Vibrational spectroscopic study of ferroelectric SbNbO4, antiferroelectric BiNbO4, and their solid solutions

Raman scattering and infrared absorption studies of ferroelectric SbNb${\mathrm{O}}_4$, antiferroelectric BiNb${\mathrm{O}}_4$, and the solid solution $\left({\mathrm{Sb}}_{1-x}{\mathrm{Bi}}_x\right)\mathrm{Nb}{\mathrm{O}}_4$ for $0<x<\mathrm{}1\mathrm{}$ in the range 20-1200 ${\mathrm{cm}}^{-1\mathrm{}}$ are reported. These are the first vibrational spectroscopic investigations in these stibiotantalite-type systems. Symmetry assignments are proposed for the internal modes. Such an assignment is shown to lead to a consistent set of values for the local force constants calculated under the general quadratic valence force field. The nature of the variations of lattice and internal frequencies with changes in $A^{3+}$-ion concentration is qualitatively discussed with particular reference to the ferroelectric soft mode.