ba}}_2$Na${\mathrm{Nb}}_5$ ${\mathrm{O}}_{15}$ crystals between 600 and 1.5 K. All the optical modes at $k=0\mathrm{}$ which are Raman active have been systematically investigated. No soft mode has been observed concerning the paraelastic-ferroelastic transition at $T_0\simeq 575$ K. The most obvious effect of this transition is the splitting of the $E$ modes of the quadratic phase into the $B_1\mathrm{}\left(x\right)\mathrm{}$ and $B_2\mathrm{}\left(y\right)\mathrm{}$ modes of the orthorhombic phase. The lowest-frequency $B_2\mathrm{}\left(y\right)\mathrm{}$ line gives rise to a Raman-inactive mode above $T_0$. Besides, this line is a feature characteristic of the orthorhombic structure. The evolution of its frequency (which softens markedly on cooling) as well as its intensity (which gives an estimate of the magnitude of the crystal orthorhombicity) is given versus temperature from $T_0$ to 1.5 K. The investigations are extended to ${\mathrm{Ba}}_{2+x}{\mathrm{Na}}_{1-2x}{\mathrm{Nb}}_5{\mathrm{O}}_{15}$ with respect to the variable $x$, these solid solutions being studied as single crystals or as polycrystalline ceramics. Substitutions of the cations (Ba by Sr and Na by K) and the Nb atoms by Ta, Ti, and W are also examined in ceramic samples by means of Raman scattering, the study being principally focused on the low-frequency vibrational spectra.